Author name code: keller ADS astronomy entries on 2022-09-14 author:"Keller, Christoph U." ------------------------------------------------------------------------ Title: Upgrading the high contrast imaging facility SPHERE: science drivers and instrument choices Authors: Boccaletti, A.; Chauvin, G.; Wildi, F.; Milli, J.; Stadler, E.; Diolaiti, E.; Gratton, R.; Vidal, F.; Loupias, M.; Langlois, M.; Cantalloube, F.; N'Diaye, M.; Gratadour, D.; Ferreira, F.; Tallon, M.; Mazoyer, J.; Segransan, D.; Mouillet, D.; Beuzit, J. -L.; Bonnefoy, M.; Galicher, R.; Vigan, A.; Snellen, I.; Feldt, M.; Desidera, S.; Rousseau, S.; Baruffolo, A.; Goulas, C.; Baudoz, P.; Bechet, C.; Benisty, M.; Bianco, A.; Carry, B.; Cascone, E.; Charnay, B.; Choquet, E.; Christiaens, V.; Cortecchia, F.; de Caprio, V.; De Rosa, A.; Desgrange, C.; D'Orazi, V.; Douté, S.; Frangiamore, M.; Gendron, E.; Ginski, C.; Huby, E.; Keller, C.; Kulcsár, C.; Landman, R.; Lagarde, S.; Lagadec, E.; Lagrange, A. -M.; Kasper, M. Lombini M.; Ménard, F.; Magnard, Y.; Malaguti, G.; Maurel, D.; Mesa, D.; Morgante, G.; Pantin, E.; Pichon, T.; Potier, A.; Rabou, P.; Rochat, S.; Terenzi, L.; Thiébaut, E.; Tallon-Bosc, I.; Raynaud, H. -F.; Rouan, D.; Sevin, A.; Schiavone, F.; Schreiber, L.; Zanutta, A. Bibcode: 2022arXiv220902092B Altcode: SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT, which is intended to boost the current performances of detection and characterization for exoplanets and disks. SPHERE+ will also serve as a demonstrator for the future planet finder (PCS) of the European ELT. The main science drivers for SPHERE+ are 1/ to access the bulk of the young giant planet population down to the snow line ($3-10$ au), to bridge the gap with complementary techniques (radial velocity, astrometry); 2/ to observe fainter and redder targets in the youngest ($1-10$\,Myr) associations compared to those observed with SPHERE to directly study the formation of giant planets in their birth environment; 3/ to improve the level of characterization of exoplanetary atmospheres by increasing the spectral resolution in order to break degeneracies in giant planet atmosphere models. Achieving these objectives requires to increase the bandwidth of the xAO system (from $\sim$1 to 3\,kHz) as well as the sensitivity in the infrared (2 to 3\,mag). These features will be brought by a second stage AO system optimized in the infrared with a pyramid wavefront sensor. As a new science instrument, a medium resolution integral field spectrograph will provide a spectral resolution from 1000 to 5000 in the J and H bands. This paper gives an overview of the science drivers, requirements and key instrumental trade-off that were done for SPHERE+ to reach the final selected baseline concept. Title: Joint optimization of wavefront sensing and reconstruction with automatic differentiation Authors: Landman, Rico; Keller, Christoph; Por, Emiel H.; Haffert, Sebastiaan; Doelman, David; Stockmans, Thijs Bibcode: 2022arXiv220905904L Altcode: High-contrast imaging instruments need extreme wavefront control to directly image exoplanets. This requires highly sensitive wavefront sensors which optimally make use of the available photons to sense the wavefront. Here, we propose to numerically optimize Fourier-filtering wavefront sensors using automatic differentiation. First, we optimize the sensitivity of the wavefront sensor for different apertures and wavefront distributions. We find sensors that are more sensitive than currently used sensors and close to the theoretical limit, under the assumption of monochromatic light. Subsequently, we directly minimize the residual wavefront error by jointly optimizing the sensing and reconstruction. This is done by connecting differentiable models of the wavefront sensor and reconstructor and alternatingly improving them using a gradient-based optimizer. We also allow for nonlinearities in the wavefront reconstruction using Convolutional Neural Networks, which extends the design space of the wavefront sensor. Our results show that optimization can lead to wavefront sensors that have improved performance over currently used wavefront sensors. The proposed approach is flexible, and can in principle be used for any wavefront sensor architecture with free design parameters. Title: Spectropolarimetry of life: airborne measurements from a hot air balloon Authors: Mulder, Willeke; Patty, C. H. Lucas; Spadaccia, Stefano; Pommerol, Antoine; Demory, Brice-Olivier; Keller, Christoph U.; Kühn, Jonas G.; Snik, Frans; Stam, Daphne M. Bibcode: 2022arXiv220802317M Altcode: Does life exist outside our Solar System? A first step towards searching for life outside our Solar System is detecting life on Earth by using remote sensing applications. One powerful and unambiguous biosignature is the circular polarization resulting from the homochirality of biotic molecules and systems. We aim to investigate the possibility of identifying and characterizing life on Earth by using airborne spectropolarimetric observations from a hot air balloon during our field campaign in Switzerland, May 2022. In this work we present the optical-setup and the data obtained from aerial circular spectropolarimetric measurements of farmland, forests, lakes and urban sites. We make use of the well-calibrated FlyPol instrument that measures the fractionally induced circular polarization ($V/I$) of (reflected) light with a sensitivity of $<10^{-4}$. The instrument operates in the visible spectrum, ranging from 400 to 900 nm. We demonstrate the possibility to distinguish biotic from abiotic features using circular polarization spectra and additional broadband linear polarization information. We review the performance of our optical-setup and discuss potential improvements. This sets the requirements on how to perform future airborne spectropolarimetric measurements of the Earth's surface features from several elevations. Title: Detecting life outside our solar system with a large high-contrast-imaging mission Authors: Snellen, Ignas A. G.; Snik, F.; Kenworthy, M.; Albrecht, S.; Anglada-Escudé, G.; Baraffe, I.; Baudoz, P.; Benz, W.; Beuzit, J. -L.; Biller, B.; Birkby, J. L.; Boccaletti, A.; van Boekel, R.; de Boer, J.; Brogi, Matteo; Buchhave, L.; Carone, L.; Claire, M.; Claudi, R.; Demory, B. -O.; Désert, J. -M.; Desidera, S.; Gaudi, B. S.; Gratton, R.; Gillon, M.; Grenfell, J. L.; Guyon, O.; Henning, T.; Hinkley, S.; Huby, E.; Janson, M.; Helling, C.; Heng, K.; Kasper, M.; Keller, C. U.; Krause, O.; Kreidberg, L.; Madhusudhan, N.; Lagrange, A. -M.; Launhardt, R.; Lenton, T. M.; Lopez-Puertas, M.; Maire, A. -L.; Mayne, N.; Meadows, V.; Mennesson, B.; Micela, G.; Miguel, Y.; Milli, J.; Min, M.; de Mooij, E.; Mouillet, D.; N'Diaye, M.; D'Orazi, V.; Palle, E.; Pagano, I.; Piotto, G.; Queloz, D.; Rauer, H.; Ribas, I.; Ruane, G.; Selsis, F.; Sozzetti, A.; Stam, D.; Stark, C. C.; Vigan, A.; de Visser, Pieter Bibcode: 2021ExA...tmp..124S Altcode: In this White Paper, which was submitted in response to the European Space Agency (ESA) Voyage 2050 Call, we recommend the ESA plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We provide an overview of relevant European expertise, and advocate ESA to start a technology development program towards detecting life outside the Solar System. Title: LOUPE: Observing the Earth from the Moon to prepare for detecting life on Earth-like exoplanets Authors: Klindžić, Dora; Stam, Daphne; Snik, Frans; Keller, Christoph; Pallichadath, Vidhya; van Dijk, Chris; Esposito, Marco; van Dam, Dirk Bibcode: 2021EPSC...15..657K Altcode: LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as if it were an exoplanet, designed to accompany any landing, roving or orbiting mission to the Moon. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterizing such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour life. These data will be used to test numerical codes to predict signals of Earth-like exoplanets, to test algorithms that retrieve planet properties, and to fine-tune the design and observational strategies of future space observatories.We present a novel spectropolarimetric instrument design: LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth (Klindžić, 2020), which aims to observe the Earth from the Moon as if it were an exoplanet and perform spectropolarimetric measurements spanning the full range of phase angles. Various reasons make observing the Earth from the Moon or from a Lunar orbit, rather than a low Earth orbit, crucial to the experiment:The Moon is sufficiently far away to allow a spatially unresolved view of the whole Earth. For a lander on the Lunar surface, the Earth is always visible in a confined area in the sky. From the Moon, the Earth can be observed at all phase angles during a month. From the Moon, the Earth's daily rotation can be captured. LOUPE's science requirements include:Perform near-instantaneous (snapshot) spectropolarimetry of the entire Earth. Detect the presence of liquid water oceans and clouds. Derive and monitor atmospheric properties, e.g. via Rayleigh scattering, for potential climate research applications. Detect the O₂A band in flux and polarization and its variance with cloud cover, altitude and phase angle. Detect the Chlorophyll Green Bump and Vegetation Red Edge, the spectroscopic signature of plant life. Derive a map of continents from the disk-integrated signal and identify notable features, such as rainforests, deserts and ice caps. LOUPE shall perform its science goals by recording and demodulating the disk-integrated Stokes vector of sunlight reflected from the Earth. The leading instrument design principle adopted for LOUPE is to create a compact, low-mass, low-volume, space-ready hyperspectropolarimeter with no moving parts. These constraints require creative solutions from the cutting edge of hyperspectral and polarimetric instrument design, where polarimeters traditionally used active rotating optics (temporal modulation) or beam-splitting (spatial modulation).The latest LOUPE concept (Fig. 1.) utilizes Patterned Liquid Crystal (PLC) plates for encoding polarization information as a modulation orthogonal to the spectral flux measurement, enabling the linear-Stokes vector of a target to be recorded in one single "snapshot", as shown in Fig. 2. Unlike a traditional rotating-retarder polarimeter, polarization is modulated in the cross-spectral direction, meaning polarimetry can be performed at full spectral resolution, which is not possible in the case of channeled spectropolarimetry with spectral modulation. This pioneering use of Patterned Liquid Crystals makes it possible to forgo the use of moving elements, resulting in a compact, space-ready instrument with versatile options of installation on a range of landing, roving and orbiting missions.Here we discuss our detailed design process and the challenges involved in creating a unique space-qualified spectropolarimeter with no moving parts, whilst maintaining flexibility for different usage scenarios: rovers, landers, orbiters, and more. We present a performance trade-off, optical design informed by ray tracing with polarization effects, and the development of methods for spectral and polarimetric demodulation of simulated Earth observation data.Figure 1: Tentative design of LOUPE. Figure 2: Simulated LOUPE measurement. Wavelength filtering is applied in the y-direction, and polarization modulation in the x-direction. Each dot represents an unresolved image of the Earth. Title: Pale polarized dots: spectropolarimetry of the Earth as an exoplanet with LOUPE Authors: Klindžić, Dora; Snik, Frans; Stam, Daphne M.; Keller, Christoph U.; Stockmans, Thijs; Hoeijmakers, H. Jens; van Dam, Dirk M.; Willebrands, Michele; Karalidi, Theodora; Pallichadath, Vidhya; van Dijk, Chris N.; Esposito, M. Bibcode: 2021SPIE11833E..06K Altcode: We present LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, a compact snapshot spectropolarimeter designed to observe the Earth from the Moon as if it were an exoplanet. Viewing the Earth as it would be seen by a faraway observer will offer novel insight into the spectropolarimetric signatures of planets harboring life, as well as a chance to refine algorithms for the retrieval of exoplanetary properties such as the presence of liquid water, clouds, vegetation, and more. LOUPE boasts a novel solid-state design based on patterned liquid crystal optics built atop the cosine HyperScout®, a flight-proven hyperspectral imager. Uniquely to LOUPE, a microlens array creates a two- dimensional grid of unresolved Earth-images on the detector, resulting in an array of "pale (blue) dots" filtered spectrally along one direction, with polarization modulation applied in the perpendicular direction. The clever use of custom-patterned liquid crystals as a passive modulator thus replaces the need for classical dispersion elements and polarization modulation optics. This pioneering approach enables LOUPE to simultaneously obtain spectral and Stokes measurements for the entire Earth, whilst the position of the Earth-dots also has the benefit of providing input for angle-dependent spectral and polarization calibration. Here we discuss our detailed design process and the challenges involved in creating a unique, space-qualified spectropolarimeter with no moving parts and no bulky optics, whilst maintaining flexibility for different usage scenarios: rovers, landers, orbiters, and more. We present a performance trade-off and optical design informed by ray tracing with polarization effects, to prepare for the demodulation of simulated Earth observation data. Title: Spatial polarization modulators: distinguishing diffraction effects from spatial polarization modulation Authors: Mulder, Willeke; Doelman, David S.; Keller, Christoph U.; Patty, C. H. Lucas; Snik, Frans Bibcode: 2021SPIE11833E..0MM Altcode: 2021arXiv210802538M Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump, as well as circular polarization resulting from the homochirality of biotic molecules. LSDpol is optimized for sensing circular polarization. To this end, LSDpol employs a spatial light modulator in the entrance slit of the spectrograph, a liquid-crystal quarter-wave retarder where the fast axis rotates as a function of slit position. The original design of LSDpol implemented a dual-beam spectropolarimeter by combining a quarter-wave plate with a polarization grating. Unfortunately, this design causes significant linear-to-circular cross-talk. In addition, it revealed spurious polarization modulation effects. Here, we present numerical simulations that illustrate how Fresnel diffraction effects can create these spurious modulations. We verified the simulations with accurate polarization state measurements in the lab using 100% linearly and circularly polarized light. Title: Spectropolarimetry Authors: Keller, Christoph U.; Snik, Frans Bibcode: 2021hai3.book..239K Altcode: No abstract at ADS Title: Vector-apodizing phase plate coronagraph: design, current performance, and future development [Invited] Authors: Doelman, D. S.; Snik, F.; Por, E. H.; Bos, S. P.; Otten, G. P. P. L.; Kenworthy, M.; Haffert, S. Y.; Wilby, M.; Bohn, A. J.; Sutlieff, B. J.; Miller, K.; Ouellet, M.; de Boer, J.; Keller, C. U.; Escuti, M. J.; Shi, S.; Warriner, N. Z.; Hornburg, K.; Birkby, J. L.; Males, J.; Morzinski, K. M.; Close, L. M.; Codona, J.; Long, J.; Schatz, L.; Lumbres, J.; Rodack, A.; Van Gorkom, K.; Hedglen, A.; Guyon, O.; Lozi, J.; Groff, T.; Chilcote, J.; Jovanovic, N.; Thibault, S.; de Jonge, C.; Allain, G.; Vallée, C.; Patel, D.; Côté, O.; Marois, C.; Hinz, P.; Stone, J.; Skemer, A.; Briesemeister, Z.; Boehle, A.; Glauser, A. M.; Taylor, W.; Baudoz, P.; Huby, E.; Absil, O.; Carlomagno, B.; Delacroix, C. Bibcode: 2021ApOpt..60D..52D Altcode: 2021arXiv210411211D Over the last decade, the vector-apodizing phase plate (vAPP) coronagraph has been developed from concept to on-sky application in many high-contrast imaging systems on 8-m class telescopes. The vAPP is an geometric-phase patterned coronagraph that is inherently broadband, and its manufacturing is enabled only by direct-write technology for liquid-crystal patterns. The vAPP generates two coronagraphic PSFs that cancel starlight on opposite sides of the point spread function (PSF) and have opposite circular polarization states. The efficiency, that is the amount of light in these PSFs, depends on the retardance offset from half-wave of the liquid-crystal retarder. Using different liquid-crystal recipes to tune the retardance, different vAPPs operate with high efficiencies ($>96\%$) in the visible and thermal infrared (0.55 $\mu$m to 5 $\mu$m). Since 2015, seven vAPPs have been installed in a total of six different instruments, including Magellan/MagAO, Magellan/MagAO-X, Subaru/SCExAO, and LBT/LMIRcam. Using two integral field spectrographs installed on the latter two instruments, these vAPPs can provide low-resolution spectra (R$\sim$30) between 1 $\mu$m and 5 $\mu$m. We review the design process, development, commissioning, on-sky performance, and first scientific results of all commissioned vAPPs. We report on the lessons learned and conclude with perspectives for future developments and applications. Title: Self-optimizing adaptive optics control with reinforcement learning for high-contrast imaging Authors: Landman, Rico; Haffert, Sebastiaan Y.; Radhakrishnan, Vikram M.; Keller, Christoph U. Bibcode: 2021JATIS...7c9002L Altcode: 2021arXiv210811332L Current and future high-contrast imaging instruments require extreme adaptive optics systems to reach contrasts necessary to directly imaged exoplanets. Telescope vibrations and the temporal error induced by the latency of the control loop limit the performance of these systems. One way to reduce these effects is to use predictive control. We describe how model-free reinforcement learning can be used to optimize a recurrent neural network controller for closed-loop predictive control. First, we verify our proposed approach for tip-tilt control in simulations and a lab setup. The results show that this algorithm can effectively learn to mitigate vibrations and reduce the residuals for power-law input turbulence as compared to an optimal gain integrator. We also show that the controller can learn to minimize random vibrations without requiring online updating of the control law. Next, we show in simulations that our algorithm can also be applied to the control of a high-order deformable mirror. We demonstrate that our controller can provide two orders of magnitude improvement in contrast at small separations under stationary turbulence. Furthermore, we show more than an order of magnitude improvement in contrast for different wind velocities and directions without requiring online updating of the control law. Title: Biosignatures of the Earth. I. Airborne spectropolarimetric detection of photosynthetic life Authors: Patty, C. H. Lucas; Kühn, Jonas G.; Lambrev, Petar H.; Spadaccia, Stefano; Jens Hoeijmakers, H.; Keller, Christoph; Mulder, Willeke; Pallichadath, Vidhya; Poch, Olivier; Snik, Frans; Stam, Daphne M.; Pommerol, Antoine; Demory, Brice-Olivier Bibcode: 2021A&A...651A..68P Altcode: 2021arXiv210600493P Context. Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. Additionally, circular polarization can be a valuable tool in Earth remote sensing because the circular polarization signal directly relates to vegetation physiology.
Aims: While high-quality circular polarization measurements can be obtained in the laboratory and under semi-static conditions in the field, there has been a significant gap to more realistic remote sensing conditions.
Methods: In this study, we present sensitive circular spectropolarimetric measurements of various landscape elements taken from a fast-moving helicopter.
Results: We demonstrate that during flight, within mere seconds of measurements, we can differentiate (S∕N > 5) between grass fields, forests, and abiotic urban areas. Importantly, we show that with only nonzero circular polarization as a discriminant, photosynthetic organisms can even be measured in lakes.
Conclusions: Circular spectropolarimetry can be a powerful technique to detect life beyond Earth, and we emphasize the potential of utilizing circular spectropolarimetry as a remote sensing tool to characterize and monitor in detail the vegetation physiology and terrain features of Earth itself. Title: A MUSE view of the asymmetric jet from HD 163296 Authors: Xie, C.; Haffert, S. Y.; de Boer, J.; Kenworthy, M. A.; Brinchmann, J.; Girard, J.; Snellen, I. A. G.; Keller, C. U. Bibcode: 2021A&A...650L...6X Altcode: 2021arXiv210601661X Context. Jets and outflows are thought to play important roles in regulating star formation and disk evolution. An important question is how the jets are launched. HD 163296 is a well-studied Herbig Ae star that hosts proto-planet candidates, a protoplanetary disk, a protostellar jet, and a molecular outflow, which makes it an excellent laboratory for studying jets.
Aims: We aim to characterize the jet at the inner regions and check if there are large differences with the features at large separations. A secondary objective is to demonstrate the performance of Multi Unit Spectroscopic Explorer (MUSE) in high-contrast imaging of extended line emission.
Methods: MUSE in the narrow field mode (NFM) can provide observations at optical wavelengths with high spatial (∼75 mas) and medium spectral (R ∼ 2500) resolution. With the high-resolution spectral differential imaging technique, we can characterize the kinematic structures and physical conditions of jets down to 100 mas.
Results: We detect multiple atomic lines in two new knots, B3 and A4, at distances of < 4″ from the host star with MUSE. The derived \Mdot;jet/\Mdot;acc is about 0.08 and 0.06 for knots B3 and A4, respectively. The observed [Ca II]/[S II] ratios indicate that there is no sign of dust grains at distances of < 4″. Assuming the A4 knot traced the streamline, we can estimate a jet radius at the origin by fitting the half width half maximum of the jet, which sets an upper limit of 2.2 au on the size of the launching region. Although MUSE has the ability to detect the velocity shifts caused by high- and low-velocity components, we found no significant evidence of velocity decrease transverse to the jet direction in our 500 s MUSE observation.
Conclusions: Our work demonstrates the capability of using MUSE NFM observations for the detailed study of stellar jets in the optical down to 100 mas. The derived \Mdot;jet/\Mdot;acc, no dust grain, and jet radius at the star support the magneto-centrifugal models as a launching mechanism for the jet. Title: First light of a holographic aperture mask: Observation at the Keck OSIRIS Imager Authors: Doelman, David S.; Wardenier, Joost P.; Tuthill, Peter; Fitzgerald, Michael P.; Lyke, Jim; Sallum, Steph; Norris, Barnaby; Warriner, N. Zane; Keller, Christoph; Escuti, Michael J.; Snik, Frans Bibcode: 2021A&A...649A.168D Altcode: 2021arXiv210411210D Context. As an interferometric technique, sparse aperture masking (SAM) is capable of imaging beyond the diffraction limit of single telescopes. This makes SAM an important technique for studying processes such as planet formation at Solar System scales. However, it comes at the cost of a reduction in throughput, typically by 80-90%.
Aims: We report on the design, construction, and commissioning of a prototype aperture masking technology implemented at the Keck OH-Suppressing Infrared Integral Field Spectrograph (OSIRIS) Imager: the holographic aperture mask. Holographic aperture masking (HAM) aims at (i) increasing the throughput of SAM by selectively combining all subapertures across a telescope pupil in multiple interferograms using a phase mask, and (ii) adding low-resolution spectroscopic capabilities.
Methods: Using liquid-crystal geometric phase patterns, we manufacture a HAM mask that uses an 11-hole SAM design as the central component and a holographic component comprising 19 different subapertures. Thanks to a multilayer liquid-crystal implementation, the mask has a diffraction efficiency higher than 96% from 1.1 to 2.5 micron. We create a pipeline that extracts monochromatic closure phases from the central component as well as multiwavelength closure phases from the holographic component. We test the performance of the HAM mask in the laboratory and on-sky.
Results: The holographic component yields 26 closure phases with spectral resolutions between R ∼ 6.5 and R ∼ 15, depending on the interferogram positions. On April 19, 2019, we observed the binary star HDS 1507 in the Hbb filter (λ0 = 1638 nm and Δλ = 330 nm) and retrieved a constant separation of 120.9 ± 0.5 mas for the independent wavelength bins, which is in excellent agreement with literature values. For both the laboratory measurements and the observations of unresolved reference stars, we recorded nonzero closure phases - a potential source of systematic error that we traced to polarization leakage of the HAM optic. We propose a future upgrade that improves the performance, reducing this effect to an acceptable level.
Conclusions: Holographic aperture masking is a simple upgrade of SAM with increased throughput and a new capability of simultaneous low-resolution spectroscopy that provides new differential observables (e.g., differential phases with wavelength). Title: PCS — A Roadmap for Exoearth Imaging with the ELT Authors: Kasper, M.; Cerpa Urra, N.; Pathak, P.; Bonse, M.; Nousiainen, J.; Engler, B.; Heritier, C. T.; Kammerer, J.; Leveratto, S.; Rajani, C.; Bristow, P.; Le Louarn, M.; Madec, P. -Y.; Ströbele, S.; Verinaud, C.; Glauser, A.; Quanz, S. P.; Helin, T.; Keller, C.; Snik, F.; Boccaletti, A.; Chauvin, G.; Mouillet, D.; Kulcsár, C.; Raynaud, H. -F. Bibcode: 2021Msngr.182...38K Altcode: 2021arXiv210311196K The Planetary Camera and Spectrograph (PCS) for the Extremely Large Telescope (ELT) will be dedicated to detecting and characterising nearby exoplanets with sizes from sub-Neptune to Earth-size in the neighbourhood of the Sun. This goal is achieved by a combination of eXtreme Adaptive Optics (XAO), coronagraphy and spectroscopy. PCS will allow us not only to take images, but also to look for biosignatures such as molecular oxygen in the exoplanets' atmospheres. This article describes the PCS primary science goals, the instrument concept and the research and development activities that will be carried out over the coming years. Title: A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS. First polarimetric detections revealing disks around DH Tau B and GSC 6214-210 B Authors: van Holstein, R. G.; Stolker, T.; Jensen-Clem, R.; Ginski, C.; Milli, J.; de Boer, J.; Girard, J. H.; Wahhaj, Z.; Bohn, A. J.; Millar-Blanchaer, M. A.; Benisty, M.; Bonnefoy, M.; Chauvin, G.; Dominik, C.; Hinkley, S.; Keller, C. U.; Keppler, M.; Langlois, M.; Marino, S.; Ménard, F.; Perrot, C.; Schmidt, T. O. B.; Vigan, A.; Zurlo, A.; Snik, F. Bibcode: 2021A&A...647A..21V Altcode: 2021arXiv210104033V Context. Young giant planets and brown dwarf companions emit near-infrared radiation that can be linearly polarized up to several percent. This polarization can reveal the presence of an (unresolved) circumsubstellar accretion disk, rotation-induced oblateness of the atmosphere, or an inhomogeneous distribution of atmospheric dust clouds.
Aims: We aim to measure the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions.
Methods: We observed the companions with the high-contrast imaging polarimeter SPHERE-IRDIS at the Very Large Telescope. We reduced the data using the IRDAP pipeline to correct for the instrumental polarization and crosstalk of the optical system with an absolute polarimetric accuracy <0.1% in the degree of polarization. We employed aperture photometry, angular differential imaging, and point-spread-function fitting to retrieve the polarization of the companions.
Results: We report the first detection of polarization originating from substellar companions, with a polarization of several tenths of a percent for DH Tau B and GSC 6214-210 B in H-band. By comparing the measured polarization with that of nearby stars, we find that the polarization is unlikely to be caused by interstellar dust. Because the companions have previously measured hydrogen emission lines and red colors, the polarization most likely originates from circumsubstellar disks. Through radiative transfer modeling, we constrain the position angles of the disks and find that the disks must have high inclinations. For the 18 other companions, we do not detect significant polarization and place subpercent upper limits on their degree of polarization. We also present images of the circumstellar disks of DH Tau, GQ Lup, PDS 70, β Pic, and HD 106906. We detect a highly asymmetric disk around GQ Lup and find evidence for multiple scattering in the disk of PDS 70. Both disks show spiral-like features that are potentially induced by GQ Lup B and PDS 70 b, respectively.
Conclusions: The presence of the disks around DH Tau B and GSC 6214-210 B as well as the misalignment of the disk of DH Tau B with the disk around its primary star suggest in situ formation of the companions. The non-detections of polarization for the other companions may indicate the absence of circumsubstellar disks, a slow rotation rate of young companions, the upper atmospheres containing primarily submicron-sized dust grains, and/or limited cloud inhomogeneity.

Based on observations collected at the European Southern Observatory under ESO programs 098.C-0790, 0101.C-0502, 0101.C-0635, 0101.C-0855, 0102.C-0453, 0102.C-0466, 0102.C-0871, 0102.C-0916, and 0104.C-0265. Title: LOUPE: observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets Authors: Klindžić, D.; Stam, D. M.; Snik, F.; Keller, C. U.; Hoeijmakers, H. J.; van Dam, D. M.; Willebrands, M.; Karalidi, T.; Pallichadath, V.; van Dijk, C. N.; Esposito, M. Bibcode: 2021RSPTA.37990577K Altcode: 2020arXiv200716078K LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as an exoplanet. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterizing such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour life. These data will be used to test numerical codes to predict signals of Earth-like exoplanets, to test algorithms that retrieve planet properties, and to fine-tune the design and observational strategies of future space observatories. From the Moon, LOUPE will continuously see the entire Earth, enabling it to monitor the signal changes due to the planet's daily rotation, weather patterns and seasons, across all phase angles. Here, we present both the science case and the technology behind LOUPE's instrumental and mission design.

This article is part of a discussion meeting issue `Astronomy from the Moon: the next decades'. Title: Using the generalised-optical differentiation wavefront sensor for laser guide star wavefront sensing Authors: Haffert, Sebastiaan Y.; Keller, Christoph U.; Dekany, Richard Bibcode: 2020SPIE11448E..1TH Altcode: 2020arXiv201202336H Laser guide stars (LGS) are used in many adaptive optics systems to extend sky coverage. The most common wavefront sensor used in combination with a LGS is a Shack-Hartmann wavefront sensor (SHWFS). The ShackHartmann has a major disadvantage for extended source wavefront sensing because it directly samples the image. In this proceeding we propose to use the generalized-Optical Differentation Wavefront Sensor (g-ODWFS) a wavefront sensor for wavefront sensing of LGS. The g-ODWFS uses only 4 pixels per sub-aperture, has little to no aliasing noise and therefore no spurious low-order errors and has no need for centroid gain calibrations. In this proceeding we show the results of simulations that compare the g-ODWFS with the SHWFS. Title: Prediction of the planet yield of the MaxProtoPlanetS high-contrast survey for H-alpha protoplanets with MagAO-X based on first light contrasts Authors: Close, Laird M.; Males, Jared; Long, Joseph D.; Van Gorkom, Kyle; Hedglen, Alexander D.; Kautz, Maggie; Lumbres, Jennifer; Haffert, Sebastiaan Y.; Follette, Katherine; Wagner, Kevin; Miller, Kelsey; Apai, Daniel; Wu, Ya-Lin; Guyon, Olivier; Schatz, Lauren; Rodack, Alex; Doelman, David; Snik, Frans; Knight, Justin M.; Morzinski, Katie; Gasho, Victor; Keller, Christoph; Pearce, Logan; Weinberger, Alycia; Pérez, Laura; Doyon, René Bibcode: 2020SPIE11448E..0UC Altcode: Our past GAPplanetS survey over the last 5 years with the MagAO visible AO system discovered the first examples of accreting protoplanets (by direct observation of H-alpha emission). Examples include LkCa15 b (Sallum et al. 2015) and PDS70 b (Wagner et al. 2018). In this paper we review the science performance of the newly (Dec. 2019) commissioned MagAO-X extreme AO system. In particular, we use the vAPP coronagraphic contrasts measured during MagAO-X first light. We use the Massive Accreting Gap (MAG) protoplanet model of Close 2020 to predict the H-alpha contrasts of 19 of the best transitional disk systems (ages 1-5 Myr) for the direct detection of H-alpha from accretion of hydrogen onto these protoplanets. The MAG protoplanet model applied to the observed first light MagAO-X contrasts predict a maximum yield of 46+/-7 planets from 19 stars (42 of these planets would be new discoveries). This suggests that there is a large, yet, unexplored reservoir of protoplanets that can be discovered with an extreme AO coronagraphic survey of 19 of the best transitional disk systems. Based on our first light contrasts we predict a healthy yield of protoplanets from our MaxProtoPlanetS survey of 19 transitional disks with MagAO-X. Title: Focal plane wavefront sensing on SUBARU/SCExAO Authors: Vievard, S.; Bos, S. P.; Cassaing, F.; Currie, T.; Deo, V.; Guyon, O.; Jovanovic, N.; Keller, C. U.; Lamb, M.; Lopez, C.; Lozi, J.; Martinache, F.; Miller, K.; Montmerle-Bonnefois, A.; Mugnier, L. M.; N'Diaye, M.; Norris, B.; Sahoo, A.; Sauvage, J. -F.; Skaf, N.; Snik, F.; Wilby, M. J.; Wong, A. Bibcode: 2020SPIE11448E..6DV Altcode: 2020arXiv201212417V Focal plane wavefront sensing is an elegant solution for wavefront sensing since near-focal images of any source taken by a detector show distortions in the presence of aberrations. Non-Common Path Aberrations and the Low Wind Effect both have the ability to limit the achievable contrast of the finest coronagraphs coupled with the best extreme adaptive optics systems. To correct for these aberrations, the Subaru Coronagraphic Extreme Adaptive Optics instrument hosts many focal plane wavefront sensors using detectors as close to the science detector as possible. We present seven of them and compare their implementation and efficiency on SCExAO. This work will be critical for wavefront sensing on next generation of extremely large telescopes that might present similar limitations. Title: MagAO-X first light Authors: Males, Jared R.; Close, Laird M.; Guyon, Olivier; Hedglen, Alexander D.; Van Gorkom, Kyle; Long, Joseph D.; Kautz, Maggie; Lumbres, Jennifer; Schatz, Lauren; Rodack, Alexander; Miller, Kelsey; Doelman, David; Snik, Frans; Bos, Steven; Knight, Justin M.; Morzinski, Katie; Gasho, Victor; Keller, Christoph; Haffert, Sebastiaan; Pearce, Logan Bibcode: 2020SPIE11448E..4LM Altcode: MagAO-X is a new "extreme" adaptive optics system for the Magellan Clay 6.5 m telescope which began commissioning in December, 2019. MagAO-X is based around a 2040 actuator deformable mirror, controlled by a pyramid wavefront sensor operating at up to 3.6 kHz. When fully optimized, MagAO-X will deliver high Strehls (< 70%), high resolution (19 mas), and high contrast (< 1 × 10-4) at Hα (656 nm). We present a brief review of the instrument design and operations, and then report on the results of the first-light run. Title: Self-optimizing adaptive optics control with reinforcement learning Authors: Landman, R.; Haffert, S. Y.; Radhakrishnan, V. M.; Keller, C. U. Bibcode: 2020SPIE11448E..49L Altcode: 2020arXiv201201997L Current and future high-contrast imaging instruments require extreme Adaptive Optics (XAO) systems to reach contrasts necessary to directly image exoplanets. Telescope vibrations and the temporal error induced by the latency of the control loop limit the performance of these systems. Optimization of the (predictive) control algorithm is crucial in reducing these effects. We describe how model-free Reinforcement Learning can be used to optimize a Recurrent Neural Network controller for closed-loop adaptive optics control. We verify our proposed approach for tip-tilt control in simulations and a lab setup. The results show that this algorithm can effectively learn to suppress a combination of tip-tilt vibrations. Furthermore, we report decreased residuals for power-law input turbulence compared to an optimal gain integrator. Finally, we demonstrate that the controller can learn to identify the parameters of a varying vibration without requiring online updating of the control law. We conclude that Reinforcement Learning is a promising approach towards data-driven predictive control; future research will apply this approach to the control of high-order deformable mirrors. Title: Planet formation with all flavors of adaptive optics: VLT/MUSE's laser tomography adaptive optics to directly image young accreting exoplanets Authors: Girard, Julien H.; Haffert, Sebastiaan Y.; Bae, Jaehan; Zeidler, Peter; de Boer, Jozua; Bohn, Alexander; van Holstein, Rob G.; Brinchmann, Jarle; Snellen, Ignas; Bacon, Rolan; Keller, Christoph Bibcode: 2020SPIE11448E..08G Altcode: We present recent results obtained with the VLT/MUSE Integral Field Spectrograph fed by the 4LGSF and its laser tomography adaptive optics module GALACSI. While this so-called narrow-field mode of MUSE was not designed to perform directly imaging of exoplanets and outflows, we show that it can be a game changer to detect and characterize young exoplanets with a prominent emission lines (i.e Hα, tracer of accretion), at moderate contrasts. These performances are achieved thanks to the combo of a near-diffraction limited PSF and a medium resolution spectrograph and a cross-correlation approach in post-processing . We discuss this in the context of ground and space, infrared and visible wavelengths, preparing for missions like JWST and WFIRST in great synergy and as pathfinder for future ELT/GSMT (Extremely Large and/or Giant Segmented Mirror Telescopes) instruments. Title: Multi-core fibre-fed integral-field unit (MCIFU): overview and first-light Authors: Haffert, Sebastiaan Y.; Harris, Robert J.; Zanutta, Alessio; Pike, Fraser A.; Bianco, Andrea; Redaelli, Edoardo; Benoît, Aurélien; MacLachlan, David G.; Ross, Calum A.; Gris-Sánchez, Itandehui; Trappen, Mareike D.; Xu, Yilin; Blaicher, Matthias; Maier, Pascal; Riva, Giulio; Sinquin, Baptiste; Kulcsár, Caroline; Bharmal, Nazim Ali; Gendron, Eric; Staykov, Lazar; Morris, Tim J.; Barboza, Santiago; Muench, Norbert; Bardour, Lisa; Prengère, Léonard; Raynaud, Henri-François; Hottinger, Philipp; Anagnos, Theodoros; Osborn, James; Koos, Christian; Thompson, Robert R.; Birks, Tim A.; Snellen, Ignas A. G.; Keller, Christoph U.; Close, Laird; Males, Jared R. Bibcode: 2020SPIE11448E..4MH Altcode: 2021arXiv210109766A The Multi-Core Integral-Field Unit (MCIFU) is a new diffraction-limited near-infrared integral-field unit for exoplanet atmosphere characterization with extreme adaptive optics (xAO) instruments. It has been developed as an experimental pathfinder for spectroscopic upgrades for SPHERE+/VLT and other xAO systems. The wavelength range covers 1.0 um to 1.6um at a resolving power around 5000 for 73 points on-sky. The MCIFU uses novel astrophotonic components to make this very compact and robust spectrograph. We performed the first successful on-sky test with CANARY at the 4.2 meter William Herschel Telescope in July 2019, where observed standard stars and several stellar binaries. An improved version of the MCIFU will be used with MagAO-X, the new extreme adaptive optics system at the 6.5 meter Magellan Clay telescope in Chile. We will show and discuss the first-light performance and operations of the MCIFU at CANARY and discuss the integration of the MCIFU with MagAO-X. Title: Design of the life signature detection polarimeter LSDpol Authors: Keller, Christoph U.; Snik, Frans; Patty, C. H. Lucas; Klindžic, Dora; Krasteva, Mariya; Doelman, David S.; Wijnen, Thomas; Pallichadath, Vidhya; Stam, Daphne M.; Demory, Brice-Olivier; Kühn, Jonas G.; Hoeijmakers, H. Jens; Pommerol, Antoine; Poch, Olivier Bibcode: 2020SPIE11443E..3RK Altcode: 2020arXiv201209105K Many biologically produced chiral molecules such as amino acids and sugars show a preference for left or right handedness (homochirality). Light reflected by biological materials such as algae and leaves therefore exhibits a small amount of circular polarization that strongly depends on wavelength. Our Life Signature Detection polarimeter (LSDpol) is optimized to measure these signatures of life. LSDpol is a compact spectropolarimeter concept with no moving parts that instantaneously measures linear and circular polarization averaged over the field of view with a sensitivity of better than 10-4. We expect to launch the instrument into orbit after validating its performance on the ground and from aircraft. LSDpol is based on a spatially varying quarter-wave retarder that is implemented with a patterned liquid-crystal. It is the first optical element to maximize the polarimetric sensitivity. Since this pattern as well as the entrance slit of the spectrograph have to be imaged onto the detector, the slit serves as the aperture, and an internal field stop limits the field of view. The retarder's fast axis angle varies linearly along one spatial dimension. A fixed quarter-wave retarder combined with a polarization grating act as the disperser and the polarizing beam-splitter. Circular and linear polarization are thereby encoded at incompatible modulation frequencies across the spectrum, which minimizes the potential cross-talk from linear into circular polarization. Title: Searching for proto-planets with MUSE Authors: Xie, C.; Haffert, S. Y.; de Boer, J.; Kenworthy, M. A.; Brinchmann, J.; Girard, J.; Snellen, I. A. G.; Keller, C. U. Bibcode: 2020A&A...644A.149X Altcode: 2020arXiv201108043X Context. Protoplanetary disks contain structures such as gaps, rings, and spirals, which are thought to be produced by the interaction between the disk and embedded protoplanets. However, only a few planet candidates are found orbiting within protoplanetary disks, and most of them are being challenged as having been confused with disk features.
Aims: The VLT/MUSE discovery of PDS 70 c demonstrated a powerful way of searching for still-forming protoplanets by targeting accretion signatures with medium-resolution integral field spectroscopy. We aim to discover more proto-planetary candidates with MUSE, with a secondary aim of improving the high-resolution spectral differential imaging (HRSDI) technique by analyzing the instrumental residuals of MUSE.
Methods: We analyzed MUSE observations of five young stars with various apparent brightnesses and spectral types. We applied the HRSDI technique to perform high-contrast imaging. The detection limits were estimated using fake planet injections.
Results: With a 30 min integration time, MUSE can reach 5σ detection limits in apparent Hα line flux down to 10-14 and 10-15 erg s-1 cm-2 at 0.075'' and 0.25'', respectively. In addition to PDS 70 b and c, we did not detect any clear accretion signatures in PDS 70, J1850-3147, and V1094 Sco down to 0.1''. MUSE avoids the small sample statistics problem by measuring the noise characteristics in the spatial direction at multiple wavelengths. We detected two asymmetric atomic jets in HD 163296 with a very high spatial resolution (down to 8 au) and medium spectral resolution (R ~ 2500).
Conclusions: The HRSDI technique when applied to MUSE data allows us to reach the photon noise limit at small separations (i.e., <0.5''). With the combination of high-contrast imaging and medium spectral resolution, MUSE can achieve fainter detection limits in apparent line flux than SPHERE/ZIMPOL by a factor of ~5. MUSE has some instrumental issues that limit the contrast that appear in cases with strong point sources, which can be either a spatial point source due to high Strehl observations or a spectral point source due to a high line-to-continuum ratio. We modified the HRSDI technique to better handle the instrumental artifacts and improve the detection limits. To avoid the instrumental effects altogether, we suggest faint young stars with relatively low Hα line-to-continuum ratio to be the most suitable targets for MUSE to search for potential protoplanets. Title: Diffraction-limited integral-field spectroscopy for extreme adaptive optics systems with the multicore fiber-fed integral-field unit Authors: Haffert, Sebastiaan Y.; Harris, Robert J.; Zanutta, Alessio; Pike, Fraser A.; Bianco, Andrea; Redaelli, Eduardo; Benoît, Aurélien; MacLachlan, David G.; Ross, Calum A.; Gris-Sánchez, Itandehui; Trappen, Mareike D.; Xu, Yilin; Blaicher, Matthias; Maier, Pascal; Riva, Giulio; Sinquin, Baptiste; Kulcsár, Caroline; Bharmal, Nazim Ali; Gendron, Eric; Staykov, Lazar; Morris, Tim J.; Barboza, Santiago; Muench, Norbert; Bardou, Lisa; Prengère, Léonard; Raynaud, Henri-François; Hottinger, Phillip; Anagnos, Theodoros; Osborn, James; Koos, Christian; Thomson, Robert R.; Birks, Tim A.; Snellen, Ignas A. G.; Keller, Christoph U. Bibcode: 2020JATIS...6d5007H Altcode: 2020arXiv200903529H Direct imaging instruments have the spatial resolution to resolve exoplanets from their host star. This enables direct characterization of the exoplanets atmosphere, but most direct imaging instruments do not have spectrographs with high enough resolving power for detailed atmospheric characterization. We investigate the use of a single-mode diffraction-limited integral-field unit that is compact and easy to integrate into current and future direct imaging instruments for exoplanet characterization. This achieved by making use of recent progress in photonic manufacturing to create a single-mode fiber-fed image reformatter. The fiber link is created with three-dimensional printed lenses on top of a single-mode multicore fiber that feeds an ultrafast laser inscribed photonic chip that reformats the fiber into a pseudoslit. We then couple it to a first-order spectrograph with a triple stacked volume phase holographic grating for a high efficiency over a large bandwidth. The prototype system has had a successful first-light observing run at the 4.2-m William Herschel Telescope. The measured on-sky resolving power is between 2500 and 3000, depending on the wavelength. With our observations, we show that single-mode integral-field spectroscopy is a viable option for current and future exoplanet imaging instruments. Title: CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion Authors: Haffert, S. Y.; van Holstein, R. G.; Ginski, C.; Brinchmann, J.; Snellen, I. A. G.; Milli, J.; Stolker, T.; Keller, C. U.; Girard, J. Bibcode: 2020A&A...640L..12H Altcode: 2020arXiv200707831H Context. Direct imaging provides a steady flow of newly discovered giant planets and brown dwarf companions. These multi-object systems can provide information about the formation of low-mass companions in wide orbits and/or help us to speculate about possible migration scenarios. Accurate classification of companions is crucial for testing formation pathways.
Aims: In this work we further characterise the recently discovered candidate for a planetary-mass companion CS Cha b and determine if it is still accreting.
Methods: MUSE is a four-laser-adaptive-optics-assisted medium-resolution integral-field spectrograph in the optical part of the spectrum. We observed the CS Cha system to obtain the first spectrum of CS Cha b. The companion is characterised by modelling both the spectrum from 6300 Å to 9300 Å and the photometry using archival data from the visible to the near-infrared (NIR).
Results: We find evidence of accretion and outflow signatures in Hα and OI emission. The atmospheric models with the highest likelihood indicate an effective temperature of 3450 ± 50 K with a log g of 3.6 ± 0.5 dex. Based on evolutionary models, we find that the majority of the object is obscured. We determine the mass of the faint companion with several methods to be between 0.07 M and 0.71 M with an accretion rate of Ṁ = 4 × 10-11±0.4 M yr-1.
Conclusions: Our results show that CS Cha B is most likely a mid-M-type star that is obscured by a highly inclined disc, which has led to its previous classification using broadband NIR photometry as a planetary-mass companion. This shows that it is important and necessary to observe over a broad spectral range to constrain the nature of faint companions.

The extracted spectrum of CS Cha B is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/L12 Title: Two Directly Imaged, Wide-orbit Giant Planets around the Young, Solar Analog TYC 8998-760-1 Authors: Bohn, Alexander J.; Kenworthy, Matthew A.; Ginski, Christian; Rieder, Steven; Mamajek, Eric E.; Meshkat, Tiffany; Pecaut, Mark J.; Reggiani, Maddalena; de Boer, Jozua; Keller, Christoph U.; Snik, Frans; Southworth, John Bibcode: 2020ApJ...898L..16B Altcode: 2020arXiv200710991B Even though tens of directly imaged companions have been discovered in the past decades, the number of directly confirmed multiplanet systems is still small. Dynamical analysis of these systems imposes important constraints on formation mechanisms of these wide-orbit companions. As part of the Young Suns Exoplanet Survey we report the detection of a second planetary-mass companion around the 17 Myr-old, solar-type star TYC 8998-760-1 that is located in the Lower Centaurus Crux subgroup of the Scorpius-Centaurus association. The companion has a projected physical separation of 320 au and several individual photometric measurements from 1.1 to 3.8 microns constrain a companion mass of 6 ± 1 MJup, which is equivalent to a mass ratio of q = 0.57 ± 0.10% with respect to the primary. With the previously detected 14 ± 3 MJup companion that is orbiting the primary at 160 au, TYC 8998-760-1 is the first directly imaged multiplanet system that is detected around a young, solar analog. We show that circular orbits are stable, but that mildly eccentric orbits for either/both components (e > 0.1) are chaotic on gigayear timescales, implying in situ formation or a very specific ejection by an unseen third companion. Due to the wide separations of the companions TYC 8998-760-1 is an excellent system for spectroscopic and photometric follow-up with space-based observatories such as the James Webb Space Telescope. * Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 099.C-0698(A), 0101.C-0341(A), 2103.C-5012(B), and 0104.C-0265(A). Title: On-sky verification of Fast and Furious focal-plane wavefront sensing: Moving forward toward controlling the island effect at Subaru/SCExAO Authors: Bos, S. P.; Vievard, S.; Wilby, M. J.; Snik, F.; Lozi, J.; Guyon, O.; Norris, B. R. M.; Jovanovic, N.; Martinache, F.; Sauvage, J. -F.; Keller, C. U. Bibcode: 2020A&A...639A..52B Altcode: 2020arXiv200512097B Context. High-contrast imaging (HCI) observations of exoplanets can be limited by the island effect (IE). The IE occurs when the main wavefront sensor (WFS) cannot measure sharp phase discontinuities across the telescope's secondary mirror support structures (also known as spiders). On the current generation of telescopes, the IE becomes a severe problem when the ground wind speed is below a few meters per second. During these conditions, the air that is in close contact with the spiders cools down and is not blown away. This can create a sharp optical path length difference between light passing on opposite sides of the spiders. Such an IE aberration is not measured by the WFS and is therefore left uncorrected. This is referred to as the low-wind effect (LWE). The LWE severely distorts the point spread function (PSF), significantly lowering the Strehl ratio and degrading the contrast.
Aims: In this article, we aim to show that the focal-plane wavefront sensing (FPWFS) algorithm, Fast and Furious (F&F), can be used to measure and correct the IE/LWE. The F&F algorithm is a sequential phase diversity algorithm and a software-only solution to FPWFS that only requires access to images of non-coronagraphic PSFs and control of the deformable mirror.
Methods: We deployed the algorithm on the SCExAO HCI instrument at the Subaru Telescope using the internal near-infrared camera in H-band. We tested with the internal source to verify that F&F can correct a wide variety of LWE phase screens. Subsequently, F&F was deployed on-sky to test its performance with the full end-to-end system and atmospheric turbulence. The performance of the algorithm was evaluated by two metrics based on the PSF quality: (1) the Strehl ratio approximation (SRA), and (2) variance of the normalized first Airy ring (VAR). The VAR measures the distortion of the first Airy ring, and is used to quantify PSF improvements that do not or barely affect the PSF core (e.g., during challenging atmospheric conditions).
Results: The internal source results show that F&F can correct a wide range of LWE phase screens. Random LWE phase screens with a peak-to-valley wavefront error between 0.4 μm and 2 μm were all corrected to a SRA > 90% and an VAR ⪅ 0.05. Furthermore, the on-sky results show that F&F is able to improve the PSF quality during very challenging atmospheric conditions (1.3-1.4″seeing at 500 nm). Closed-loop tests show that F&F is able to improve the VAR from 0.27-0.03 and therefore significantly improve the symmetry of the PSF. Simultaneous observations of the PSF in the optical (λ = 750 nm, Δλ = 50 nm) show that during these tests we were correcting aberrations common to the optical and NIR paths within SCExAO. We could not conclusively determine if we were correcting the LWE and/or (quasi-)static aberrations upstream of SCExAO.
Conclusions: The F&F algorithm is a promising focal-plane wavefront sensing technique that has now been successfully tested on-sky. Going forward, the algorithm is suitable for incorporation into observing modes, which will enable PSFs of higher quality and stability during science observations. Title: VizieR Online Data Catalog: CS Cha B spectrum (Haffert+, 2020) Authors: Haffert, S. Y.; van Holstein, R. G.; Ginski, C.; Brinchmann, J.; Snellen, I. A. G.; Milli, J.; Stolker, T.; Keller, C. U.; Girard, J. Bibcode: 2020yCat..36409012H Altcode: The extracted MUSE spectrum of CS Cha B.

(1 data file). Title: A universal smartphone add-on for portable spectroscopy and polarimetry: iSPEX 2 Authors: Burggraaff, Olivier; Perduijn, Armand B.; van Hek, Robert F.; Schmidt, Norbert; Keller, Christoph U.; Snik, Frans Bibcode: 2020SPIE11389E..2KB Altcode: 2020arXiv200601519B Spectropolarimetry is a powerful technique for remote sensing of the environment. It enables the retrieval of particle shape and size distributions in air and water to an extent that traditional spectroscopy cannot. SPEX is an instrument concept for spectropolarimetry through spectral modulation, providing snapshot, and hence accurate, hyperspectral intensity and degree and angle of linear polarization. Successful SPEX instruments have included groundSPEX and SPEX airborne, which both measure aerosol optical thickness with high precision, and soon SPEXone, which will fly on PACE. Here, we present a low-cost variant for consumer cameras, iSPEX 2, with universal smartphone support. Smartphones enable citizen science measurements which are significantly more scaleable, in space and time, than professional instruments. Universal smartphone support is achieved through a modular hardware design and SPECTACLE data processing. iSPEX 2 will be manufactured through injection molding and 3D printing. A smartphone app for data acquisition and processing is in active development. Production, calibration, and validation will commence in the summer of 2020. Scientific applications will include citizen science measurements of aerosol optical thickness and surface water reflectance, as well as low-cost laboratory and portable spectroscopy. Title: IRDAP: SPHERE-IRDIS polarimetric data reduction pipeline Authors: van Holstein, R. G.; Girard, J. H.; de Boer, J.; Snik, F.; Milli, J.; Stam, D. M.; Ginski, C.; Mouillet, D.; Wahhaj, Z.; Schmid, H. M.; Keller, C. U.; Langlois, M.; Dohlen, K.; Vigan, A.; Pohl, A.; Carbillet, M.; Fantinel, D.; Maurel, D.; Origné, A.; Petit, C. Ramos, J.; Rigal, F.; Sevin, A.; Boccaletti, A.; Le Coroller, H.; Dominik, C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.; Udry, S.; Chauvin, G.; Feldt, M.; Beuzit, J. -L. Bibcode: 2020ascl.soft04015V Altcode: IRDAP (IRDIS Data reduction for Accurate Polarimetry) accurately reduces SPHERE-IRDIS polarimetric data. It is a highly-automated end-to-end pipeline; its core feature is model-based correction of the instrumental polarization effects. IRDAP handles data taken both in field- and pupil-tracking mode and using the broadband filters Y, J, H and Ks. Data taken with the narrowband filters can be reduced as well, although with a somewhat worse accuracy. For pupil-tracking observations IRDAP can additionally apply angular differential imaging. Title: The Single-mode Complex Amplitude Refinement (SCAR) coronagraph. II. Lab verification, and toward the characterization of Proxima b Authors: Haffert, S. Y.; Por, E. H.; Keller, C. U.; Kenworthy, M. A.; Doelman, D. S.; Snik, F.; Escuti, M. J. Bibcode: 2020A&A...635A..56H Altcode: We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8-2.4λ/D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of 2-3 × 10-4, and the 180 SCAR reaches a null of 1 × 10-4. Their monochromatic contrast is maintained within a range of ±0.16λ/D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood. Title: Original use of MUSE's laser tomography adaptive optics to directly image young accreting exoplanets Authors: Girard, Julien H.; de Boer, Jozua; Haffert, Sebastiaan; Zeidler, Peter; Bohn, Alexander; van Holstein, Rob G.; Snellen, Ignas; Brinchmann, Jarle; Keller, Christoph; Bacon, Roland; Bae, Jaehan Bibcode: 2020arXiv200302145G Altcode: We present recent results obtained with the VLT/MUSE Integral Field Spectrograph fed by the 4LGSF and its laser tomography adaptive optics module GALACSI. While this so-called narrow-field mode of MUSE was not designed to perform directly imaging of exoplanets and outflows, we show that it can be a game changer to detect and characterize young exoplanets with a prominent emission lines (i.e H{\alpha}, tracer of accretion), at moderate contrasts. These performances are achieved thanks to the combo of a near-diffraction limited PSF and a medium resolution spectrograph and a cross-correlation approach in post-processing . We discuss this in the context of ground and space, infrared and visible wavelengths, preparing for missions like JWST and WFIRST in great synergy and as pathfinder for future ELT/GSMT (Extremely Large and/or Giant Segmented Mirror Telescopes) instruments. Title: SPHERE+: Imaging young Jupiters down to the snowline Authors: Boccaletti, A.; Chauvin, G.; Mouillet, D.; Absil, O.; Allard, F.; Antoniucci, S.; Augereau, J. -C.; Barge, P.; Baruffolo, A.; Baudino, J. -L.; Baudoz, P.; Beaulieu, M.; Benisty, M.; Beuzit, J. -L.; Bianco, A.; Biller, B.; Bonavita, B.; Bonnefoy, M.; Bos, S.; Bouret, J. -C.; Brandner, W.; Buchschache, N.; Carry, B.; Cantalloube, F.; Cascone, E.; Carlotti, A.; Charnay, B.; Chiavassa, A.; Choquet, E.; Clenet, Y.; Crida, A.; De Boer, J.; De Caprio, V.; Desidera, S.; Desert, J. -M.; Delisle, J. -B.; Delorme, P.; Dohlen, K.; Doelman, D.; Dominik, C.; Orazi, V. D; Dougados, C.; Doute, S.; Fedele, D.; Feldt, M.; Ferreira, F.; Fontanive, C.; Fusco, T.; Galicher, R.; Garufi, A.; Gendron, E.; Ghedina, A.; Ginski, C.; Gonzalez, J. -F.; Gratadour, D.; Gratton, R.; Guillot, T.; Haffert, S.; Hagelberg, J.; Henning, T.; Huby, E.; Janson, M.; Kamp, I.; Keller, C.; Kenworthy, M.; Kervella, P.; Kral, Q.; Kuhn, J.; Lagadec, E.; Laibe, G.; Langlois, M.; Lagrange, A. -M.; Launhardt, R.; Leboulleux, L.; Le Coroller, H.; Li Causi, G.; Loupias, M.; Maire, A. L.; Marleau, G.; Martinache, F.; Martinez, P.; Mary, D.; Mattioli, M.; Mazoyer, J.; Meheut, H.; Menard, F.; Mesa, D.; Meunier, N.; Miguel, Y.; Milli, J.; Min, M.; Molliere, P.; Mordasini, C.; Moretto, G.; Mugnier, L.; Muro Arena, G.; Nardetto, N.; Diaye, M. N; Nesvadba, N.; Pedichini, F.; Pinilla, P.; Por, E.; Potier, A.; Quanz, S.; Rameau, J.; Roelfsema, R.; Rouan, D.; Rigliaco, E.; Salasnich, B.; Samland, M.; Sauvage, J. -F.; Schmid, H. -M.; Segransan, D.; Snellen, I.; Snik, F.; Soulez, F.; Stadler, E.; Stam, D.; Tallon, M.; Thebault, P.; Thiebaut, E.; Tschudi, C.; Udry, S.; van Holstein, R.; Vernazza, P.; Vidal, F.; Vigan, A.; Waters, R.; Wildi, F.; Willson, M.; Zanutta, A.; Zavagno, A.; Zurlo, A. Bibcode: 2020arXiv200305714B Altcode: SPHERE (Beuzit et al,. 2019) has now been in operation at the VLT for more than 5 years, demonstrating a high level of performance. SPHERE has produced outstanding results using a variety of operating modes, primarily in the field of direct imaging of exoplanetary systems, focusing on exoplanets as point sources and circumstellar disks as extended objects. The achievements obtained thus far with SPHERE (~200 refereed publications) in different areas (exoplanets, disks, solar system, stellar physics...) have motivated a large consortium to propose an even more ambitious set of science cases, and its corresponding technical implementation in the form of an upgrade. The SPHERE+ project capitalizes on the expertise and lessons learned from SPHERE to push high contrast imaging performance to its limits on the VLT 8m-telescope. The scientific program of SPHERE+ described in this document will open a new and compelling scientific window for the upcoming decade in strong synergy with ground-based facilities (VLT/I, ELT, ALMA, and SKA) and space missions (Gaia, JWST, PLATO and WFIRST). While SPHERE has sampled the outer parts of planetary systems beyond a few tens of AU, SPHERE+ will dig into the inner regions around stars to reveal and characterize by mean of spectroscopy the giant planet population down to the snow line. Building on SPHERE's scientific heritage and resounding success, SPHERE+ will be a dedicated survey instrument which will strengthen the leadership of ESO and the European community in the very competitive field of direct imaging of exoplanetary systems. With enhanced capabilities, it will enable an even broader diversity of science cases including the study of the solar system, the birth and death of stars and the exploration of the inner regions of active galactic nuclei. Title: The Young Suns Exoplanet Survey: Detection of a wide-orbit planetary-mass companion to a solar-type Sco-Cen member Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Manara, C. F.; Pecaut, M. J.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Reggiani, M.; Todorov, K. O.; Snik, F. Bibcode: 2020MNRAS.492..431B Altcode: 2019MNRAS.tmp.3127B; 2019arXiv191204284B The Young Suns Exoplanet Survey consists of a homogeneous sample of 70 young, solar-mass stars located in the Lower Centaurus-Crux subgroup of the Scorpius-Centaurus association with an average age of 15 ± 3 Myr. We report the detection of a co-moving companion around the K3IV star TYC 8998-760-1 (2MASSJ13251211-6456207) that is located at a distance of 94.6 ± 0.3 pc using SPHERE/IRDIS on the VLT. Spectroscopic observations with VLT/X-SHOOTER constrain the mass of the star to 1.00± 0.02 M_{⊙ } and an age of 16.7± 1.4 Myr. The companion TYC 8998-760-1 b is detected at a projected separation of 1.71″, which implies a projected physical separation of 162 au. Photometric measurements ranging from Y to M band provide a mass estimate of 14± 3 M_jup by comparison to BT-Settl and AMES-dusty isochrones, corresponding to a mass ratio of q = 0.013 ± 0.003 with respect to the primary. We rule out additional companions to TYC 8998-760-1 that are more massive than 12 M_jup and farther than 12 au away from the host. Future polarimetric and spectroscopic observations of this system with ground and space based observatories will facilitate testing of formation and evolution scenarios shaping the architecture of the circumstellar environment around this `young Sun'. Title: RefPlanets: Search for reflected light from extrasolar planets with SPHERE/ZIMPOL Authors: Hunziker, S.; Schmid, H. M.; Mouillet, D.; Milli, J.; Zurlo, A.; Delorme, P.; Abe, L.; Avenhaus, H.; Baruffolo, A.; Bazzon, A.; Boccaletti, A.; Baudoz, P.; Beuzit, J. L.; Carbillet, M.; Chauvin, G.; Claudi, R.; Costille, A.; Daban, J. -B.; Desidera, S.; Dohlen, K.; Dominik, C.; Downing, M.; Engler, N.; Feldt, M.; Fusco, T.; Ginski, C.; Gisler, D.; Girard, J. H.; Gratton, R.; Henning, Th.; Hubin, N.; Kasper, M.; Keller, C. U.; Langlois, M.; Lagadec, E.; Martinez, P.; Maire, A. L.; Menard, F.; Meyer, M. R.; Pavlov, A.; Pragt, J.; Puget, P.; Quanz, S. P.; Rickman, E.; Roelfsema, R.; Salasnich, B.; Sauvage, J. -F.; Siebenmorgen, R.; Sissa, E.; Snik, F.; Suarez, M.; Szulágyi, J.; Thalmann, Ch.; Turatto, M.; Udry, S.; van Holstein, R. G.; Vigan, A.; Wildi, F. Bibcode: 2020A&A...634A..69H Altcode: 2019arXiv191112759H
Aims: RefPlanets is a guaranteed time observation programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of Spectro-Polarimetric High-contrast Exoplanet REsearch instrument at the Very Large Telescope to perform a blind search for exoplanets in wavelengths from 600 to 900 nm. The goals of this study are the characterisation of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarised reflected light around some of the closest bright stars to the Sun, and potentially the direct detection of an evolved cold exoplanet for the first time.
Methods: For our observations of α Cen A and B, Sirius A, Altair, ɛ Eri and τ Ceti we used the polarimetricdifferential imaging (PDI) mode of ZIMPOL which removes the speckle noise down to the photon noise limit for angular separations ≿0.6''. We describe some of the instrumental effects that dominate the noise for smaller separations and explain how to remove these additional noise effects in post-processing. We then combine PDI with angular differential imaging as a final layer of post-processing to further improve the contrast limits of our data at these separations.
Results: For good observing conditions we achieve polarimetric contrast limits of 15.0-16.3 mag at the effective inner working angle of ~0.13'', 16.3-18.3 mag at 0.5'', and 18.8-20.4 mag at 1.5''. The contrast limits closer in (≾0.6'') display a significant dependence on observing conditions, while in the photon-noise-dominated regime (≿0.6'') the limits mainly depend on the brightness of the star and the total integration time. We compare our results with contrast limits from other surveys and review the exoplanet detection limits obtained with different detection methods. For all our targets we achieve unprecedented contrast limits. Despite the high polarimetric contrasts we are not able to find any additional companions or extended polarised light sources in the data obtained so far.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs: 095.C-0312(B), 096.C-0326(A), 097.C-0524(A), 097.C-0524(B), 098.C-0197(A), 099.C-0127(A), 099.C-0127(B), 0102.C-0435(A). Title: VizieR Online Data Catalog: A planetary-mass companion to a solar-type star (Bohn+, 2020) Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Manara, C. F.; Pecaut, M. J.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Reggiani, M.; Todorov, K. O.; Snik, F. Bibcode: 2020yCat..74920431B Altcode: Fits images corresponding to the imagery presented in the paper. The data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed with version 0.8.1 of PynPoint. We detect a co-moving companion around The solar-type Sco-Cen member TYC 8998-760-1 with a mass of 14+/-3 Jupiter masses. The extracted astrometry and photometry is presented in this catalogue.

(4 data files). Title: Polarimetric imaging mode of VLT/SPHERE/IRDIS. I. Description, data reduction, and observing strategy Authors: de Boer, J.; Langlois, M.; van Holstein, R. G.; Girard, J. H.; Mouillet, D.; Vigan, A.; Dohlen, K.; Snik, F.; Keller, C. U.; Ginski, C.; Stam, D. M.; Milli, J.; Wahhaj, Z.; Kasper, M.; Schmid, H. M.; Rabou, P.; Gluck, L.; Hugot, E.; Perret, D.; Martinez, P.; Weber, L.; Pragt, J.; Sauvage, J. -F.; Boccaletti, A.; Le Coroller, H.; Dominik, C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.; Udry, S.; Chauvin, G.; Feldt, M.; Beuzit, J. -L. Bibcode: 2020A&A...633A..63D Altcode: 2019arXiv190913107D Context. Polarimetric imaging is one of the most effective techniques for high-contrast imaging and for the characterization of protoplanetary disks, and it has the potential of becoming instrumental in the characterization of exoplanets. The Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument installed on the Very Large Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph (IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers the capability of obtaining linear polarization images at high contrast and resolution.
Aims: We aim to provide an overview of the polarimetric imaging mode of VLT/SPHERE/IRDIS and study its optical design to improve observing strategies and data reduction.
Methods: For H-band observations of TW Hydrae, we compared two data reduction methods that correct for instrumental polarization effects in different ways: a minimization of the "noise" image (Uϕ), and a correction method based on a polarimetric model that we have developed, as presented in Paper II of this study.
Results: We use observations of TW Hydrae to illustrate the data reduction. In the images of the protoplanetary disk around this star, we detect variability in the polarized intensity and angle of linear polarization that depend on the pointing-dependent instrument configuration. We explain these variations as instrumental polarization effects and correct for these effects using our model-based correction method.
Conclusions: The polarimetric imaging mode of IRDIS has proven to be a very successful and productive high-contrast polarimetric imaging system. However, the instrument performance is strongly dependent on the specific instrument configuration. We suggest adjustments to future observing strategies to optimize polarimetric efficiency in field-tracking mode by avoiding unfavorable derotator angles. We recommend reducing on-sky data with the pipeline called IRDAP, which includes the model-based correction method (described in Paper II) to optimally account for the remaining telescope and instrumental polarization effects and to retrieve the true polarization state of the incident light.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 095.C-0273(D). Title: Polarimetric imaging mode of VLT/SPHERE/IRDIS. II. Characterization and correction of instrumental polarization effects Authors: van Holstein, R. G.; Girard, J. H.; de Boer, J.; Snik, F.; Milli, J.; Stam, D. M.; Ginski, C.; Mouillet, D.; Wahhaj, Z.; Schmid, H. M.; Keller, C. U.; Langlois, M.; Dohlen, K.; Vigan, A.; Pohl, A.; Carbillet, M.; Fantinel, D.; Maurel, D.; Origné, A.; Petit, C.; Ramos, J.; Rigal, F.; Sevin, A.; Boccaletti, A.; Le Coroller, H.; Dominik, C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.; Udry, S.; Chauvin, G.; Feldt, M.; Beuzit, J. -L. Bibcode: 2020A&A...633A..64V Altcode: 2019arXiv190913108V Context. Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct-imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy.
Aims: We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope and SPHERE/IRDIS.
Methods: We created a detailed Mueller matrix model in the broadband filters Y, J, H, and Ks and calibrated the model using measurements with SPHERE's internal light source and observations of two unpolarized stars. We developed a data-reduction method that uses the model to correct for the instrumental polarization effects, and applied it to observations of the circumstellar disk of T Cha.
Results: The instrumental polarization is almost exclusively produced by the telescope and SPHERE's first mirror and varies with telescope altitude angle. The crosstalk primarily originates from the image derotator (K-mirror). At some orientations, the derotator causes severe loss of signal (> 90% loss in the H- and Ks-band) and strongly offsets the angle of linear polarization. With our correction method we reach, in all filters, a total polarimetric accuracy of ≲0.1% in the degree of linear polarization and an accuracy of a few degrees in angle of linear polarization.
Conclusions: The correction method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks, and is a vital tool for detecting spatially unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automated end-to-end data-reduction pipeline called IRDAP, which we made publicly available online.

Based on observations made with ESO telescopes at the La Silla Paranal Observatory under program ID 60.A-9800(S), 60.A-9801(S) and 096.C-0248(C).

The data-reduction pipeline IRDAP is available at https://irdap.readthedocs.io Title: Overview of focal plane wavefront sensors to correct for the Low Wind Effect on SUBARU/SCExAO Authors: Vievard, Sebastien; Bos, Steven; Cassaing, Frederic; Ceau, Alban; Guyon, Olivier; Jovanovic, Nemanja; Keller, Christoph U.; Lozi, Julien; Martinache, Frantz; Montmerle-Bonnefois, Aurelie; Mugnier, Laurent; NDiaye, Mamadou; Norris, Barnaby; Sahoo, Ananya; Sauvage, Jean-Francois; Snik, Frans; Wilby, Michael J.; Wong, Alisson Bibcode: 2019arXiv191210179V Altcode: The Low Wind Effect (LWE) refers to a phenomenon that occurs when the wind speed inside a telescope dome drops below $3$m/s creating a temperature gradient near the telescope spider. This produces phase discontinuities in the pupil plane that are not detected by traditional Adaptive Optics (AO) systems such as the pyramid wavefront sensor or the Shack-Hartmann. Considering the pupil as divided in 4 quadrants by regular spiders, the phase discontinuities correspond to piston, tip and tilt aberrations in each quadrant of the pupil. Uncorrected, it strongly decreases the ability of high contrast imaging instruments utilizing coronagraphy to detect exoplanets at small angular separations. Multiple focal plane wavefront sensors are currently being developed and tested on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument at Subaru Telescope: Among them, the Zernike Asymmetric Pupil (ZAP) wavefront sensor already showed on-sky that it could measure the LWE induced aberrations in focal plane images. The Fast and Furious algorithm, using previous deformable mirror commands as temporal phase diversity, showed in simulations its efficiency to improve the wavefront quality in the presence of LWE. A Neural Network algorithm trained with SCExAO telemetry showed promising PSF prediction on-sky. The Linearized Analytic Phase Diversity (LAPD) algorithm is a solution for multi-aperture cophasing and is studied to correct for the LWE aberrations by considering the Subaru Telescope as a 4 sub-aperture instrument. We present the different algorithms, show the latest results and compare their implementation on SCExAO/SUBARU as real-time wavefront sensors for the LWE compensation. Title: Focal-plane wavefront sensing with the vector-Apodizing Phase Plate Authors: Bos, S. P.; Doelman, D. S.; Lozi, J.; Guyon, O.; Keller, C. U.; Miller, K. L.; Jovanovic, N.; Martinache, F.; Snik, F. Bibcode: 2019A&A...632A..48B Altcode: 2019arXiv190908317B Context. One of the key limitations of the direct imaging of exoplanets at small angular separations are quasi-static speckles that originate from evolving non-common path aberrations (NCPA) in the optical train downstream of the instrument's main wavefront sensor split-off.
Aims: In this article we show that the vector-Apodizing Phase Plate (vAPP) coronagraph can be designed such that the coronagraphic point spread functions (PSFs) can act as wavefront sensors to measure and correct the (quasi-)static aberrations without dedicated wavefront sensing holograms or modulation by the deformable mirror. The absolute wavefront retrieval is performed with a non-linear algorithm.
Methods: The focal-plane wavefront sensing (FPWFS) performance of the vAPP and the algorithm are evaluated via numerical simulations to test various photon and read noise levels, the sensitivity to the 100 lowest Zernike modes, and the maximum wavefront error (WFE) that can be accurately estimated in one iteration. We apply these methods to the vAPP within SCExAO, first with the internal source and subsequently on-sky.
Results: In idealized simulations we show that for 107 photons the root mean square (rms) WFE can be reduced to ∼λ/1000, which is 1 nm rms in the context of the SCExAO system. We find that the maximum WFE that can be corrected in one iteration is ∼λ/8 rms or ∼200 nm rms (SCExAO). Furthermore, we demonstrate the SCExAO vAPP capabilities by measuring and controlling the 30 lowest Zernike modes with the internal source and on-sky. On-sky, we report a raw contrast improvement of a factor ∼2 between 2 and 4 λ/D after five iterations of closed-loop correction. When artificially introducing 150 nm rms WFE, the algorithm corrects it within five iterations of closed-loop operation.
Conclusions: FPWFS with the vAPP coronagraphic PSFs is a powerful technique since it integrates coronagraphy and wavefront sensing, eliminating the need for additional probes and thus resulting in a 100% science duty cycle and maximum throughput for the target. Title: Spatial linear dark field control and holographic modal wavefront sensing with a vAPP coronagraph on MagAO-X Authors: Miller, Kelsey; Males, Jared R.; Guyon, Olivier; Close, Laird M.; Doelman, David; Snik, Frans; Por, Emiel; Wilby, Michael J.; Keller, Christoph; Bohlman, Chris; Van Gorkom, Kyle; Rodack, Alexander; Knight, Justin; Lumbres, Jennifer; Bos, Steven; Jovanovic, Nemanja Bibcode: 2019JATIS...5d9004M Altcode: The Magellan Extreme Adaptive Optics (MagAO-X) Instrument is an extreme AO system coming online at the end of 2019 that will be operating within the visible and near-IR. With state-of-the-art wavefront sensing and coronagraphy, MagAO-X will be optimized for high-contrast direct exoplanet imaging at challenging visible wavelengths, particularly Hα. To enable high-contrast imaging, the instrument hosts a vector apodizing phase plate (vAPP) coronagraph. The vAPP creates a static region of high contrast next to the star that is referred to as a dark hole; on MagAO-X, the expected dark hole raw contrast is ∼4 × 10 - 6. The ability to maintain this contrast during observations, however, is limited by the presence of non-common path aberrations (NCPA) and the resulting quasi-static speckles that remain unsensed and uncorrected by the primary AO system. These quasi-static speckles within the dark hole degrade the high contrast achieved by the vAPP and dominate the light from an exoplanet. The aim of our efforts here is to demonstrate two focal plane wavefront sensing (FPWFS) techniques for sensing NCPA and suppressing quasi-static speckles in the final focal plane. To sense NCPA to which the primary AO system is blind, the science image is used as a secondary wavefront sensor. With the vAPP, a static high-contrast dark hole is created on one side of the PSF, leaving the opposite side of the PSF unocculted. In this unobscured region, referred to as the bright field, the relationship between modulations in intensity and low-amplitude pupil plane phase aberrations can be approximated as linear. The bright field can therefore be used as a linear wavefront sensor to detect small NCPA and suppress quasi-static speckles. This technique, known as spatial linear dark field control (LDFC), can monitor the bright field for aberrations that will degrade the high-contrast dark hole. A second form of FPWFS, known as holographic modal wavefront sensing (hMWFS), is also employed with the vAPP. This technique uses hologram-generated PSFs in the science image to monitor the presence of low-order aberrations. With LDFC and the hMWFS, high contrast across the dark hole can be maintained over long observations, thereby allowing planet light to remain visible above the stellar noise over the course of observations on MagAO-X. Here, we present simulations and laboratory demonstrations of both spatial LDFC and the hMWFS with a vAPP coronagraph at the University of Arizona Extreme Wavefront Control Laboratory. We show both in simulation and in the lab that the hMWFS can be used to sense low-order aberrations and reduce the wavefront error (WFE) by a factor of 3 - 4 × . We also show in simulation that, in the presence of a temporally evolving pupil plane phase aberration with 27-nm root-mean-square (RMS) WFE, LDFC can reduce the WFE to 18-nm RMS, resulting in factor of 6 to 10 gain in contrast that is kept stable over time. This performance is also verified in the lab, showing that LDFC is capable of returning the dark hole to the average contrast expected under ideal lab conditions. These results demonstrate the power of the hMWFS and spatial LDFC to improve MagAO-X's high-contrast imaging capabilities for direct exoplanet imaging. Title: A snapshot full-Stokes spectropolarimeter for detecting life on Earth Authors: Snik, Frans; Keller, Christoph U.; Doelman, David S.; Kühn, Jonas; Patty, C. H. Lucas; Hoeijmakers, H. Jens; Pallichadath, Vidhya; Stam, Daphne M.; Pommerol, Antoine; Poch, Olivier; Demory, Brice-Olivier Bibcode: 2019SPIE11132E..0AS Altcode: 2019arXiv190902283S We present the design of a point-and-shoot non-imaging full-Stokes spectropolarimeter dedicated to detecting life on Earth from an orbiting platform like the ISS. We specifically aim to map circular polarization in the spectral features of chorophyll and other biopigments for our planet as a whole. These non-zero circular polarization signatures are caused by homochirality of the molecular and supramolecular configurations of organic matter, and are considered the most unambiguous biomarker. To achieve a fully solid-state snapshot design, we implement a novel spatial modulation that completely separates the circular and linear polarization channels. The polarization modulator consists of a patterned liquid-crystal quarter-wave plate inside the spectrograph slit, which also constitutes the first optical element of the instrument. This configuration eliminates cross-talk between linear and circular polarization, which is crucial because linear polarization signals are generally much stronger than the circular polarization signals. This leads to a quite unorthodox optical concept for the spectrograph, in which the object and the pupil are switched. We discuss the general design requirements and trade-offs of LSDpol (Life Signature Detection polarimeter), a prototype instrument that is currently under development. Title: Insights into Terrestrial Planet Compositions and Geophysics from Observations of Magma Worlds Authors: Ridden-Harper, Andrew; Snellen, Ignas; Keller, Christoph; Mollière, Paul; De Mooij, Ernst J. W.; Jayawardhana, Ray; de Kok, Remco; Hoeijmakers, H. Jens; Brogi, Matteo; Fridlund, Carl Malcolm; Vermeersen, Bert; Westrenen, Wim Bibcode: 2019ESS.....431305R Altcode: There exists a remarkable population of short period transiting rocky exoplanets with temperatures >2,000 K, and masses ranging from about 8 Earth masses, such as the hot super-Earth 55 Cancri e, to that of Mercury or smaller, such as K2-22b. These planets are thought to have mineral atmospheres that are produced by the vaporisation of their magma surfaces, or large exospheres that are produced by sputtering of their atmospheres or exposed surfaces by intense stellar winds. Additionally, the smaller, low surface gravity hot rocky exoplanets have been found to be actively disintegrating and forming 'comet-like' dust tails.

Since their atmospheres and released gas and dust can be observationally constrained, these planets present the tantalising prospect of directly probing the composition of rocky planets. Sodium and calcium are promising species to detect given their low sublimation temperatures, large absorption cross-sections, likely presence in terrestrial planet compositions, and presence in Mercury's exosphere.

This poster presents the insights we gained from using high-resolution transmission spectroscopy to search for Na and Ca around 55 Cnc e and K2-22 b using several ground based telescopes. For 55 Cne e, we detected a tantilizing 5 σ signal of Ca+ on one night of observation, but a similar signal has not been detected since (despite our unprecedented limits). This may be related to variability of the star-planet system and the planet's magnetic field.

For K2-22 b, we did not detect absorption by Na or Ca+, but found lower-limits that are smaller than the expected magnitude of the signal based on the planet's estimated mass-loss rate and assuming a terrestrial composition. We attribute this non-detection to the probed gases being accelerated by the stellar wind and radiation pressure to high velocities, resulting in very broad Doppler shifted absorption signals that are hard to detect.

The implications of these results on probing rocky exoplanet compositions, constraining planetary magnetic fields, and understanding the environment around short-period rocky exoplanets are also outlined. Title: ESA Voyage 2050 White Paper: Detecting life outside our solar system with a large high-contrast-imaging mission Authors: Snellen, Ignas; Albrecht, Simon; Anglada-Escude, Guillem; Baraffe, Isabelle; Baudoz, Pierre; Benz, Willy; Beuzit, Jean-Luc; Biller, Beth; Birkby, Jayne; Boccaletti, Anthony; van Boekel, Roy; de Boer, Jos; Brogi, Matteo; Buchhave, Lars; Carone, Ludmila; Claire, Mark; Claudi, Riccardo; Demory, Brice-Olivier; Desert, Jean-Michel; Desidera, Silvano; Gaudi, Scott; Gratton, Raffaele; Gillon, Michael; Grenfell, John Lee; Guyon, Olivier; Henning, Thomas; Hinkley, Sasha; Huby, Elsa; Janson, Markus; Helling, Christiane; Heng, Kevin; Kasper, Markus; Keller, Christoph; Kenworthy, Matthew; Krause, Oliver; Kreidberg, Laura; Madhusudhan, Nikku; Lagrange, Anne-Marie; Launhardt, Ralf; Lenton, Tim; Lopez-Puertas, Manuel; Maire, Anne-Lise; Mayne, Nathan; Meadows, Victoria; Mennesson, Bertrand; Micela, Giuseppina; Miguel, Yamila; Milli, Julien; Min, Michiel; de Mooij, Ernst; Mouillet, David; N'Diaye, Mamadou; D'Orazi, Valentina; Palle, Enric; Pagano, Isabella; Piotto, Giampaolo; Queloz, Didier; Rauer, Heike; Ribas, Ignasi; Ruane, Garreth; Selsis, Franck; Snik, Frans; Sozzetti, Alessandro; Stam, Daphne; Stark, Christopher; Vigan, Arthur; de Visser, Pieter Bibcode: 2019arXiv190801803S Altcode: In this white paper, we recommend the European Space Agency plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We provide an overview of relevant European expertise, and advocate ESA to start a technology development program towards detecting life outside the Solar system. Title: Search for gas from the disintegrating rocky exoplanet K2-22b Authors: Ridden-Harper, A. R.; Snellen, I. A. G.; Keller, C. U.; Mollière, P. Bibcode: 2019A&A...628A..70R Altcode: 2019arXiv190608795R Context. The red dwarf star K2-22 is transited every 9.14 h by an object which is best explained by being a disintegrating rocky exoplanet featuring a variable comet-like dust tail. While the dust is thought to dominate the transit light curve, gas is also expected to be present, either from being directly evaporated off the planet or by being produced by the sublimation of dust particles in the tail.
Aims: Both ionized calcium and sodium have large cross-sections, and although present at low abundance, exhibit the strongest atomic absorption features in comets. We therefore also identify these species as the most promising tracers of circumplanetary gas in evaporating rocky exoplanets and search for them in the tail of K2-22 b to constrain the gas-loss and sublimation processes in this enigmatic object.
Methods: We observed four transits of K2-22 b with X-shooter on the Very Large Telescope operated by ESO to obtain time series of intermediate-resolution (R 11 400) spectra. Our analysis focussed on the two sodium D lines (588.995 and 589.592 nm) and the Ca+ triplet (849.802, 854.209, and 866.214 nm). The stellar calcium and sodium absorption was removed using the out-of-transit spectra. We searched for planet-related absorption in the velocity rest frame of the planet, which changes from approximately -66 to +66 km s-1 during the transit.
Results: Since K2-22 b exhibits highly variable transit depths, we analysed the individual nights and their average. By injecting signals we reached 5σ upper limits on the individual nights that range from 11-13% and 1.7-2.0% for the sodium and ionized calcium absorption of the tail, respectively. Night 1 was contaminated by its companion star so we considered weighted averages with and without Night 1 and quote conservative 5σ limits without Night 1 of 9 and 1.4%, respectively. Assuming their mass fractions to be similar to those in the Earth's crust, these limits correspond to scenarios in which 0.04 and 35% of the transiting dust is sublimated and observed as absorbing gas. However, this assumes the gas to be co-moving with the planet. We show that for the high irradiation environment of K2-22 b, sodium and ionized calcium could be quickly accelerated to 100s of km s-1 owing to radiation pressure and entrainment by the stellar wind, making these species much more difficult to detect. No evidence for such possibly broad and blue-shifted signals are seen in our data.
Conclusions: Future observations aimed at observing circumplanetary gas should take into account the possible broad and blue-shifted velocity field of atomic and ionized species. Title: Polarimetric imaging of circumstellar disks. I. Artifacts due to limited angular resolution Authors: Heikamp, S.; Keller, C. U. Bibcode: 2019A&A...627A.156H Altcode: Context. Polarimetric images of circumstellar environments, even when corrected with adaptive optics, have a limited angular resolution. Finite resolution greatly affects polarimetric images because of the canceling of adjacent polarization signals with opposite signs. In radio astronomy this effect is called beam depolarization and is well known. However, radio techniques to mitigate beam depolarization are not directly applicable to optical images as a consequence of the inherent lack of phase information at optical wavelengths.
Aims: We explore the effects of a finite point-spread function (PSF) on polarimetric images and the application of Richardson-Lucy deconvolution to polarimetric images.
Methods: We simulated polarimetric images of highly simplified, circumstellar disk models and convolved these with simulated and actual SPHERE/ZIMPOL PSFs. We attempted to deconvolve simulated images in orthogonal linear polarizations and polarized intensity images.
Results: The most significant effect of finite angular resolution is the loss of polarimetric signal close to the central star where large polarization signals of opposite signs average out. The finite angular resolution can also introduce polarized light in areas beyond the original, polarized signal such as outside of disks. These effects are particularly severe for disks that are not rotationally symmetric. The deconvolution of polarimetric images is far from trivial. Richardson-Lucy deconvolution applied to images in opposite linear polarization states, which are subsequently subtracted from each other, cannot recover the signal close to the star. Sources that lack rotational symmetry cannot be recovered with this deconvolution approach. Title: Two accreting protoplanets around the young star PDS 70 Authors: Haffert, S. Y.; Bohn, A. J.; de Boer, J.; Snellen, I. A. G.; Brinchmann, J.; Girard, J. H.; Keller, C. U.; Bacon, R. Bibcode: 2019NatAs...3..749H Altcode: 2019NatAs.tmp..329H; 2019arXiv190601486H Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks1-3, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques4,5. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 706,7. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection8, and another located close to the outer edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission6,7,9. We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets10,11, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics-assisted, medium-resolution integral field spectroscopy with MUSE12 targeting accretion signatures will be a powerful way to trace ongoing planet formation in transitional disks at different stages of their evolution. Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System13. Title: Discovery of a directly imaged disk in scattered light around the Sco-Cen member Wray 15-788 Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Benisty, M.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Muro-Arena, G. A.; Pecaut, M. J.; Snik, F.; Wolff, S. G.; Reggiani, M. Bibcode: 2019A&A...624A..87B Altcode: 2019arXiv190207723B Context. Protoplanetary disks are the birth environments of planetary systems. Therefore, the study of young, circumstellar environments is essential in understanding the processes taking place in planet formation and the evolution of planetary systems.
Aims: We detect and characterize circumstellar disks and potential companions around solar-type, pre-main sequence stars in the Scorpius-Centaurus association (Sco-Cen).
Methods: As part of our ongoing survey we carried out high-contrast imaging with VLT/SPHERE/IRDIS to obtain polarized and total intensity images of the young (11-7+16 Myr old) K3IV star Wray 15-788 within the Lower Centaurus Crux subgroup of Sco-Cen. For the total intensity images, we remove the stellar halo via an approach based on reference star differential imaging in combination with principal component analysis.
Results: Both total intensity and polarimetric data resolve a disk around the young, solar-like Sco-Cen member Wray 15-788. Modeling of the stellar spectral energy distribution suggests that this is a protoplanetary disk at a transition stage. We detect a bright outer ring at a projected separation of 370 mas (≈56 au), hints of inner substructures at 170 mas (≈28 au), and a gap in between. Within a position angle range of only 60° < ϕ < 240°, we are confident at the 5σ level that we detect actual scattered light flux from the outer ring of the disk; the remaining part is indistinguishable from background noise. For the detected part of the outer ring we determine a disk inclination of i = 21° ± 6° and a position angle of φ = 76° ± 16°. Furthermore, we find that Wray 15-788 is part of a binary system with the A2V star HD 98363 at a separation of 50'' (≈6900 au).
Conclusions: The detection of only half of the outer ring might be due to shadowing by a misaligned inner disk. A potential substellar companion can cause the misalignment of the inner structures and can be responsible for clearing the detected gap from scattering material. However, we cannot rule out the possibility of a non-detection due to our limited signal-to-noise ratio (S/N), combined with brightness azimuthal asymmetry. From our data we can exclude companions more massive than 10 Mjup within the gap at a separation of 230 mas (≈35 au). Additional data are required to characterize the disk's peculiar morphology and to set tighter constraints on the potential perturber's orbital parameters and mass.

Reduced images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A87Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 099.C-0698(A), 0101.C-0153(A), and 0101.C-0464(A). Title: VizieR Online Data Catalog: Discovery of a resolved disk around Wray 15-788 (Bohn+, 2019) Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Benisty, M.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Muro-Arena, G. A.; Pecaut, M. J.; Snik, F.; Wolff, S. G.; Reggiani, M. Bibcode: 2019yCat..36240087B Altcode: Fits images corresponding to the imagery presented in the paper. The data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed with the latest version of PynPoint. In both total and polarized intensity we resolve a disk around the K-type pre-main sequence star Wray 15-788. SED analysis suggests that this is a protoplanetary disk at a transition stage.

(4 data files). Title: SPHERE/ZIMPOL high resolution polarimetric imager. I. System overview, PSF parameters, coronagraphy, and polarimetry Authors: Schmid, H. M.; Bazzon, A.; Roelfsema, R.; Mouillet, D.; Milli, J.; Menard, F.; Gisler, D.; Hunziker, S.; Pragt, J.; Dominik, C.; Boccaletti, A.; Ginski, C.; Abe, L.; Antoniucci, S.; Avenhaus, H.; Baruffolo, A.; Baudoz, P.; Beuzit, J. L.; Carbillet, M.; Chauvin, G.; Claudi, R.; Costille, A.; Daban, J. -B.; de Haan, M.; Desidera, S.; Dohlen, K.; Downing, M.; Elswijk, E.; Engler, N.; Feldt, M.; Fusco, T.; Girard, J. H.; Gratton, R.; Hanenburg, H.; Henning, Th.; Hubin, N.; Joos, F.; Kasper, M.; Keller, C. U.; Langlois, M.; Lagadec, E.; Martinez, P.; Mulder, E.; Pavlov, A.; Podio, L.; Puget, P.; Quanz, S. P.; Rigal, F.; Salasnich, B.; Sauvage, J. -F.; Schuil, M.; Siebenmorgen, R.; Sissa, E.; Snik, F.; Suarez, M.; Thalmann, Ch.; Turatto, M.; Udry, S.; van Duin, A.; van Holstein, R. G.; Vigan, A.; Wildi, F. Bibcode: 2018A&A...619A...9S Altcode: 2018arXiv180805008S Context. The SPHERE "planet finder" is an extreme adaptive optics (AO) instrument for high resolution and high contrast observations at the Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance.
Aims: We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an optimal planning of observations and for improving the data reduction and calibration. We aim to provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging.
Methods: We have analyzed SPHERE/ZIMPOL point spread functions (PSFs) and measure the normalized peak surface brightness, the encircled energy, and the full width half maximum (FWHM) for different wavelengths, atmospheric conditions, star brightness, and instrument modes. Coronagraphic images are described and the peak flux attenuation and the off-axis flux transmission are determined. Simultaneous images of the coronagraphic focal plane and the pupil plane are analyzed and the suppression of the diffraction rings by the pupil stop is investigated. We compared the performance at small separation for different coronagraphs with tests for the binary α Hyi with a separation of 92 mas and a contrast of Δm ≈ 6m. For the polarimetric mode we made the instrument calibrations using zero polarization and high polarization standard stars and here we give a recipe for the absolute calibration of polarimetric data. The data show small (< 1 mas) but disturbing differential polarimetric beam shifts, which can be explained as Goos-Hähnchen shifts from the inclined mirrors, and we discuss how to correct this effect. The polarimetric sensitivity is investigated with non-coronagraphic and deep, coronagraphic observations of the dust scattering around the symbiotic Mira variable R Aqr.
Results: SPHERE/ZIMPOL reaches routinely an angular resolution (FWHM) of 22-28 mas, and a normalized peak surface brightness of SB0 - mstar ≈ -6.5m arcsec-2 for the V-, R- and I-band. The AO performance is worse for mediocre ≳1.0″ seeing conditions, faint stars mR ≳ 9m, or in the presence of the "low wind" effect (telescope seeing). The coronagraphs are effective in attenuating the PSF peak by factors of > 100, and the suppression of the diffracted light improves the contrast performance by a factor of approximately two in the separation range 0.06″-0.20″. The polarimetric sensitivity is Δp < 0.01% and the polarization zero point can be calibrated to better than Δp ≈ 0.1%. The contrast limits for differential polarimetric imaging for the 400 s I-band data of R Aqr at a separation of ρ = 0.86″ are for the surface brightness contrast SBpol( ρ)-mstar ≈ 8m arcsec-2 and for the point source contrast mpol( ρ)-mstar ≈ 15m and much lower limits are achievable with deeper observations.
Conclusions: SPHERE/ZIMPOL achieves imaging performances in the visual range with unprecedented characteristics, in particular very high spatial resolution and very high polarimetric contrast. This instrument opens up many new research opportunities for the detailed investigation of circumstellar dust, in scattered and therefore polarized light, for the investigation of faint companions, and for the mapping of circumstellar Hα emission.

Based on observations collected at La Silla and Paranal Observatory, ESO (Chile), Program ID: 60.A-9249 and 60.A-9255. Title: Chromatic transit light curves of disintegrating rocky planets Authors: Ridden-Harper, A. R.; Keller, C. U.; Min, M.; van Lieshout, R.; Snellen, I. A. G. Bibcode: 2018A&A...618A..97R Altcode: 2018arXiv180707973R Context. Kepler observations have revealed a class of short-period exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust tails thought to be the result of small, rocky planets losing mass. The shape and chromaticity of the transits constrain the properties of the dust particles originating from the planet's surface, offering a unique opportunity to probe the composition and geophysics of rocky exoplanets.
Aims: We aim to approximate the average Kepler long-cadence light curve of Kepler-1520 b and investigate how the optical thickness and transit cross section of a general dust tail can affect the observed wavelength dependence and depth of transit light curves.
Methods: We developed a new 3D model that ejects sublimating particles from the planet surface to build up a dust tail, assuming it to be optically thin, and used 3D radiative transfer computations that fully treat scattering using the distribution of hollow spheres (DHS) method, to generate transit light curves between 0.45 and 2.5 μm.
Results: We show that the transit depth is wavelength independent of optically thick tails, potentially explaining why only some observations indicate a wavelength dependence. From the 3D nature of our simulated tails, we show that their transit cross sections are related to the component of particle ejection velocity perpendicular to the planets orbital plane and use this to derive a minimum ejection velocity of 1.2 km s-1. To fit the average transit depth of Kepler-1520 b of 0.87%, we require a high dust mass-loss rate of 7-80 M Gyr-1 which implies planet lifetimes that may be inconsistent with the observed sample. Therefore, these mass loss rates should be considered to be upper limits. Title: Review of high-contrast imaging systems for current and future ground- and space-based telescopes I: coronagraph design methods and optical performance metrics Authors: Ruane, G.; Riggs, A.; Mazoyer, J.; Por, E. H.; N'Diaye, M.; Huby, E.; Baudoz, P.; Galicher, R.; Douglas, E.; Knight, J.; Carlomagno, B.; Fogarty, K.; Pueyo, L.; Zimmerman, N.; Absil, O.; Beaulieu, M.; Cady, E.; Carlotti, A.; Doelman, D.; Guyon, O.; Haffert, S.; Jewell, J.; Jovanovic, N.; Keller, C.; Kenworthy, M. A.; Kuhn, J.; Miller, K.; Sirbu, D.; Snik, F.; Wallace, J. Kent; Wilby, M.; Ygouf, M. Bibcode: 2018SPIE10698E..2SR Altcode: 2018arXiv180707042R The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this first installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of design methods and optical performance metrics developed for coronagraph instruments. The design and optimization of coronagraphs for future telescopes has progressed rapidly over the past several years in the context of space mission studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based telescopes. Design tools have been developed at several institutions to optimize a variety of coronagraph mask types. We aim to give a broad overview of the approaches used, examples of their utility, and provide the optimization tools to the community. Though it is clear that the basic function of coronagraphs is to suppress starlight while maintaining light from off-axis sources, our community lacks a general set of standard performance metrics that apply to both detecting and characterizing exoplanets. The attendees of the OOC workshop agreed that it would benefit our community to clearly define quantities for comparing the performance of coronagraph designs and systems. Therefore, we also present a set of metrics that may be applied to theoretical designs, testbeds, and deployed instruments. We show how these quantities may be used to easily relate the basic properties of the optical instrument to the detection significance of the given point source in the presence of realistic noise. Title: First direct detection of a polarized companion outside a resolved circumbinary disk around CS Chamaeleonis Authors: Ginski, C.; Benisty, M.; van Holstein, R. G.; Juhász, A.; Schmidt, T. O. B.; Chauvin, G.; de Boer, J.; Wilby, M.; Manara, C. F.; Delorme, P.; Ménard, F.; Pinilla, P.; Birnstiel, T.; Flock, M.; Keller, C.; Kenworthy, M.; Milli, J.; Olofsson, J.; Pérez, L.; Snik, F.; Vogt, N. Bibcode: 2018A&A...616A..79G Altcode: 2018arXiv180502261G
Aims: To understand planet formation it is necessary to study the birth environment of planetary systems. Resolved imaging of young planet forming disks allows us to study this environment in great detail and find signs of planet-disk interaction and disk evolution. In the present study we aim to investigate the circumstellar environment of the spectroscopic binary T Tauri star CS Cha. From unresolved mid-to far-infrared photometry it is predicted that CS Cha hosts a disk with a large cavity. In addition, spectral energy distribution modeling suggests significant dust settling, pointing toward an evolved disk that may show signs of ongoing or completed planet formation.
Methods: We observed CS Cha with the high contrast imager SPHERE at the ESO Very Large Telescope (VLT) in polarimetric differential imaging mode to resolve the circumbinary disk in near-infrared scattered light. These observations were followed up by VLT/NACO L-band observations and complemented by archival VLT/NACO K-band and Hubble Space Telescope WFPC2 I-band data.
Results: We resolve the compact circumbinary disk around CS Cha for the first time in scattered light. We find a smooth, low inclination disk with an outer radius of 55 au (at 165 pc). We do not detect the inner cavity but find an upper limit for the cavity size of 15 au. Furthermore, we find a faint comoving companion with a projected separation of 210 au from the central binary outside of the circumbinary disk. The companion is detected in polarized light and shows an extreme degree of polarization (13.7 ± 0.4% in the J band). The J- and H-band magnitudes of the companion are compatible with masses of a few MJup. However, K-, L-, and I-band data draw this conclusion into question. We explore with radiative transfer modeling whether an unresolved circum-companion disk can be responsible for the high polarization and complex photometry. We find that the set of observations is best explained by a heavily extincted low-mass ( 20 MJup) brown dwarf or high-mass planet with an unresolved disk and dust envelope.

Based on observations performed with VLT/SPHERE under program ID 098.C-0760(B) and 099.C-0891(B) and VLT/NACO under program ID 298.C-5054(B) and 076.C-0292(A)The reduced images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/616/A79 Title: High contrast imaging for the enhanced resolution imager and spectrometer (ERIS) Authors: Kenworthy, Matthew A.; Snik, Frans; Keller, Christoph U.; Doelman, David; Por, Emiel H.; Absil, Olivier; Carlomagno, Brunella; Karlsson, Mikael; Huby, Elsa; Glauser, Adrian M.; Quanz, Sascha P.; Taylor, William D. Bibcode: 2018SPIE10702E..46K Altcode: 2020arXiv201201963K ERIS is a diffraction limited thermal infrared imager and spectrograph for the Very Large Telescope UT4. One of the science cases for ERIS is the detection and characterization of circumstellar structures and exoplanets around bright stars that are typically much fainter than the stellar diffraction halo. Enhanced sensitivity is provided through the combination of (i) suppression of the diffraction halo of the target star using coronagraphs, and (ii) removal of any residual diffraction structure through focal plane wavefront sensing and subsequent active correction. In this paper we present the two coronagraphs used for diffraction suppression and enabling high contrast imaging in ERIS. Title: On-sky results of the Leiden EXoplanet Instrument (LEXI) Authors: Haffert, S. Y.; Wilby, M. J.; Keller, C. U.; Snellen, I. A. G.; Doelman, D. S.; Por, E. H.; van Kooten, M.; Bos, S. P.; Wardenier, J. Bibcode: 2018SPIE10703E..23H Altcode: The Leiden EXoplanet Instrument (LEXI) is a bench-mounted high-contrast spectrograph (HCS) and high- contrast imager(HCI). Both science instruments are mounted behind a common adaptive optics (AO) system. The AO can be controlled by several new wavefront sensors for which we will show the first on-sky results. There is a new pupil-plane wavefront sensors; the Generalized Optical Differentiation Wavefront sensor(g-ODWFS). LEXI can switch between two observing modes, the HCI mode or the HD-IFS mode. The spectrograph is very compact because it is fed by single-mode fiber. The HD-IFS is an IFS that covers the spectral range of 600 800 nm with a constant spectral resolving power of 96000. The 2kx3k detector makes it possible to deliver diffraction limited spectra of up to 20 input fibers. The high-dispersion spectra of the HD-IFS allow for robust post-processing technique to remove residual stellar speckles and allows for direct characterization of the faint stellar environment. We will show the first sucesful on-sky results of the injection into a single-mode fiber with LEXI. In HCI mode an Apodizing Phase Plate (APP) is used to create a dark region around the star with an average design contrast of 1E-4. The APP is multiplexed with holographic modes to create the Coronagraphic Modal Wavefront sensor (cMWS) for non-common path error (NCPE) correction. The cMWS creates holographic copies in the focal plane that react linearly to aberrations. The holographic copies are measured simultaneously with the science target. There is no downtime for NCPE correction. We will show the first on-sky closed-loop correction of (NCPEs) with the cMWS. Title: Optimization of contrast in adaptive optics for exoplanet imaging Authors: Radhakrishnan, Vikram Mark; Keller, Christoph U.; Doelman, Niek Bibcode: 2018SPIE10703E..4NR Altcode: Direct imaging of exoplanets requires overcoming the enormous contrast between the exoplanet and its host star, to distinguish the reflected light from the exoplanet, from the diffracted light of the star. Direct optimization of contrast, using nonlinear optimization techniques of the Adaptive Optics (AO) system coupled with coronagraphy, shows significant promise in achieving high contrast, beyond the limits of what can be achieved with traditional AO systems. Using a coronagraph optic as a "static" phase modifying element, and a deformable mirror as a "dynamic" element, we create an adaptive coronagraph, capable of engineering the point spread function (PSF) of the imaging system, to create a deep, dark hole in the focal plane, within which the exoplanet can be imaged. We present the results of simulations of a system, consisting of a vector Apodizing Phase Plate (vAPP) coronagraph, a deformable mirror (DM), and an imaging camera. The vAPP coronagraph reroutes starlight within the pupil plane, to create a designated dark hole region, which in the ideal case would be devoid of starlight in the focal plane. Off-axis exoplanet light is transmitted through to the dark hole and hence can be imaged. Atmospheric turbulence is simulated to generate a distorted wavefront, and a nonlinear, gradient climbing based optimization algorithm is implemented to drive the DM to optimize a merit function. This merit function is chosen with a dual objective to maximize average raw contrast in the dark hole, while maintaining a sufficiently high Strehl ratio. Preliminary results show that in a setup with a coronagraph designed to create a 6×6 (λ/D)2 rectangular dark hole with a raw contrast of 10-5, the optimization procedure results in a raw contrast of 10-7 at the dark hole while maintaining a Strehl ratio above 40%. It is observed that by tweaking the merit function, this non-linear optimization procedure can be adjusted to result in either higher Strehl or higher contrast. We discuss potential strategies to extend the non-linear optimization techniques to real-time, non-linear control for the AO system, thereby achieving a real-time, dynamic, adaptive coronagraph. Toward this end, we investigate the results of using the fast wavefront sensor data to reconstruct the wavefront phase, virtually propagate this through the science optical path, and optimize contrast on this virtual science image, as opposed to using the slower science camera to optimize contrast on the true science image. One potential approach to implement true real-time control would be to use deep neural networks, trained using deep deterministic policy gradients, to identify and remove speckles of diffracted starlight in the dark hole region in real-time. Title: Review of high-contrast imaging systems for current and future ground-based and space-based telescopes: Part II. Common path wavefront sensing/control and coherent differential imaging Authors: Jovanovic, Nemanja; Absil, Olivier; Baudoz, Pierre; Beaulieu, Mathilde; Bottom, Michael; Cady, Eric; Carlomagno, Brunella; Carlotti, Alexis; Doelman, David; Fogarty, Kevin; Galicher, Raphaël.; Guyon, Olivier; Haffert, Sebastiaan; Huby, Elsa; Jewell, Jeffrey; Keller, Christoph; Kenworthy, Matthew A.; Knight, Justin; Kühn, Jonas; Miller, Kelsey; Mazoyer, Johan; N'Diaye, Mamadou; Por, Emiel; Pueyo, Laurent; Riggs, A. J. E.; Ruane, Garreth; Sirbu, Dan; Snik, Frans; Wallace, J. K.; Wilby, Michael; Ygouf, Marie Bibcode: 2018SPIE10703E..1UJ Altcode: 2018arXiv180707043J The Optimal Optical Coronagraph (OOC) Workshop held at the Lorentz Center in September 2017 in Leiden, the Netherlands, gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this second installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of common path wavefront sensing/control and Coherent Differential Imaging techniques, highlight the latest results, and expose their relative strengths and weaknesses. We layout critical milestones for the field with the aim of enhancing future ground/space based high contrast imaging platforms. Techniques like these will help to bridge the daunting contrast gap required to image a terrestrial planet in the zone where it can retain liquid water, in reflected light around a G type star from space. Title: Laboratory verification of Fast & Furious phase diversity: Towards controlling the low wind effect in the SPHERE instrument Authors: Wilby, M. J.; Keller, C. U.; Sauvage, J. -F.; Dohlen, K.; Fusco, T.; Mouillet, D.; Beuzit, J. -L. Bibcode: 2018A&A...615A..34W Altcode: 2018arXiv180303258W Context. The low wind effect (LWE) refers to a characteristic set of quasi-static wavefront aberrations seen consistently by the SPHERE instrument when dome-level wind speeds drop below 3 ms-1. The LWE produces bright low-order speckles in the stellar point-spread function (PSF), which severely limit the contrast performance of SPHERE under otherwise optimal observing conditions.
Aims: In this paper we propose the Fast & Furious (F&F) phase diversity algorithm as a viable software-only solution for real-time LWE compensation, which would utilise image sequences from the SPHERE differential tip-tilt sensor (DTTS) and apply corrections via reference slope offsets on the AO system's Shack-Hartmann wavefront sensor.
Methods: We evaluated the closed-loop performance of F&F on the MITHIC high-contrast test-bench, under conditions emulating LWE-affected DTTS images. These results were contrasted with predictive simulations for a variety of convergence tests, in order to assess the expected performance of an on-sky implementation of F&F in SPHERE.
Results: The algorithm was found to be capable of returning LWE-affected images to Strehl ratios of greater than 90% within five iterations, for all appropriate laboratory test cases. These results are highly representative of predictive simulations, and demonstrate stability of the algorithm against a wide range of factors including low image signal-to-noise ratio (S/N), small image field of view, and amplitude errors. It was also found in simulation that closed-loop stability can be preserved down to image S/N as low as five while still improving overall wavefront quality, allowing for reliable operation even on faint targets.
Conclusions: The Fast & Furious algorithm is an extremely promising solution for real-time compensation of the LWE, which can operate simultaneously with science observations and may be implemented in SPHERE without requiring additional hardware. The robustness and relatively large effective dynamic range of F&F also make it suitable for general wavefront optimisation applications, including the co-phasing of segmented ELT-class telescopes. Title: MagAO-X: project status and first laboratory results Authors: Males, Jared R.; Close, Laird M.; Miller, Kelsey; Schatz, Lauren; Doelman, David; Lumbres, Jennifer; Snik, Frans; Rodack, Alex; Knight, Justin; Van Gorkom, Kyle; Long, Joseph D.; Hedglen, Alex; Kautz, Maggie; Jovanovic, Nemanja; Morzinski, Katie; Guyon, Olivier; Douglas, Ewan; Follette, Katherine B.; Lozi, Julien; Bohlman, Chris; Durney, Olivier; Gasho, Victor; Hinz, Phil; Ireland, Michael; Jean, Madison; Keller, Christoph; Kenworthy, Matt; Mazin, Ben; Noenickx, Jamison; Alfred, Dan; Perez, Kevin; Sanchez, Anna; Sauve, Corwynn; Weinberger, Alycia; Conrad, Al Bibcode: 2018SPIE10703E..09M Altcode: 2018arXiv180704315M MagAO-X is an entirely new extreme adaptive optics system for the Magellan Clay 6.5 m telescope, funded by the NSF MRI program starting in Sep 2016. The key science goal of MagAO-X is high-contrast imaging of accreting protoplanets at Hα. With 2040 actuators operating at up to 3630 Hz, MagAO-X will deliver high Strehls (> 70%), high resolution (19 mas), and high contrast (< 1 × 10-4 ) at Hα (656 nm). We present an overview of the MagAO-X system, review the system design, and discuss the current project status. Title: ERIS: revitalising an adaptive optics instrument for the VLT Authors: Davies, R.; Esposito, S.; Schmid, H. -M.; Taylor, W.; Agapito, G.; Agudo Berbel, A.; Baruffolo, A.; Biliotti, V.; Biller, B.; Black, M.; Boehle, A.; Briguglio, B.; Buron, A.; Carbonaro, L.; Cortes, A.; Cresci, G.; Deysenroth, M.; Di Cianno, A.; Di Rico, G.; Doelman, D.; Dolci, M.; Dorn, R.; Eisenhauer, F.; Fantinel, D.; Ferruzzi, D.; Feuchtgruber, H.; Förster Schreiber, N.; Gao, X.; Gemperlein, H.; Genzel, R.; George, E.; Gillessen, S.; Giordano, C.; Glauser, A.; Glindemann, A.; Grani, P.; Hartl, M.; Heijmans, J.; Henry, D.; Huber, H.; Kasper, M.; Keller, C.; Kenworthy, M.; Kühn, J.; Kuntschner, H.; Lightfoot, J.; Lunney, D.; MacIntosh, M.; Mannucci, F.; March, S.; Neeser, M.; Patapis, P.; Pearson, D.; Plattner, M.; Puglisi, A.; Quanz, S.; Rau, C.; Riccardi, A.; Salasnich, B.; Schubert, J.; Snik, F.; Sturm, E.; Valentini, A.; Waring, C.; Wiezorrek, E.; Xompero, M. Bibcode: 2018SPIE10702E..09D Altcode: 2018arXiv180705089D ERIS is an instrument that will both extend and enhance the fundamental diffraction limited imaging and spectroscopy capability for the VLT. It will replace two instruments that are now being maintained beyond their operational lifetimes, combine their functionality on a single focus, provide a new wavefront sensing module that makes use of the facility Adaptive Optics System, and considerably improve their performance. The instrument will be competitive with respect to JWST in several regimes, and has outstanding potential for studies of the Galactic Center, exoplanets, and high redshift galaxies. ERIS had its final design review in 2017, and is expected to be on sky in 2020. This contribution describes the instrument concept, outlines its expected performance, and highlights where it will most excel. Title: Review of high-contrast imaging systems for current and future ground-based and space-based telescopes III: technology opportunities and pathways Authors: Snik, Frans; Absil, Olivier; Baudoz, Pierre; Beaulieu, Mathilde; Bendek, Eduardo; Cady, Eric; Carlomagno, Brunella; Carlotti, Alexis; Cvetojevic, Nick; Doelman, David; Fogarty, Kevin; Galicher, Raphaël.; Guyon, Olivier; Haffert, Sebastiaan; Huby, Elsa; Jewell, Jeffrey; Jovanovic, Nemanja; Keller, Christoph; Kenworthy, Matthew A.; Knight, Justin; Kuhn, Jonas; Mazoyer, Johan; Miller, Kelsey; N'Diaye, Mamadou; Norris, Barnaby; Por, Emiel; Pueyo, Laurent; Riggs, A. J. Eldorado; Ruane, Garreth; Sirbu, Dan; Wallace, J. Kent; Wilby, Michael; Ygouf, Marie Bibcode: 2018SPIE10706E..2LS Altcode: 2018arXiv180707100S The Optimal Optical CoronagraphWorkshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 30 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. This contribution is the final part of a series of three papers summarizing the outcomes of the workshop, and presents an overview of novel optical technologies and systems that are implemented or considered for high-contrast imaging instruments on both ground-based and space telescopes. The overall objective of high contrast instruments is to provide direct observations and characterizations of exoplanets at contrast levels as extreme as 10-10. We list shortcomings of current technologies, and identify opportunities and development paths for new technologies that enable quantum leaps in performance. Specifically, we discuss the design and manufacturing of key components like advanced deformable mirrors and coronagraphic optics, and their amalgamation in "adaptive coronagraph" systems. Moreover, we discuss highly integrated system designs that combine contrast-enhancing techniques and characterization techniques (like high-resolution spectroscopy) while minimizing the overall complexity. Finally, we explore extreme implementations using all-photonics solutions for ground-based telescopes and dedicated huge apertures for space telescopes. Title: Multiplexed holographic aperture masking with liquid-crystal geometric phase masks Authors: Doelman, D. S.; Tuthill, P.; Norris, B.; Wilby, M. J.; Por, E. H.; Keller, C. U.; Escuti, M. J.; Snik, F. Bibcode: 2018SPIE10701E..0TD Altcode: 2018arXiv180800751D Sparse Aperture Masking (SAM) allows for high-contrast imaging at small inner working angles, however the performance is limited by the small throughput and the number of baselines. We present the concept and first lab results of Holographic Aperture Masking (HAM) with extreme liquid-crystal geometric phase patterns. We multiplex subapertures using holographic techniques to combine the same subaperture in multiple non-redundant PSFs in combination with a non-interferometric reference spot. This way arbitrary subaperture combinations and PSF configurations can be realized, giving HAM more uv-coverage, better throughput and improved calibration as compared to SAM, at the cost of detector space. Title: A Planet with a Disc? A Surprising Detection in Polarised Light with VLT/SPHERE Authors: Ginski, C.; van Holstein, R.; Juhász, A.; Benisty, M.; Schmidt, T.; Chauvin, G.; de Boer, J.; Wilby, M.; Manara, C. F.; Delorme, P.; Ménard, F.; Muro-Arena, G.; Pinilla, P.; Birnstiel, T.; Flock, M.; Keller, C.; Kenworthy, M.; Milli, J.; Olofsson, J.; Pérez, L.; Snik, F.; Vogt, N. Bibcode: 2018Msngr.172...27G Altcode: With the Spectro-Polarimetric High- contrast Exoplanet REsearch (SPHERE) instrument at ESO's Very Large Telescope (VLT) we can study the linear polarisation of directly detected planets and brown dwarfs, to learn about their atmospheres and immediate environments. We summarise here the recent discovery of a low-mass companion in polarised light by Ginski et al. (2018). The object shows an extreme degree of polarisation, indicating the presence of a circumplanetary disc. Title: The Single-mode Complex Amplitude Refinement (SCAR) coronagraph: II. Lab verification, and toward the characterization of Proxima b Authors: Haffert, S. Y.; Por, E. H.; Keller, C. U.; Kenworthy, M. A.; Doelman, D. S.; Snik, F.; Escuti, M. J. Bibcode: 2018arXiv180310693H Altcode: We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8-2.4 \lambda/D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of $2-3 \times 10^{-4}$ , and the 180 SCAR reaches a null of $1 \times 10^{-4}$ . Their monochromatic contrast is maintained within a range of $\pm$ 0.16 \lambda/D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood. Title: SPEX: a highly accurate spectropolarimeter for atmospheric aerosol characterization Authors: Rietjens, J. H. H.; Smit, J. M.; di Noia, A.; Hasekamp, O. P.; van Harten, G.; Snik, F.; Keller, C. U. Bibcode: 2017SPIE10563E..44R Altcode: Global characterization of atmospheric aerosol in terms of the microphysical properties of the particles is essential for understanding the role aerosols in Earth climate [1]. For more accurate predictions of future climate the uncertainties of the net radiative forcing of aerosols in the Earth's atmosphere must be reduced [2]. Essential parameters that are needed as input in climate models are not only the aerosol optical thickness (AOT), but also particle specific properties such as the aerosol mean size, the single scattering albedo (SSA) and the complex refractive index. The latter can be used to discriminate between absorbing and non-absorbing aerosol types, and between natural and anthropogenic aerosol. Classification of aerosol types is also very important for air-quality and health-related issues [3]. Remote sensing from an orbiting satellite platform is the only way to globally characterize atmospheric aerosol at a relevant timescale of 1 day [4]. One of the few methods that can be employed for measuring the microphysical properties of aerosols is to observe both radiance and degree of linear polarization of sunlight scattered in the Earth atmosphere under different viewing directions [5][6][7]. The requirement on the absolute accuracy of the degree of linear polarization PL is very stringent: the absolute error in PL must be smaller then 0.001+0.005.PL in order to retrieve aerosol parameters with sufficient accuracy to advance climate modelling and to enable discrimination of aerosol types based on their refractive index for air-quality studies [6][7]. In this paper we present the SPEX instrument, which is a multi-angle spectropolarimeter that can comply with the polarimetric accuracy needed for characterizing aerosols in the Earth's atmosphere. We describe the implementation of spectral polarization modulation in a prototype instrument of SPEX and show results of ground based measurements from which aerosol microphysical properties are retrieved. Title: The Spectropolarimeter for Planetary Exploration: SPEX Authors: Laan, Erik; Stam, Daphne; Snik, Frans; Karalidi, Theodora; Keller, Christoph; ter Horst, Rik; Navarro, Ramon; Oomen, Gijsbert; de Vries, Johan; Hoogeveen, Ruud Bibcode: 2017SPIE10566E..2GL Altcode: SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact remotesensing instrument for measuring and characterizing aerosols in the atmosphere. The shoebox size instrument is capable of accurate full linear spectropolarimetry without moving parts or liquid crystals. High precision polarimetry is performed through encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the spectrum. Measuring this intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization. Polarimetry has proven to be an excellent tool to study microphysical properties of atmospheric particles. Such information is essential to better understand the weather and climate of a planet. Although SPEX can be used to study any planetary atmosphere, including the Earth's, the current design of SPEX is tailored to study Martian dust and clouds from an orbiting platform. SPEX' 9 entrance pupils can simultaneously measure intensity spectra from 0.4 to 0.8 microns, in different directions along the flight direction (including two limb viewing directions). This way, the scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying over it. SPEX can provide synergy with instruments on rovers and landers, as it provides an overview of spatial and temporal variations of the Martian atmosphere. Title: SPEX: the Spectropolarimeter for Planetary Exploration Authors: Rietjens, J. H. H.; Snik, F.; Stam, D. M.; Smit, J. M.; van Harten, G.; Keller, C. U.; Verlaan, A. L.; Laan, E. C.; ter Horst, R.; Navarro, R.; Wielinga, K.; Moon, S. G.; Voors, R. Bibcode: 2017SPIE10565E..1CR Altcode: We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass spectropolarimeter designed to operate from an orbiting or in situ platform. Its purpose is to simultaneously measure the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere and/or reflected by a planetary surface with high accuracy. The degree of linear polarization is extremely sensitive to the microphysical properties of atmospheric or surface particles (such as size, shape, and composition), and to the vertical distribution of atmospheric particles, such as cloud top altitudes. Measurements as those performed by SPEX are therefore crucial and often the only tool for disentangling the many parameters that describe planetary atmospheres and surfaces. SPEX uses a novel, passive method for its radiance and polarization observations that is based on a carefully selected combination of polarization optics. This method, called spectral modulation, is the modulation of the radiance spectrum in both amplitude and phase by the degree and angle of linear polarization, respectively. The polarization optics consists of an achromatic quarter-wave retarder, an athermal multiple-order retarder, and a polarizing beam splitter. We will show first results obtained with the recently developed prototype of the SPEX instrument, and present a performance analysis based on a dedicated vector radiative transport model together with a recently developed SPEX instrument simulator. Title: Polarization modeling and predictions for DKIST part 2: application of the Berreman calculus to spectral polarization fringes of beamsplitters and crystal retarders Authors: Harrington, David M.; Snik, Frans; Keller, Christoph U.; Sueoka, Stacey R.; van Harten, Gerard Bibcode: 2017JATIS...3d8001H Altcode: 2017arXiv171006067H We outline polarization fringe predictions derived from an application of the Berreman calculus for the Daniel K. Inouye Solar Telescope (DKIST) retarder optics. The DKIST retarder baseline design used six crystals, single-layer antireflection coatings, thick cover windows, and oil between all optical interfaces. This tool estimates polarization fringes and optic Mueller matrices as functions of all optical design choices. The amplitude and period of polarized fringes under design changes, manufacturing errors, tolerances, and several physical factors can now be estimated. This tool compares well with observations of fringes for data collected with the spectropolarimeter for infrared and optical regions at the Dunn Solar Telescope using bicrystalline achromatic retarders as well as laboratory tests. With this tool, we show impacts of design decisions on polarization fringes as impacted by antireflection coatings, oil refractive indices, cover window presence, and part thicknesses. This tool helped DKIST decide to remove retarder cover windows and also recommends reconsideration of coating strategies for DKIST. We anticipate this tool to be essential in designing future retarders for mitigation of polarization and intensity fringe errors in other high spectral resolution astronomical systems. Title: Combining angular differential imaging and accurate polarimetry with SPHERE/IRDIS to characterize young giant exoplanets Authors: van Holstein, Rob G.; Snik, Frans; Girard, Julien H.; de Boer, Jozua; Ginski, C.; Keller, Christoph U.; Stam, Daphne M.; Beuzit, Jean-Luc; Mouillet, David; Kasper, Markus; Langlois, Maud; Zurlo, Alice; de Kok, Remco J.; Vigan, Arthur Bibcode: 2017SPIE10400E..15V Altcode: 2017arXiv170907519V Young giant exoplanets emit infrared radiation that can be linearly polarized up to several percent. This linear polarization can trace: 1) the presence of atmospheric cloud and haze layers, 2) spatial structure, e.g. cloud bands and rotational flattening, 3) the spin axis orientation and 4) particle sizes and cloud top pressure. We introduce a novel high-contrast imaging scheme that combines angular differential imaging (ADI) and accurate near-infrared polarimetry to characterize self-luminous giant exoplanets. We implemented this technique at VLT/SPHEREIRDIS and developed the corresponding observing strategies, the polarization calibration and the data-reduction approaches. The combination of ADI and polarimetry is challenging, because the field rotation required for ADI negatively affects the polarimetric performance. By combining ADI and polarimetry we can characterize planets that can be directly imaged with a very high signal-to-noise ratio. We use the IRDIS pupil-tracking mode and combine ADI and principal component analysis to reduce speckle noise. We take advantage of IRDIS' dual-beam polarimetric mode to eliminate differential effects that severely limit the polarimetric sensitivity (flat-fielding errors, differential aberrations and seeing), and thus further suppress speckle noise. To correct for instrumental polarization effects, we apply a detailed Mueller matrix model that describes the telescope and instrument and that has an absolute polarimetric accuracy <= 0.1%. Using this technique we have observed the planets of HR 8799 and the (sub-stellar) companion PZ Tel B. Unfortunately, we do not detect a polarization signal in a first analysis. We estimate preliminary 1σ upper limits on the degree of linear polarization of ∼ 1% and ∼ 0.1% for the planets of HR 8799 and PZ Tel B, respectively. The achieved sub-percent sensitivity and accuracy show that our technique has great promise for characterizing exoplanets through direct-imaging polarimetry Title: Three Years of SPHERE: The Latest View of the Morphology and Evolution of Protoplanetary Discs Authors: Garufi, A.; Benisty, M.; Stolker, T.; Avenhaus, H.; de Boer, J. .; Pohl, A.; Quanz, S. P.; Dominik, C.; Ginski, C.; Thalmann, C.; van Boekel, R.; Boccaletti, A.; Henning, T.; Janson, M.; Salter, G.; Schmid, H. M.; Sissa, E.; Langlois, M.; Beuzit, J. -L.; Chauvin, G.; Mouillet, D.; Augereau, J. -C.; Bazzon, A.; Biller, B.; Bonnefoy, M.; Buenzli, E.; Cheetham, A.; Daemgen, S.; Desidera, S.; Engler, N.; Feldt, M.; Girard, J.; Gratton, R.; Hagelberg, J.; Keller, C.; Keppler, M.; Kenworthy, M.; Kral, Q.; Lopez, B.; Maire, A. -L.; Menard, F.; Mesa, D.; Messina, S.; Meyer, M. R.; Milli, J.; Min, M.; Muller, A.; Olofsson, J.; Pawellek, N.; Pinte, C.; Szulagyi, J.; Vigan, A.; Wahhaj, Z.; Waters, R.; Zurlo, A. Bibcode: 2017Msngr.169...32G Altcode: 2017arXiv171002795G Spatially resolving the immediate surroundings of young stars is a key challenge for the planet formation community. SPHERE on the VLT represents an important step forward by increasing the opportunities offered by optical or near-infrared imaging instruments to image protoplanetary discs. The Guaranteed Time Observation Disc team has concentrated much of its efforts on polarimetric differential imaging, a technique that enables the efficient removal of stellar light and thus facilitates the detection of light scattered by the disc within a few au from the central star. These images reveal intriguing complex disc structures and diverse morphological features that are possibly caused by ongoing planet formation in the disc. An overview of the recent advances enabled by SPHERE is presented. Title: Investigating circular patterns in linear polarization observations of Venus Authors: Mahapatra, G.; Stam, D. M.; Rossi, L.; Rodenhuis, M.; Snik, F.; Keller, C. U. Bibcode: 2017EPSC...11..885M Altcode: In this work, we report our observations of Venus using the polarized flux. Our observations show certain curious looking concentric rings around the sub-solar point of Venus. We use our radiative transfer model to explain these fluctuations and discuss what the possible explanations might be. Title: Rigorous vector wave propagation for arbitrary flat media Authors: Bos, Steven P.; Haffert, Sebastiaan Y.; Keller, Christoph U. Bibcode: 2017SPIE10407E..09B Altcode: 2018arXiv181109777B Precise modelling of the (off-axis) point spread function (PSF) to identify geometrical and polarization aberrations is important for many optical systems. In order to characterise the PSF of the system in all Stokes parameters, an end-to-end simulation of the system has to be performed in which Maxwell's equations are rigorously solved. We present the first results of a python code that we are developing to perform multiscale end-to-end wave propagation simulations that include all relevant physics. Currently we can handle plane-parallel near- and far-field vector diffraction effects of propagating waves in homogeneous isotropic and anisotropic materials, refraction and reflection of flat parallel surfaces, interference effects in thin films and unpolarized light. We show that the code has a numerical precision on the order of 10-16 for non-absorbing isotropic and anisotropic materials. For absorbing materials the precision is on the order of 10-8. The capabilities of the code are demonstrated by simulating a converging beam reflecting from a flat aluminium mirror at normal incidence. Title: BP Piscium: its flaring disc imaged with SPHERE/ZIMPOL Authors: de Boer, J.; Girard, J. H.; Canovas, H.; Min, M.; Sitko, M.; Ginski, C.; Jeffers, S. V.; Mawet, D.; Milli, J.; Rodenhuis, M.; Snik, F.; Keller, C. U. Bibcode: 2017MNRAS.466L...7D Altcode: 2016arXiv161006609D Whether BP Piscium (BP Psc) is either a pre-main sequence T Tauri star at d ≈ 80 pc, or a post-main sequence G giant at d ≈ 300 pc is still not clear. As a first-ascent giant, it is the first to be observed with a molecular and dust disc. Alternatively, BP Psc would be among the nearest T Tauri stars with a protoplanetary disc (PPD). We investigate whether the disc geometry resembles typical PPDs, by comparing polarimetric images with radiative transfer models. Our Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)/Zurich IMaging Polarimeter (ZIMPOL) observations allow us to perform polarimetric differential imaging, reference star differential imaging, and Richardson-Lucy deconvolution. We present the first visible light polarization and intensity images of the disc of BP Psc. Our deconvolution confirms the disc shape as detected before, mainly showing the southern side of the disc. In polarized intensity the disc is imaged at larger detail and also shows the northern side, giving it the typical shape of high-inclination flared discs. We explain the observed disc features by retrieving the large-scale geometry with MCMAX radiative transfer modelling, which yields a strongly flared model, atypical for discs of T Tauri stars. Title: On-sky Performance Analysis of the Vector Apodizing Phase Plate Coronagraph on MagAO/Clio2 Authors: Otten, Gilles P. P. L.; Snik, Frans; Kenworthy, Matthew A.; Keller, Christoph U.; Males, Jared R.; Morzinski, Katie M.; Close, Laird M.; Codona, Johanan L.; Hinz, Philip M.; Hornburg, Kathryn J.; Brickson, Leandra L.; Escuti, Michael J. Bibcode: 2017ApJ...834..175O Altcode: 2017arXiv170204193O We report on the performance of a vector apodizing phase plate coronagraph that operates over a wavelength range of 2-5 μm and is installed in MagAO/Clio2 at the 6.5 m Magellan Clay telescope at Las Campanas Observatory, Chile. The coronagraph manipulates the phase in the pupil to produce three beams yielding two coronagraphic point-spread functions (PSFs) and one faint leakage PSF. The phase pattern is imposed through the inherently achromatic geometric phase, enabled by liquid crystal technology and polarization techniques. The coronagraphic optic is manufactured using a direct-write technique for precise control of the liquid crystal pattern and multitwist retarders for achromatization. By integrating a linear phase ramp to the coronagraphic phase pattern, two separated coronagraphic PSFs are created with a single pupil-plane optic, which makes it robust and easy to install in existing telescopes. The two coronagraphic PSFs contain a 180° dark hole on each side of a star, and these complementary copies of the star are used to correct the seeing halo close to the star. To characterize the coronagraph, we collected a data set of a bright (mL = 0-1) nearby star with ∼1.5 hr of observing time. By rotating and optimally scaling one PSF and subtracting it from the other PSF, we see a contrast improvement by 1.46 magnitudes at 3.5 λ /D. With regular angular differential imaging at 3.9 μm, the MagAO vector apodizing phase plate coronagraph delivers a 5σ {{Δ }}{mag} contrast of 8.3 (={10}-3.3) at 2 λ /D and 12.2 (={10}-4.8) at 3.5 λ /D. Title: The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments Authors: Wilby, M. J.; Keller, C. U.; Snik, F.; Korkiakoski, V.; Pietrow, A. G. M. Bibcode: 2017A&A...597A.112W Altcode: 2016arXiv161004235W The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental Non-Common Path Errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10-6 to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor suitable for real-time NCPE correction. This combines the Apodizing Phase Plate (APP) coronagraph with a holographic modal wavefront sensor to provide simultaneous coronagraphic imaging and focal-plane wavefront sensing with the science point-spread function. We first characterise the baseline performance of the cMWS via idealised closed-loop simulations, showing that the sensor is able to successfully recover diffraction-limited coronagraph performance over an effective dynamic range of ±2.5 radians root-mean-square (rms) wavefront error within 2-10 iterations, with performance independent of the specific choice of mode basis. We then present the results of initial on-sky testing at the William Herschel Telescope, which demonstrate that the sensor is capable of NCPE sensing under realistic seeing conditions via the recovery of known static aberrations to an accuracy of 10 nm (0.1 radians) rms error in the presence of a dominant atmospheric speckle foreground. We also find that the sensor is capable of real-time measurement of broadband atmospheric wavefront variance (50% bandwidth, 158 nm rms wavefront error) at a cadence of 50 Hz over an uncorrected telescope sub-aperture. When combined with a suitable closed-loop adaptive optics system, the cMWS holds the potential to deliver an improvement of up to two orders of magnitude over the uncorrected QSS floor. Such a sensor would be eminently suitable for the direct imaging and spectroscopy of exoplanets with both existing and future instruments, including EPICS and METIS for the E-ELT. Title: Dusty tails of evaporating exoplanets. II. Physical modelling of the KIC 12557548b light curve Authors: van Lieshout, R.; Min, M.; Dominik, C.; Brogi, M.; de Graaff, T.; Hekker, S.; Kama, M.; Keller, C. U.; Ridden-Harper, A.; van Werkhoven, T. I. M. Bibcode: 2016A&A...596A..32V Altcode: 2016arXiv160900275V Context. Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of dust, which can trail the planet in a comet-like tail. When such objects occult their host star, the resulting transit signal contains information about the dust in the tail.
Aims: We aim to use the detailed shape of the Kepler light curve of KIC 12557548b to constrain the size and composition of the dust grains that make up the tail, as well as the mass loss rate of the planet.
Methods: Using a self-consistent numerical model of the dust dynamics and sublimation, we calculated the shape of the tail by following dust grains from their ejection from the planet to their destruction due to sublimation. From this dust cloud shape, we generated synthetic light curves (incorporating the effects of extinction and angle-dependent scattering), which were then compared with the phase-folded Kepler light curve. We explored the free-parameter space thoroughly using a Markov chain Monte Carlo method.
Results: Our physics-based model is capable of reproducing the observed light curve in detail. Good fits are found for initial grain sizes between 0.2 and 5.6 μm and dust mass loss rates of 0.6 to 15.6 M Gyr-1 (2σ ranges). We find that only certain combinations of material parameters yield the correct tail length. These constraints are consistent with dust made of corundum (Al2O3), but do not agree with a range of carbonaceous, silicate, or iron compositions.
Conclusions: Using a detailed, physically motivated model, it is possible to constrain the composition of the dust in the tails of evaporating rocky exoplanets. This provides a unique opportunity to probe to interior composition of the smallest known exoplanets. Title: Direct detection of scattered light gaps in the transitional disk around HD 97048 with VLT/SPHERE Authors: Ginski, C.; Stolker, T.; Pinilla, P.; Dominik, C.; Boccaletti, A.; de Boer, J.; Benisty, M.; Biller, B.; Feldt, M.; Garufi, A.; Keller, C. U.; Kenworthy, M.; Maire, A. L.; Ménard, F.; Mesa, D.; Milli, J.; Min, M.; Pinte, C.; Quanz, S. P.; van Boekel, R.; Bonnefoy, M.; Chauvin, G.; Desidera, S.; Gratton, R.; Girard, J. H. V.; Keppler, M.; Kopytova, T.; Lagrange, A. -M.; Langlois, M.; Rouan, D.; Vigan, A. Bibcode: 2016A&A...595A.112G Altcode: 2016arXiv160904027G
Aims: We studied the well-known circumstellar disk around the Herbig Ae/Be star HD 97048 with high angular resolution to reveal undetected structures in the disk which may be indicative of disk evolutionary processes such as planet formation.
Methods: We used the IRDIS near-IR subsystem of the extreme adaptive optics imager SPHERE at the ESO/VLT to study the scattered light from the circumstellar disk via high resolution polarimetry and angular differential imaging.
Results: We imaged the disk in unprecedented detail and revealed four ring-like brightness enhancements and corresponding gaps in the scattered light from the disk surface with radii between 39 au and 341 au. We derived the inclination and position angle as well as the height of the scattering surface of the disk from our observational data. We found that the surface height profile can be described by a single power law up to a separation 270 au. Using the surface height profile we measured the scattering phase function of the disk and found that it is consistent with theoretical models of compact dust aggregates. We discuss the origin of the detected features and find that low mass (≤1 MJup) nascent planets are a possible explanation.

Based on data collected at the European Southern Observatory, Chile (ESO Programs 096.C-0248, 096.C-0241, 077.C-0106). Title: Multiple rings in the transition disk and companion candidates around RX J1615.3-3255. High contrast imaging with VLT/SPHERE Authors: de Boer, J.; Salter, G.; Benisty, M.; Vigan, A.; Boccaletti, A.; Pinilla, P.; Ginski, C.; Juhasz, A.; Maire, A. -L.; Messina, S.; Desidera, S.; Cheetham, A.; Girard, J. H.; Wahhaj, Z.; Langlois, M.; Bonnefoy, M.; Beuzit, J. -L.; Buenzli, E.; Chauvin, G.; Dominik, C.; Feldt, M.; Gratton, R.; Hagelberg, J.; Isella, A.; Janson, M.; Keller, C. U.; Lagrange, A. -M.; Lannier, J.; Menard, F.; Mesa, D.; Mouillet, D.; Mugrauer, M.; Peretti, S.; Perrot, C.; Sissa, E.; Snik, F.; Vogt, N.; Zurlo, A.; SPHERE Consortium Bibcode: 2016A&A...595A.114D Altcode: 2016arXiv161004038D Context. The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found.
Aims: We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at the surface of the transition disk around RX J1615.3-3255 (RX J1615).
Methods: We observed RX J1615 with VLT/SPHERE. From these observations, we obtained polarimetric imaging with ZIMPOL (R'-band) and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous spatially resolved spectra with the IFS (YJ).
Results: We image the disk for the first time in scattered light and detect two arcs, two rings, a gap and an inner disk with marginal evidence for an inner cavity. The shapes of the arcs suggest that they are probably segments of full rings. Ellipse fitting for the two rings and inner disk yield a disk inclination I = 47 ± 2° and find semi-major axes of 1.50 ± 0.01'' (278 au), 1.06 ± 0.01'' (196 au) and 0.30 ± 0.01'' (56 au), respectively. We determine the scattering surface height above the midplane, based on the projected ring center offsets. Nine point sources are detected between 2.1'' and 8.0'' separation and considered as companion candidates. With NACO data we recover four of the nine point sources, which we determine to be not co-moving, and therefore unbound to the system.
Conclusions: We present the first detection of the transition disk of RX J1615 in scattered light. The height of the rings indicate limited flaring of the disk surface, which enables partial self-shadowing in the disk. The outermost arc either traces the bottom of the disk or it is another ring with semi-major axis ≳ 2.35'' (435 au). We explore both scenarios, extrapolating the complete shape of the feature, which will allow us to distinguish between the two in future observations. The most attractive scenario, where the arc traces the bottom of the outer ring, requires the disk to be truncated at r ≈ 360 au. If the closest companion candidate is indeed orbiting the disk at 540 au, then it would be the most likely cause for such truncation. This companion candidate, as well as the remaining four, all require follow up observations to determine if they are bound to the system.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 095.C-0298(A), 095.C-0298(B), and 095.C-0693(A) during guaranteed and open time observations of the SPHERE consortium, and on NACO observations: program IDs: 085.C-0012(A), 087.C-0111(A), and 089.C-0133(A). The reduced images as FITS files are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114 Title: The Hera Saturn entry probe mission Authors: Mousis, O.; Atkinson, D. H.; Spilker, T.; Venkatapathy, E.; Poncy, J.; Frampton, R.; Coustenis, A.; Reh, K.; Lebreton, J. -P.; Fletcher, L. N.; Hueso, R.; Amato, M. J.; Colaprete, A.; Ferri, F.; Stam, D.; Wurz, P.; Atreya, S.; Aslam, S.; Banfield, D. J.; Calcutt, S.; Fischer, G.; Holland, A.; Keller, C.; Kessler, E.; Leese, M.; Levacher, P.; Morse, A.; Muñoz, O.; Renard, J. -B.; Sheridan, S.; Schmider, F. -X.; Snik, F.; Waite, J. H.; Bird, M.; Cavalié, T.; Deleuil, M.; Fortney, J.; Gautier, D.; Guillot, T.; Lunine, J. I.; Marty, B.; Nixon, C.; Orton, G. S.; Sánchez-Lavega, A. Bibcode: 2016P&SS..130...80M Altcode: 2015arXiv151007685M The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems. Title: Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e Authors: Ridden-Harper, A. R.; Snellen, I. A. G.; Keller, C. U.; de Kok, R. J.; Di Gloria, E.; Hoeijmakers, H. J.; Brogi, M.; Fridlund, M.; Vermeersen, B. L. A.; van Westrenen, W. Bibcode: 2016A&A...593A.129R Altcode: 2016arXiv160608447R Context. The atmospheric and surface characterization of rocky planets is a key goal of exoplanet science. Unfortunately, the measurements required for this are generally out of reach of present-day instrumentation. However, the planet Mercury in our own solar system exhibits a large exosphere composed of atomic species that have been ejected from the planetary surface by the process of sputtering. Since the hottest rocky exoplanets known so far are more than an order of magnitude closer to their parent star than Mercury is to the Sun, the sputtering process and the resulting exospheres could be orders of magnitude larger and potentially detectable using transmission spectroscopy, indirectly probing their surface compositions.
Aims: The aim of this work is to search for an absorption signal from exospheric sodium (Na) and singly ionized calcium (Ca+) in the optical transmission spectrum of the hot rocky super-Earth 55 Cancri e. Although the current best-fitting models to the planet mass and radius require a possible atmospheric component, uncertainties in the radius exist, making it possible that 55 Cancri e could be a hot rocky planet without an atmosphere.
Methods: High resolution (R ~ 110 000) time-series spectra of five transits of 55 Cancri e, obtained with three different telescopes (UVES/VLT, HARPS/ESO 3.6 m and HARPS-N/TNG) were analysed. Targeting the sodium D lines and the calcium H and K lines, the potential planet exospheric signal was filtered out from the much stronger stellar and telluric signals, making use of the change of the radial component of the orbital velocity of the planet over the transit from -57 to +57 km s-1.
Results: Combining all five transit data sets, we detect a signal potentially associated with sodium in the planet exosphere at a statistical significance level of 3σ. Combining the four HARPS transits that cover the calcium H and K lines, we also find a potential signal from ionized calcium (4.1σ). Interestingly, this latter signal originates from just one of the transit measurements - with a 4.9σ detection at this epoch. Unfortunately, due to the low significance of the measured sodium signal and the potentially variable Ca+ signal, we estimate the p-values of these signals to be too high (corresponding to <4σ) to claim unambiguous exospheric detections. By comparing the observed signals with artificial signals injected early in the analysis, the absorption by Na and Ca+ are estimated to be at a level of ~2.3 × 10-3 and ~7.0 × 10-2 respectively, relative to the stellar spectrum.
Conclusions: If confirmed, the 3σ signal would correspond to an optically thick sodium exosphere with a radius of 5 R, which is comparable to the Roche lobe radius of the planet. The 4.9σ detection of Ca+ in a single HARPS data set would correspond to an optically thick Ca+ exosphere approximately five times larger than the Roche lobe radius. If this were a real detection, it would imply that the exosphere exhibits extreme variability. Although no formal detection has been made, we advocate that probing the exospheres of hot super-Earths in this way has great potential, also knowing that Mercury's exosphere varies significantly over time. It may be a fast route towards the first characterization of the surface properties of this enigmatic class of planets.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 092.C-0178 and 288.C-5010 and the Telescopio Nazionale Galileo under programme CAT13B_33. Title: HERA: an atmospheric probe to unveil the depths of Saturn Authors: Mousis, Olivier; Atkinson, David H.; Amato, Michael; Aslam, Shahid; Atreya, Sushil K.; Blanc, Michel; Bolton, Scott J.; Brugger, Bastien; Calcutt, Simon; Cavalié, Thibault; Charnoz, Sébastien; Coustenis, Athena; DELEUIL, Magali; Ferri, Francesca; Fletcher, Leigh N.; Guillot, Tristan; Hartogh, Paul; Holland, Andrew; Hueso, Ricardo; Keller, Christoph; Kessler, Ernst; Lebreton, Jean-Pierre; leese, Mark; Lellouch, Emmanuel; Levacher, Patrick; Marty, Bernard; Morse, Andrew; Nixon, Conor; Reh, Kim R.; Renard, Jean-Baptiste; Sanchez-Lavega, Agustin; Schmider, François-Xavier; Sheridan, Simon; Simon, Amy A.; Snik, Frans; Spilker, Thomas R.; Stam, Daphne M.; Venkatapathy, Ethiraj; Vernazza, Pierre; Waite, J. Hunter; Wurz, Peter Bibcode: 2016DPS....4812328M Altcode: The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a significant collaboration with NASA. It consists of a Saturn atmospheric probe and a Carrier-Relay spacecraft. Hera will perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets.The primary science objectives will be addressed by an atmospheric entry probe that would descend under parachute and carry out in situ measurements beginning in the stratosphere to help characterize the location and properties of the tropopause, and continue into the troposphere to pressures of at least 10 bars. All of the science objectives, except for the abundance of oxygen, which may be only addressed indirectly via observations of species whose abundances are tied to the abundance of water, can be achieved by reaching 10 bars. As in previous highly successful collaborative efforts between ESA and NASA, the proposed mission has a baseline concept based on a NASA-provided carrier/data relay spacecraft that would deliver the ESA-provided atmospheric probe to the desired atmospheric entry point at Saturn. ESA's proposed contribution should fit well into the M5 Cosmic Vision ESA call cost envelope.A nominal mission configuration would consist of a probe that detaches from the carrier one to several months prior to probe entry. Subsequent to probe release, the carrier trajectory would be deflected to optimize the over-flight phasing of the probe descent location for both probe data relay as well as performing carrier approach and flyby science, and would allow multiple retransmissions of the probe data for redundancy. The Saturn atmospheric entry probe would in many respects resemble the Jupiter Galileo probe. It is anticipated that the probe architecture for this mission would be battery-powered and accommodate a data relay to the carrier for data collection, storage on board the carrier/data relay, for later retransmission to Earth. Title: The Leiden EXoplanet Instrument (LEXI): a high-contrast high-dispersion spectrograph Authors: Haffert, S. Y.; Wilby, M. J.; Keller, C. U.; Snellen, I. A. G. Bibcode: 2016SPIE.9908E..67H Altcode: The Leiden EXoplanet Instrument (LEXI) will be the first instrument designed for high-contrast, high-dispersion integral field spectroscopy at optical wavelengths. High-contrast imaging (HCI) and high-dispersion spectroscopy (HDS) techniques are used to reach contrasts of 10-7. LEXI will be a bench-mounted, high dispersion integral field spectrograph that will record spectra in a small area around the star with high spatial resolution and high dynamic range. A prototype is being setup to The Leiden EXoplanet Instrument (LEXI) will be the first instrument designed for high-contrast, high-dispersion integral field spectroscopy at optical wavelengths. High-contrast imaging (HCI) and high-dispersion spectroscopy (HDS) techniques are used to reach contrasts of 10-7. LEXI will be a bench-mounted, high dispersion integral field spectrograph that will record spectra in a small area around the star with high spatial resolution and high dynamic range. A prototype is being setup to test the combination of HCI+HDS and its first light is expected in 2016. Title: Novel instrument concepts for characterizing directly imaged exoplanets Authors: Keller, Christoph U. Bibcode: 2016SPIE.9908E..9VK Altcode: Current high-contrast exoplanet imagers are optimized to find new exoplanets; they minimize diffracted starlight in a large area around a star. I present four novel instrumental approaches that are optimized to characterize these discoveries by minimizing starlight in a small area around the known location of an exoplanet: 1) coronagraphs that remove virtually all starlight over an octave in wavelength while transmitting more than 90% of the exoplanet signal; 2) holographic wavefront sensors that measure aberrations in the science focal plane; 3) ultra-fast adaptive optics systems that minimize these aberrations; and 4) direct minimization of the remaining starlight. By integrating these technologies with a high spectral- resolution, integral-field spectrograph that can resolve the Doppler shift and the polarization difference between the starlight and the reflected light from the exoplanet, it will be possible to determine the atmospheric composition, temperature and velocity structures of exoplanets and their spin rotation rate and orbital velocity. This will ultimately allow the upcoming extremely large telescopes to characterize rocky exoplanets in the habitable zone to look for signatures of life. Title: End-to-end simulations of the E-ELT/METIS coronagraphs Authors: Carlomagno, Brunella; Absil, Olivier; Kenworthy, Matthew; Ruane, Garreth; Keller, Christoph U.; Otten, Gilles; Feldt, Markus; Hippler, Stefan; Huby, Elsa; Mawet, Dimitri; Delacroix, Christian; Surdej, Jean; Habraken, Serge; Forsberg, Pontus; Karlsson, Mikael; Vargas Catalan, Ernesto; Brandl, Bernhard R. Bibcode: 2016SPIE.9909E..73C Altcode: The direct detection of low-mass planets in the habitable zone of nearby stars is an important science case for future E-ELT instruments such as the mid-infrared imager and spectrograph METIS, which features vortex phase masks and apodizing phase plates (APP) in its baseline design. In this work, we present end-to-end performance simulations, using Fourier propagation, of several METIS coronagraphic modes, including focal-plane vortex phase masks and pupil-plane apodizing phase plates, for the centrally obscured, segmented E-ELT pupil. The atmosphere and the AO contributions are taken into account. Hybrid coronagraphs combining the advantages of vortex phase masks and APPs are considered to improve the METIS coronagraphic performance. Title: A "Fast and Furious'" solution to the low-wind effect for SPHERE at the VLT Authors: Wilby, M. J.; Keller, C. U.; Sauvage, J. -F.; Fusco, T.; Mouillet, D.; Beuzit, J. -L.; Dohlen, K. Bibcode: 2016SPIE.9909E..6CW Altcode: We present a potential non-invasive solution to sensing the so-called low-wind effect (LWE) seen in the SPHERE instrument at the VLT, based on the "Fast and Furious (F&F) sequential phase diversity wavefront reconstruction algorithm. This uses non-coronagraphic focal-plane images available from the near-infra-red Differential Tip-Tilt Sensor (DTTS), with the closed-loop correction cycle itself providing the necessary phase diversity between frames required to reconstruct the full wavefront phase. Crucially, this means F&F does not need to apply large artificial phase probes as required by standard phase diversity algorithms, allowing it to operate in a real-time ( 10 Hz) correction mode without impacting science observations. In this paper we present the results of realistic closed-loop AO simulations designed to emulate SPHERE/DTTS observations of the LWE. With this we demonstrate that the F&F algorithm is capable of effective removal of the characteristic point-spread function (PSF) aberrations of strongly LWE-affected images within a few closed-loop iterations, with the final wavefront quality limited only by the corrective order of the deformable mirror. The ultimate goal of this project is to provide an independent, real-time and focal-plane wavefront sensor for SPHERE which is capable of detecting and directly compensating the LWE as it arises, thus improving coronagraph performance under the best 15-20 % of observing conditions where the effect is most pronounced. Title: Focal-plane electric field sensing with pupil-plane holograms Authors: Por, Emiel H.; Keller, Christoph U. Bibcode: 2016SPIE.9909E..59P Altcode: The direct detection and spectral characterization of exoplanets requires a coronagraph to suppress the diffracted star light. Amplitude and phase aberrations in the optical train fill the dark zone of the coronagraph with quasi-static speckles that limit the achievable contrast. Focal-plane electric field sensing, such as phase diversity introduced by a deformable mirror (DM), is a powerful tool to minimize this residual star light. The residual electric field can be estimated by sequentially applying phase probes on the DM to inject star light with a well-known amplitude and phase into the dark zone and analyzing the resulting intensity images. The DM can then be used to add light with the same amplitude but opposite phase to destructively interfere with this residual star light. Using a static phase-only pupil-plane element we create holographic copies of the point spread function (PSF), each superimposed with a certain pupil-plane phase probe. We therefore obtain all intensity images simultaneously while still retaining a central, unaltered science PSF. The electric field sensing method only makes use of the holographic copies, allowing for correction of the residual electric field while retaining the central PSF for uninterrupted science data collection. In this paper we demonstrate the feasibility of this method with numerical simulations. Title: Designing and testing the coronagraphic Modal Wavefront Sensor: a fast non-common path error sensor for high-contrast imaging Authors: Wilby, M. J.; Keller, C. U.; Haffert, S.; Korkiakoski, V.; Snik, F.; Pietrow, A. G. M. Bibcode: 2016SPIE.9909E..21W Altcode: Non-Common Path Errors (NCPEs) are the dominant factor limiting the performance of current astronomical high-contrast imaging instruments. If uncorrected, the resulting quasi-static speckle noise floor limits coronagraph performance to a raw contrast of typically 10-4, a value which does not improve with increasing integration time. The coronagraphic Modal Wavefront Sensor (cMWS) is a hybrid phase optic which uses holographic PSF copies to supply focal-plane wavefront sensing information directly from the science camera, whilst maintaining a bias-free coronagraphic PSF. This concept has already been successfully implemented on-sky at the William Herschel Telescope (WHT), La Palma, demonstrating both real-time wavefront sensing capability and successful extraction of slowly varying wavefront errors under a dominant and rapidly changing atmospheric speckle foreground. In this work we present an overview of the development of the cMWS and recent first light results obtained using the Leiden EXoplanet Instrument (LEXI), a high-contrast imager and high-dispersion spectrograph pathfinder instrument for the WHT. Title: The ZIMPOL high contrast imaging polarimeter for SPHERE: polarimetric high contrast commissioning results Authors: Roelfsema, Ronald; Bazzon, Andreas; Schmid, Hans Martin; Pragt, Johan; Govaert, Alain; Gisler, Daniel; Dominik, Carsten; Baruffolo, Andrea; Beuzit, Jean-Luc; Costille, Anne; Dohlen, Kjetil; Downing, Mark; Elswijk, Eddy; de Haan, Menno; Hubin, Norbert; Kasper, Markus; Keller, Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov, Alexey; Puget, Pascal; Salasnich, Bernardo; Sauvage, Jean-Francois; Wildi, Francois Bibcode: 2016SPIE.9909E..27R Altcode: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second generation VLT instrument aimed at the direct detection of exo-planets. It has received its first light in May 2014. ZIMPOL (Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of the SPHERE instrument. It's capable of both high accuracy and high sensitivity polarimetry but can also be used as a classical imager. It is located behind an extreme AO system and a stellar coronagraph. ZIMPOL operates at visible wavelengths which is best suited to detect the very faint reflected and hence polarized visible light from extra solar planets. During the SPHERE fourth commissioning period (October 2014) we have made deep coronagraphic observations of the bright star alpha Gru (mR = 1.75) to assess the high contrast polarimetric performance of SPHERE-ZIMPOL. We have integrated on the target for a total time of about 45 minutes during the meridian transit in the Very Broad Band filter (600 - 900 nm) with a classical Lyot coronagraph with 3 λ/D radius focal mask. We reduce the data by a combination of Polarized Background subtraction, Polarimetric Differential Imaging (PDI) and Angular Differential Imaging (ADI). We reach contrasts of 10-6 and 10-7 at a radial distances of respectively 7 and 14 lambda/D from the PSF core. At these radial distances we are respectively a factor of 10 and 2 above the photon noise limit. We discuss our results by considering the temporal and spatial speckle behavior close to the PSF core in combination with low order polarimetric aberrations. Title: Instrumemtation Authors: Keller, Christoph U.; Snik, Frans; Harrington, David M.; Packham, Chris Bibcode: 2015psps.book...35K Altcode: No abstract at ADS Title: Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors Authors: Snellen, I.; de Kok, R.; Birkby, J. L.; Brandl, B.; Brogi, M.; Keller, C.; Kenworthy, M.; Schwarz, H.; Stuik, R. Bibcode: 2015A&A...576A..59S Altcode: 2015arXiv150301136S Context. Ground-based high-dispersion (R ~ 100 000) spectroscopy (HDS) is proving to be a powerful technique with which to characterize extrasolar planets. The planet signal is distilled from the bright starlight, combining ral and time-differential filtering techniques. In parallel, high-contrast imaging (HCI) is developing rapidly, aimed at spatially separating the planet from the star. While HDS is limited by the overwhelming noise from the host star, HCI is limited by residual quasi-static speckles. Both techniques currently reach planet-star contrast limits down to ~10-5, albeit for very different types of planetary systems.
Aims: In this work, we discuss a way to combine HDS and HCI (HDS+HCI). For a planet located at a resolvable angular distance from its host star, the starlight can be reduced up to several orders of magnitude using adaptive optics and/or coronography. In addition, the remaining starlight can be filtered out using high-dispersion spectroscopy, utilizing the significantly different (or Doppler shifted) high-dispersion spectra of the planet and star. In this way, HDS+HCI can in principle reach contrast limits of ~10-5 × 10-5, although in practice this will be limited by photon noise and/or sky-background. In contrast to current direct imaging techniques, such as Angular Differential Imaging and Spectral Differential Imaging, it will work well at small working angles and is much less sensitive to speckle noise. For the discovery of previously unknown planets HDS+HCI requires a high-contrast adaptive optics system combined with a high-dispersion R ~ 100 000 integral field spectrograph (IFS). This combination currently does not exist, but is planned for the European Extremely Large Telescope.
Methods: We present simulations of HDS+HCI observations with the E-ELT, both probing thermal emission from a planet at infrared wavelengths, and starlight reflected off a planet atmosphere at optical wavelengths. For the infrared simulations we use the baseline parameters of the E-ELT and METIS instrument, with the latter combining extreme adaptive optics with an R = 100 000 IFS. We include realistic models of the adaptive optics performance and atmospheric transmission and emission. For the optical simulation we also assume R = 100 000 IFS with adaptive optics capabilities at the E-ELT.
Results: One night of HDS+HCI observations with the E-ELT at 4.8 μm (Δλ = 0.07 μm) can detect a planet orbiting α Cen A with a radius of R = 1.5 Rearth and a twin-Earth thermal spectrum of Teq = 300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected light from an Earth-size planet in the habitable zone of Proxima Centauri can be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI observations have shown the potential of this technique by determining the spin-rotation of the young massive exoplanet β Pictoris b.
Conclusions: The exploration of the planetary systems of our neighbor stars is of great scientific and philosophical value. The HDS+HCI technique has the potential to detect and characterize temperate rocky planets in their habitable zones. Exoplanet scientists should not shy away from claiming a significant fraction of the future ELTs to make such observations possible. Title: Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049 Authors: Kochukhov, O.; Rusomarov, N.; Valenti, J. A.; Stempels, H. C.; Snik, F.; Rodenhuis, M.; Piskunov, N.; Makaganiuk, V.; Keller, C. U.; Johns-Krull, C. M. Bibcode: 2015A&A...574A..79K Altcode: 2014arXiv1411.7518K Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation.
Aims: Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of ~30 kG, one of the strongest known for any non-degenerate star.
Methods: We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field.
Results: Based on high-precision measurements of the mean magnetic field modulus, we refined the rotational period of HD 75049 to Prot = 4.048267 ± 0.000036 d. We also derived basic stellar parameters, Teff = 10 250 ± 250 K and log g = 4.3 ± 0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Of the chemical elements, Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole.
Conclusions: The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediate-mass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths.

Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296, 086.D-0240, 088.D-0066, 090.D-0256, 078.D-0192, 080.D-0170). Title: Surprising detection of an equatorial dust lane on the AGB star IRC+10216 Authors: Jeffers, S. V.; Min, M.; Waters, L. B. F. M.; Canovas, H.; Pols, O. R.; Rodenhuis, M.; de Juan Ovelar, M.; Keller, C. U.; Decin, L. Bibcode: 2014A&A...572A...3J Altcode: 2014arXiv1412.5063J
Aims: Understanding the formation of planetary nebulae remains elusive because in the preceding asymptotic giant branch (AGB) phase these stars are heavily enshrouded in an optically thick dusty envelope.
Methods: To further understand the morphology of the circumstellar environments of AGB stars we observe the closest carbon-rich AGB star IRC+10216 in scattered light.
Results: When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density enhancement, in contrast to the large-scale spherical rings that have been imaged much further out. We use radiative transfer models to interpret this structure in terms of two models: firstly, an equatorial density enhancement, commonly observed in the more evolved post-AGB stars, and secondly, in terms of a dust rings model, where a local enhancement of mass-loss creates a spiral ring as the star rotates.
Conclusions: We conclude that both models can be used to reproduce the dark lane in the scattered light images, which is caused by an equatorially density enhancement formed by dense dust rather than a bipolar outflow as previously thought. We are unable to place constraints on the formation of the equatorial density enhancement by a binary system.

Final reduced images (FITS) are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A3Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Title: Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters Authors: Snik, Frans; Rietjens, Jeroen H. H.; Apituley, Arnoud; Volten, Hester; Mijling, Bas; Di Noia, Antonio; Heikamp, Stephanie; Heinsbroek, Ritse C.; Hasekamp, Otto P.; Smit, J. Martijn; Vonk, Jan; Stam, Daphne M.; Harten, Gerard; Boer, Jozua; Keller, Christoph U. Bibcode: 2014GeoRL..41.7351S Altcode: To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low-cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud-free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground-based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts. Title: Drift scanning technique for mid-infrared background subtraction Authors: Heikamp, Stephanie; Brandl, Bernhard R.; Keller, Christoph U.; Venema, Lars; Pantin, Eric; Siebenmorgen, Ralf; Ives, Derek; Kerber, Florian Bibcode: 2014SPIE.9147E..9TH Altcode: Accurate calibration of ground-based, mid-infrared observations is challenging due to the strong and rapidly varying thermal background emission. The classical solution is the chopping/nodding technique where the secondary mirror and the telescope are being moved by several tens of arcseconds on the sky. However, chopping is generally inefficient and limited in accuracy and frequency by the mass and size of the secondary mirror. A more elegant solution is a drift scan where the telescope slowly drifts across or around the region of interest; the source moves on the detector by at least one FWHM of the PSF within the time over which the detector performance and the background emission can be considered stable. The final image of a drift scan is mathematically reconstructed from a series of adjacent short exposures. The drift scan approach has recently received a lot of interest, mainly for two reasons: first, some of the new, large-format mid-IR Si:As detectors (AQUARIUS) suffer from excess low frequency noise (ELFN). To reach the nominal performance limit of the detectors, chopping would have to be performed at a high frequency, faster than what most telescopes can handle; second, the next generation of extremely large telescopes will not offer chopping/nodding, and alternative methods need to be developed and tested. In this paper we present the results from simulated drift scan data. We use drift scanning to simultaneously obtain an accurate detector flat field and the sky background. The results are relevant for the future operation and calibration of VISIR at the VLT as well as for METIS, the thermal infrared instrument for the E-ELT. Title: Focal-plane wavefront sensing with high-order adaptive optics systems Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek; Kenworthy, Matthew; Otten, Gilles; Verhaegen, Michel Bibcode: 2014SPIE.9148E..5DK Altcode: 2014arXiv1407.5846K We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working with an extremely high resolution (larger than 150x150 correcting elements). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. Different approaches are considered in numerical simulations, and laboratory experiments are shown to confirm the results. We compare the performances of the standard Gerchberg-Saxton algorithm, Fast and Furious (use of small-phase assumption to take advantage of linearisation) and recently proposed phase-retrieval methods based on convex optimisation. The results indicate that the calibration task is easiest with algorithms similar to Fast and Furious, at least in the framework we considered. Title: Fast & Furious focal-plane wavefront sensing Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek; Kenworthy, Matthew; Otten, Gilles; Verhaegen, Michel Bibcode: 2014ApOpt..53.4565K Altcode: 2014arXiv1406.1006K We present two complementary algorithms suitable for using focal-plane measurements to control a wavefront corrector with an extremely high spatial resolution. The algorithms use linear approximations to iteratively minimize the aberrations seen by the focal-plane camera. The first algorithm, Fast & Furious (FF), uses a weak-aberration assumption and pupil symmetries to achieve fast wavefront reconstruction. The second algorithm, an extension to FF, can deal with an arbitrary pupil shape; it uses a Gerchberg-Saxton style error reduction to determine the pupil amplitudes. Simulations and experimental results are shown for a spatial light modulator controlling the wavefront with a resolution of 170 x 170 pixels. The algorithms increase the Strehl ratio from ~0.75 to 0.98-0.99, and the intensity of the scattered light is reduced throughout the whole recorded image of 320 x 320 pixels. The remaining wavefront rms error is estimated to be ~0.15 rad with FF and ~0.10 rad with FF-GS. Title: The ZIMPOL high contrast imaging polarimeter for SPHERE: system test results Authors: Roelfsema, Ronald; Bazzon, Andreas; Schmid, Hans Martin; Pragt, Johan; Gisler, Daniel; Dominik, Carsten; Baruffolo, Andrea; Beuzit, Jean-Luc; Costille, Anne; Dohlen, Kjetil; Downing, Mark; Elswijk, Eddy; de Haan, Menno; Hubin, Norbert; Kasper, Markus; Keller, Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov, Alexey; Puget, Pascal; Salasnich, Bernardo; Sauvage, Jean-Francois; Wildi, Francois Bibcode: 2014SPIE.9147E..3WR Altcode: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a new instrument for the VLT aimed at the direct detection of exo-planets. It has received its first light in May 2014. ZIMPOL (Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of the SPHERE instrument. It's capable of both high accuracy and high sensitivity polarimetry but can also be used as a classical imager. It is located behind an extreme AO system and a stellar coronagraph. ZIMPOL operates at visible wavelengths (600-900 nm) which is best suited to detect the very faint reflected and hence polarized visible light from extra solar planets. It has an instantaneous Field of View of 3 x 3 arcsec2 (extendable to 8 arcsec diameter) with an angular resolution of 14 mili-arcsec. We discuss the results that are obtained from the full SPHERE-ZIMPOL system testing. In particular the optical, polarimetric and high contrast performance. Title: Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo Authors: de Boer, Jozua; Girard, Julien H.; Mawet, Dimitri; Snik, Frans; Keller, Christoph U.; Milli, Julien Bibcode: 2014SPIE.9147E..87D Altcode: We propose a new calibration scheme to determine the instrumental polarization (IP) and crosstalk induced by either the telescope or an instrument at Nasmyth focus. We measure the polarized blue sky at zenith with VLT/UT4/NaCo for different NaCo derotator and telescope azimuth angles. Taking multiple measurements after rotating both the instrument and the telescope with angles of 90° allows use to determine the IP and most crosstalk components separately for the telescope and the instrument. This separation of the Mueller matrices of UT4 and the NaCo is especially important for measurements taken in the conventional polarimetric mode (field stabilized), because the rotation of the instrument with respect to M3 causes a variation in the IP and crosstalk throughout the measurement. The technique allows us to determine the IP with an accuracy of 0.4%, and constrain or determine lower or upper limits for most crosstalk components. Most notably, the UT4 U --> V crosstalk is substantially larger than theory predicts. An angular offset in NaCo's half wave plate orientation is a possible source of systematic errors. We measure this offset to be 1.8° +/- 0.5°. Title: LOUPE: Spectropolarimetry of the Earth from the surface of the Moon Authors: Hoeijmakers, H. J.; Snik, F.; Stam, D. M.; Keller, C. U. Bibcode: 2014EPSC....9..574H Altcode: We present our prototype for the LOUPE instrument: A small and robust imaging spectropolarimeter that can observe the Earth from the surface of the moon, with as primary objective to characterize the Earth's linear polarization spectrum throughout the Earth's daily rotation and monthly phase angle changes. The purpose of LOUPE is to provide benchmark data for future polarization observations of possibly habitable exoplanets. Our instrument concept has been proven to work in a laboratory setting, and efforts are being made to design and produce a flight model. Title: Towards Polarimetric Exoplanet Imaging with ELTs Authors: Keller, C. U.; Korkiakoski, V.; Rodenhuis, M.; Snik, F. Bibcode: 2014ebi..conf..4.6K Altcode: A prime science goal of Extremely Large Telescopes (ELTs) is the detection and characterization of exoplanets to answer the question: are we alone? ELTs will obtain the first direct images of rocky exoplanets in the habitable zone and search for atmospheric biomarkers. However, the required instrumental technologies are not yet at a level where an instrument could be built that would achieve this goal. Polarimetry will be an important ingredient in future high-contrast instruments as it will provide a major contrast improvement for planets located within the first two Airy rings and offers unique diagnostic capabilities for liquid water (ocean glint, water clouds and their rainbows), hazes and dust in exoplanetary atmospheres.

We will describe novel instrumental approaches to improving subsystems, in particular polarimetry, wavefront sensing and adaptive optics control. To reach contrasts of 10-9 and beyond to image rocky exoplanets from the ground, a series of individually optimized subsystems cannot succeed; rather, entire combinations of subsystems must be optimized together. We will describe our efforts at measuring and controlling wavefronts with 40'000 degrees of freedom, reaching the photon-noise limit in high-contrast polarimetric imaging at telescopes and our plans to reach a contrast of at least 10-9 in broadband light under realistic, simulated ground-based conditions in the laboratory and to test new approaches at telescopes, in particular achromatic aperture and focal-plane coronagraphs, focal-plane wavefront-sensing and speckle suppression, integral-field polarimetry and high-contrast data reduction algorithms. Title: Instrumental polarisation at the Nasmyth focus of the E-ELT Authors: de Juan Ovelar, M.; Snik, F.; Keller, C. U.; Venema, L. Bibcode: 2014A&A...562A...8D Altcode: 2013arXiv1312.6148D; 2013arXiv1312.6148O The ~39-m European Extremely Large Telescope (E-ELT) will be the largest telescope ever built. This makes it particularly suitable for sensitive polarimetric observations, as polarimetry is a photon-starved technique. However, the telescope mirrors may severely limit the polarimetric accuracy of instruments on the Nasmyth platforms by creating instrumental polarisation and/or modifying the polarisation signal of the object. In this paper we characterise the polarisation effects of the two currently considered designs for the E-ELT Nasmyth ports as well as the effect of ageing of the mirrors. By means of the Mueller matrix formalism, we compute the response matrices of each mirror arrangement for a range of zenith angles and wavelengths. We then present two techniques to correct for these effects that require the addition of a modulating device at the "polarisation-free" intermediate focus that acts either as a switch or as a part of a two-stage modulator. We find that the values of instrumental polarisation, Stokes transmission reduction and cross-talk vary significantly with wavelength, and with pointing, for the lateral Nasmyth case, often exceeding the accuracy requirements for proposed polarimetric instruments. Realistic ageing effects of the mirrors after perfect calibration of these effects may cause polarimetric errors beyond the requirements. We show that the modulation approach with a polarimetric element located in the intermediate focus reduces the instrumental polarisation effects down to tolerable values, or even removes them altogether. The E-ELT will be suitable for sensitive and accurate polarimetry, provided frequent calibrations are carried out, or a dedicated polarimetric element is installed at the intermediate focus. Title: Analysis and interpretation of 15 quarters of Kepler data of the disintegrating planet KIC 12557548 b Authors: van Werkhoven, T. I. M.; Brogi, M.; Snellen, I. A. G.; Keller, C. U. Bibcode: 2014A&A...561A...3V Altcode: 2013arXiv1311.5688V Context. The Kepler object KIC 12557548 shows irregular eclipsing behaviour with a constant 15.685 h period, but strongly varying transit depth. The object responsible for this is believed to be a disintegrating planet forming a trailing dust cloud transiting the star. A 1D model of an exponentially decaying dust tail was found to reproduce the average eclipse in intricate detail. Based on radiative hydrodynamic modelling, the upper limit for the planet mass was found to be twice the mass of the Moon.
Aims: In this paper we fit individual eclipses, in addition to fitting binned light curves, to learn more about the process underlying the eclipse depth variation. Additionally, we put forward observational constraints that any model of this planet-star system will have to match.
Methods: We manually de-correlated and de-trended 15 quarters of Kepler data, three of which were observed in short cadence mode. We determined the transit depth, egress depth, and stellar intensity for each orbit and search for dependencies between these parameters. We investigated the full orbit by comparing the flux distribution of a moving phase window of interest versus the out-of-eclipse flux distribution. We fit short cadence data on a per-orbit basis using a two-parameter tail model, allowing us to investigate potential dust tail property variations.
Results: We find two quiescent spells of ~30 orbital periods each where the transit depth is <0.1%, followed by relatively deep transits. Additionally, we find periods of on-off behaviour where >0.5% deep transits are followed by apparently no transit at all. Apart from these isolated events we find neither significant correlation between consecutive transit depths nor a correlation between transit depth and stellar intensity. We find a three-sigma upper limit for the secondary eclipse of 4.9 × 10-5, consistent with a planet candidate with a radius of less than 4600 km. Using the short cadence data we find that a 1D exponential dust tail model is insufficient to explain the data. We improved our model to a 2D, two-component dust model with an opaque core and an exponential tail. Using this model we fit individual eclipses observed in short cadence mode. We find an improved fit of the data, quantifying earlier suggestions by Budaj (2013, A&A, 557, A72) of the necessity of at least two components. We find that deep transits have most absorption in the tail, and not in a disk-shaped, opaque coma, but the transit depth and the total absorption show no correlation with the tail length. Title: Imaging the circumstellar environment of the young T Tauri star SU Aurigae Authors: Jeffers, S. V.; Min, M.; Canovas, H.; Rodenhuis, M.; Keller, C. U. Bibcode: 2014A&A...561A..23J Altcode: 2013arXiv1311.4832J The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. One method to overcome this is by using imaging polarimetry where scattered and consequently polarised starlight from the star's circumstellar disk can be separated from the unpolarised light of the central star. We present images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The images directly show that the disk extends out to 500 AU with an inclination angle of ~50°. Using interpretive models, we derived very small grains in the surface layers of its disk, with a very steep size- and surface-density distribution. Additionally, we resolved a large and extended nebulosity in our images that is most likely a remnant of the prenatal molecular cloud. The position angle of the disk, determined directly from our images, rules out a polar outflow or jet as the cause of this large-scale nebulosity. Title: Calibrating a high-resolution wavefront corrector with a static focal-plane camera Authors: Korkiakoski, Visa; Doelman, Niek; Codona, Johanan; Kenworthy, Matthew; Otten, Gilles; Keller, Christoph U. Bibcode: 2013ApOpt..52.7554K Altcode: 2013arXiv1310.1241K We present a method to calibrate a high-resolution wavefront-correcting device with a single, static camera, located in the focal plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions (dOTF) to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane wavefront sensing algorithm controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate that the locations of identical wavefront corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter. Title: Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parameters Authors: Rusomarov, N.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Makaganiuk, V.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2013A&A...558A...8R Altcode: 2013arXiv1306.0997R Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter.
Aims: The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase.
Methods: HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300-600 and resolving power exceeding 105. The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements.
Results: We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, ⟨Bz⟩, with an accuracy of 5-10 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous ⟨Bz⟩ measurements with our data allowed us to determine an improved rotational period of the star, Prot = 12.45812 ± 0.00019 d. We also measured the longitudinal magnetic field from the cores of Hα and Hβ lines. The analysis of ⟨Bz⟩ measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our ⟨Bz⟩ and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the Hα core and related it to a non-uniform surface distribution of rare earth elements.

Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296, 086.D-0240).Figure 3 and Appendix A are available in electronic form at http://www.aanda.org Title: The color dependent morphology of the post-AGB star HD 161796 Authors: Min, M.; Jeffers, S. V.; Canovas, H.; Rodenhuis, M.; Keller, C. U.; Waters, L. B. F. M. Bibcode: 2013A&A...554A..15M Altcode: 2013arXiv1303.1704M Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shells, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796.
Aims: We aim to detect signatures of an aspherical outflow, and to derive its properties.
Methods: We used the imaging polarimeter the Extreme Polarimeter (ExPo), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light that comes from circumstellar material from the bright, unpolarized, light from the central star.
Results: The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUst-Prominent Longitudinally-EXtended (DUPLEX) at short wavelengths to star-obvious low-level-elongated (SOLE) at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is a consequence of circumstellar rather than interstellar extinction.

Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsicaŋsica de Canarias. Title: Are there tangled magnetic fields on HgMn stars? Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2013A&A...554A..61K Altcode: 2013arXiv1304.6717K Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects.
Aims: We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections of weak longitudinal magnetic fields in HgMn stars based on the moment technique.
Methods: Our spectrum synthesis code calculated the Zeeman broadening of metal lines in HARPS spectra, assuming an unstructured, turbulent magnetic field. A simple line formation model with a homogeneous radial field distribution was applied to assess compatibility between previous longitudinal field measurements and the observed mean circular polarization signatures.
Results: Our analysis of the Zeeman broadening of magnetically sensitive spectral lines reveals no evidence of tangled magnetic fields in any of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the mean magnetic field modulus - much smaller than the field strengths implied by studies based on differential magnetic line intensification and quadratic field diagnostics. The new HARPSpol longitudinal field measurements for the extreme HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD 11753 shows that the recent moment technique measurements retrieved from the same data are incompatible with the lack of circular polarization signatures in the spectrum of this star.
Conclusions: We conclude that there is no evidence for substantial tangled magnetic fields on the surfaces of studied HgMn stars. We cannot independently confirm the presence of very strong quadratic or marginal longitudinal fields for these stars, so results from the moment technique are likely to be spurious.

Based on observations collected at the European Southern Observatory, Chile (ESO programmes 084.D-0338, 085.D-0296, 086.D-0240). Title: Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae Authors: Johns-Krull, Christopher M.; Chen, Wei; Valenti, Jeff A.; Jeffers, Sandra V.; Piskunov, Nikolai E.; Kochukhov, Oleg; Makaganiuk, V.; Stempels, H. C.; Snik, Frans; Keller, Christoph; Rodenhuis, M. Bibcode: 2013ApJ...765...11J Altcode: 2013arXiv1301.3182J We present high spectral resolution (R ≈ 108, 000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 Å and 6678 Å. The He I lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large-scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two He I emission lines in both stars. We observe a maximum implied field strength of 6.05 ± 0.24 kG in the 5876 Å line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative to the formation region of the narrow component of the He I lines. Title: Sterrekundig Instituut Utrecht: The Last Years Authors: Keller, C. U. Bibcode: 2013ASPC..470....3K Altcode: 2012arXiv1208.4095K I describe the last years of the 370-year long life of the Sterrekundig Instituut Utrecht, which was the second-oldest university observatory in the world and was closed in early 2012 after the Faculty of Science and the Board of Utrecht University decided, without providing qualitative or quantitative arguments, to remove astrophysics from its research and education portfolio. Title: Observing Circumstellar Neighbourhoods with the Extreme Polarimeter Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Min, M.; Keller, C. U. Bibcode: 2013ASPC..470..407R Altcode: The study of circumstellar environments at ever higher contrasts has generated considerable interest in recent years. One method to increase the contrast is to observe the linearly polarized light scattered by the circumstellar material while suppressing the unpolarized stellar flux. This paper presents some sample imaging polarimetry results obtained with the Extreme Polarimeter (ExPo). ExPo operates in the visible part of the spectrum, and currently achieves a polarimetric sensitivity of 10-4. Despite the demise of the Utrecht Astronomical Institute, where this instrument was developed, the instrument is still being used and upgraded. It has now moved to the Leiden Observatory. Title: HARPS Spectropolarimetry of the Classical T Tauri Stars GQ Lup and TW Hya Authors: Johns-Krull, Christopher M.; Chen, W.; Valenti, J. A.; Jeffers, S. V.; Piskunov, N. E.; Kochukhov, O.; Makaganiuk, V.; Stempels, H. C.; Snik, F.; Keller, C.; Rodenhuis, M. Bibcode: 2013AAS...22125614J Altcode: We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 A and 6678 A. The He I lines in both these CTTS contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large scale magnetospheric accretion flow. We detect strong polarization in the narrow component of both the He I emission lines in both stars. We observe a maximum implied field strength of 5.8 +/- 0.3 kG in the 5876 A line of GQ Lup, the highest field strength measured to date in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, unlike what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative the formation region of the narrow component of the He I lines. Title: Astronomical Polarimetry: Polarized Views of Stars and Planets Authors: Snik, Frans; Keller, Christoph U. Bibcode: 2013pss2.book..175S Altcode: Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often limit the polarimetric performance. Title: Observing the Earth as an exoplanet with LOUPE, the lunar observatory for unresolved polarimetry of Earth Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Bagnulo, S.; Sparks, W. B.; Keller, C. U. Bibcode: 2012P&SS...74..202K Altcode: 2012arXiv1203.0209K The detections of small, rocky exoplanets have surged in recent years and will likely continue to do so. To know whether a rocky exoplanet is habitable, we have to characterize its atmosphere and surface. A promising characterization method for rocky exoplanets is direct detection using spectropolarimetry. This method will be based on single pixel signals, because spatially resolving exoplanets is impossible with current and near-future instruments. Well-tested retrieval algorithms are essential to interpret these single pixel signals in terms of atmospheric composition, cloud and surface coverage. Observations of Earth itself provide the obvious benchmark data for testing such algorithms. The observations should provide signals that are integrated over the Earth's disk, that capture day and night variations, and all phase angles. The Moon is a unique platform from where the Earth can be observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust spectropolarimeter to observe our Earth as an exoplanet. Title: Electric Properties of Water Ice doped with Hydrogen Peroxide (H2O2): Implications for Icy Moons such as Europa Authors: Keller, C.; Freund, F. T.; Cruikshank, D. P. Bibcode: 2012AGUFM.P51A2026K Altcode: Large floats of ice on Jupiter's moon Europa drift and collide. The float boundaries are marked by brownish-reddish colors. The origin of these colors is poorly understood. Maybe upwelling of water along the active float boundaries brings finely divided suspended matter or organic compounds from the ocean below to the surface, where the intense, high energy environment in Jupiter's radiation belt would lead to photochemical oxidation. At the same time it has been suggested that Europa's ice contains traces of H2O2, presumably due to micro-meteorite impacts and other processes. We measured the electric currents generated in pure and H2O2-doped water ice when we subjected one end of ice blocks to uniaxial stress. Ice samples with 0%, 0.3% and 0.03% H2O2 were formed in polyethylene troughs, 4.1 x 13.5 x 3.8 cm, with Cu contacts at both ends, at 263K (-10°C), 190K (-78°C, dry ice) and 77K (-196°C,liquid N2). At 77K the ice samples detached themselves from at least one of the Cu contacts, due to thermal contraction. At 190K, when stressing one end, essentially no currents were produced in the pure water ice. By contrast, H2O2-doped ices produced several hundred picoamperes (pA) of positive currents, indicating defect electrons (holes) flowing down the stress gradient. At 263K the results are ambiguous. These (as yet preliminary) results indicate that stresses might break the peroxy bonds of imbedded H2O2 molecules, releasing the same type of positive hole charge carriers as observed during stress experiments with silicate rocks. Since positive holes are defect electrons associated with O 2sp levels at the upper edge of the valence band, they seem to have the capability to spread through the ices. Chemically positive holes are equivalent to highly oxidizing oxygen radicals. They may be responsible for oxidation reactions along the boundaries of active ice floats on Europa. Title: Unusual Stokes V profiles during flaring activity of a delta sunspot Authors: Fischer, C. E.; Keller, C. U.; Snik, F.; Fletcher, L.; Socas-Navarro, H. Bibcode: 2012A&A...547A..34F Altcode: 2012arXiv1209.0983F
Aims: We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808. The region was recorded with the Vector-Spectromagnetograph of the Synoptic Optical Long-term Investigations of the Sun facility. The active region produced several successive X-class flares between 19:00 UT and 24:00 UT on September 13, 2005 and we aim to quantify transient and permanent changes in the magnetic field and velocity field during one of the flares, which has been fully captured.
Methods: The Stokes profiles were inverted using the height-dependent inversion code LILIA to analyze magnetic field vector changes at the flaring site. We report multilobed asymmetric Stokes V profiles found in the δ-sunspot umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere consisting of two components (SIR inversions) to interpret the profile shape. The results are put in context with Michelson Doppler Imager (MDI) magnetograms and reconstructed X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager.
Results: We obtain the magnetic field vector and find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line at the flaring site. At two locations in the umbra we encounter strong fields (~3 kG), as inferred from the Stokes I profiles, which, however, exhibit a low polarization signal. During the flare we observe in addition asymmetric Stokes V profiles at one of these sites. The asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong apparent polarity reversal observed in MDI-magnetograms and a chromospheric hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes profiles result in line-of-sight velocity differences in the range of ~12 km s-1 to 14 km s-1 between the two components in the photosphere. Another possibility is that local atmospheric heating is causing the observed asymmetric Stokes V profile shape. In either case our analysis shows that a very localized patch of ~5″ in the photospheric umbra, co-spatial with a flare footpoint, exhibits a subresolution fine structure. Title: SPICES: spectro-polarimetric imaging and characterization of exoplanetary systems. From planetary disks to nearby Super Earths Authors: Boccaletti, Anthony; Schneider, Jean; Traub, Wes; Lagage, Pierre-Olivier; Stam, Daphne; Gratton, Raffaele; Trauger, John; Cahoy, Kerri; Snik, Frans; Baudoz, Pierre; Galicher, Raphael; Reess, Jean-Michel; Mawet, Dimitri; Augereau, Jean-Charles; Patience, Jenny; Kuchner, Marc; Wyatt, Mark; Pantin, Eric; Maire, Anne-Lise; Vérinaud, Christophe; Ronayette, Samuel; Dubreuil, Didier; Min, Michiel; Rodenhuis, Michiel; Mesa, Dino; Belikov, Russ; Guyon, Olivier; Tamura, Motohide; Murakami, Naoshi; Beerer, Ingrid Mary; SPICES Team; Mas, M.; Rouan, D.; Perrin, G.; Lacour, S.; Thébault, P.; Nguyen, N.; Ibgui, L.; Arenou, F.; Lestrade, J. F.; N'Diaye, M.; Dohlen, K.; Ferrari, M.; Hugot, E.; Beuzit, J. -L.; Lagrange, A. -M.; Martinez, P.; Barthelemey, M.; Mugnier, L.; Keller, C.; Marley, M.; Kalas, P.; Stapelfeldt, K.; Brown, R.; Kane, S.; Desidera, S.; Sozzetti, A.; Mura, A.; Martin, E. L.; Bouy, H.; Allan, A.; King, R.; Vigan, A.; Churcher, L.; Udry, S.; Matsuo, T.; Nishikawa, J.; Hanot, C.; Wolf, S.; Kaltenegger, L.; Klahr, H.; Pilat-Lohinger, E. Bibcode: 2012ExA....34..355B Altcode: 2012arXiv1203.0507B; 2012ExA...tmp....8B SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets and circumstellar disks in the visible (450-900 nm) at a spectral resolution of about 40 using both spectroscopy and polarimetry. By 2020/2022, present and near-term instruments will have found several tens of planets that SPICES will be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES can preferentially access exoplanets located at several AUs (0.5-10 AU) from nearby stars (<25 pc) with masses ranging from a few Jupiter masses to Super Earths (∼2 Earth radii, ∼10 M) as well as circumstellar disks as faint as a few times the zodiacal light in the Solar System. Title: A spectro-polarimetric integral field spectrograph for EPICS-EPOL Authors: Rodenhuis, M.; Sprenger, B.; Keller, C. U. Bibcode: 2012SPIE.8446E..9JR Altcode: Imaging polarimetry offers a way to increase the contrast of light scattered from circumstellar material, enabling direct observation of exoplanets -possibly rocky- with the E-ELT. To actually characterize these planets, some spectral resolution is essential. With sufficient resolution -both spectral and spatial- the spectral differential imaging technique can be used in addition to the polarimetry to detect circumstellar point sources. We present the concept for a spectro-polarimetric integral field spectrograph for the EPICS-EPOL instrument and our current efforts to demonstrate this concept with our existing imaging polarimeter ExPo. Title: Evidence for the disintegration of KIC 12557548 b Authors: Brogi, M.; Keller, C. U.; de Juan Ovelar, M.; Kenworthy, M. A.; de Kok, R. J.; Min, M.; Snellen, I. A. G. Bibcode: 2012A&A...545L...5B Altcode: 2012arXiv1208.2988B Context. The Kepler object KIC 12557548 b is peculiar. It exhibits transit-like features every 15.7 h that vary in depth between 0.2% and 1.2%. Rappaport et al. (2012, ApJ, 752, 1) explain the observations in terms of a disintegrating, rocky planet that has a trailing cloud of dust created and constantly replenished by thermal surface erosion. The variability of the transit depth is then a consequence of changes in the cloud optical depth.
Aims: We aim to validate the disintegrating-planet scenario by modeling the detailed shape of the observed light curve, and thereby constrain the cloud particle properties to better understand the nature of this intriguing object.
Methods: We analyzed the six publicly-available quarters of raw Kepler data, phase-folded the light curve and fitted it to a model for the trailing dust cloud. Constraints on the particle properties were investigated with a light-scattering code.
Results: The light curve exhibits clear signatures of light scattering and absorption by dust, including a brightening in flux just before ingress correlated with the transit depth and explained by forward scattering, and an asymmetry in the transit light curve shape, which is easily reproduced by an exponentially decaying distribution of optically thin dust, with a typical grain size of 0.1 μm.
Conclusions: Our quantitative analysis supports the hypothesis that the transit signal of KIC 12557548 b is due to a variable cloud of dust, most likely originating from a disintegrating object. Title: Searching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of Earth Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Bagnulo, S.; Sparks, W. B.; Keller, C. U. Bibcode: 2012epsc.conf..537K Altcode: 2012espc.conf..537K We present LOUPE, a novel type of spectropolarimeter to measure the flux and state of polarization of sunlight that is reflected by the Earth from 0.4 to 0.8 μm. LOUPE has been designed as payload of a lunar lander. From the moon, the Earth can be observed as a whole, during its daily rotation and at all phase angles, just as if it were an exoplanet. LOUPE will provide benchmark data for the development of instruments for Earth-like exoplanet characterization, and for the testing of numerical retrieval algorithms. Title: The extreme polarimeter: design, performance, first results and upgrades Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; de Juan Ovelar, Maria; Min, M.; Homs, L.; Keller, C. U. Bibcode: 2012SPIE.8446E..9IR Altcode: 2012arXiv1211.6300R Well over 700 exoplanets have been detected to date. Only a handful of these have been observed directly. Direct observation is extremely challenging due to the small separation and very large contrast involved. Imaging polarimetry offers a way to decrease the contrast between the unpolarized starlight and the light that has become linearly polarized after scattering by circumstellar material. This material can be the dust and debris found in circumstellar disks, but also the atmosphere or surface of an exoplanet. We present the design, calibration approach, polarimetric performance and sample observation results of the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. The polarimeter uses the beam-exchange technique, in which the two orthogonal polarization states are imaged simultaneously and a polarization modulator is swaps the polarization states of the two beams before the next image is taken. The instrument currently operates without the aid of Adaptive Optics. To reduce the effects of atmospheric seeing on the polarimetry, the images are taken at a frame rate of 35 fps, and large numbers of frames are combined to obtain the polarization images. Four successful observing runs have been performed using this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting young stars with protoplanetary disks as well as evolved stars surrounded by dusty envelopes. In terms of fractional polarization, the instrument sensitivity is better than 10-4. The contrast achieved between the central star and the circumstellar source is of the order 10-6. We show that our calibration approach yields absolute polarization errors below 1%. Title: Modeling the instrumental polarization of the VLT and E-ELT telescopes with the M&m's code Authors: de Juan Ovelar, M.; Diamantopoulou, S.; Roelfsema, R.; van Werkhoven, T.; Snik, F.; Pragt, Johan; Keller, C. Bibcode: 2012SPIE.8449E..12D Altcode: Polarimetry is a particularly powerful technique when imaging circumstellar environments. Currently most telescopes include more or less advanced polarimetric facilities and large telescopes count on it for their planet-finder instruments like SPHERE-ZIMPOL on the VLT or EPICS on the future E-ELT. One of the biggest limitations of this technique is the instrumental polarization (IP) generated in the telescope optical path, which can often be larger than the signal to be measured. In most cases this instrumental polarization changes over time and is dependent on the errors affecting the optical elements of the system. We have modeled the VLT and E-ELT telescope layouts to characterize the instrumental polarization generated on their optical paths using the M&m's code, an error budget and performance simulator for polarimetric systems. In this study we present the realistic Mueller matrices calculated with M&m's for both systems, with and without the setups to correct for the IP, showing that correction can be achieved, allowing for an accurate polarimetric performance. Title: Potential of phase-diversity for metrology of active instruments Authors: Korkiakoski, Visa; Venema, Lars; Agocs, Tibor; Keller, Christoph U.; Doelman, Niek; Fraanje, Rufus; Andrei, Raluca; Verhaegen, Michel Bibcode: 2012SPIE.8450E..5DK Altcode: We investigate the potential of phase-diversity (PD) and Gerchberg-Saxton (GS) algorithms in the calibration of active instruments. A set of images is recorded with the focal-plane scientific camera, each image having a known and unique defocus. The phase-retrieval algorithms are used, with those images, to estimate the non-common path aberration that needs to be compensated by correct alignment of the instrument. We demonstrate by numerical simulations that such algorithms, in particular GS, are sufficient detection methods to fully correct wavefronts with an rms error at least up to 6 rad — but this requires several iterative correction stages. Title: Constraining the circumbinary envelope of Z Canis Majoris via imaging polarimetry Authors: Canovas, H.; Min, M.; Jeffers, S. V.; Rodenhuis, M.; Keller, C. U. Bibcode: 2012A&A...543A..70C Altcode: 2012arXiv1205.3784C Context. Z CMa is a complex binary system composed of a Herbig Be and an FU Ori star. The Herbig star is surrounded by a dust cocoon of variable geometry, and the whole system is surrounded by an infalling envelope. Previous spectropolarimetric observations have reported a preferred orientation of the polarization angle, perpendicular to the direction of a very extended, parsec-sized jet associated with the Herbig star.
Aims: The variability in the amount of polarized light has been associated to changes in the geometry of the dust cocoon that surrounds the Herbig star. We aim to constrain the properties of Z CMa by means of imaging polarimetry at optical wavelengths.
Methods: Using ExPo, a dual-beam imaging polarimeter that operates at optical wavelengths, we have obtained imaging (linear) polarimetric data of Z CMa. Our observations were secured during the return to quiescence after the 2008 outburst.
Results: We detect three polarized features over Z CMa. Two of these features are related to the two jets reported in this system: the large jet associated to the Herbig star, and the micro-jet associated to the FU Ori star. Our results suggest that the micro-jet extends to a distance ten times longer than reported in previous studies. The third feature suggests the presence of a hole in the dust cocoon that surrounds the Herbig star of this system. According to our simulations, this hole can produce a pencil beam of light that we see scattered off the low-density envelope surrounding the system.

Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Title: Linear analytical solution to the phase diversity problem for extended objects based on the Born approximation Authors: Andrei, Raluca M.; Smith, Carlas S.; Fraanje, Rufus; Verhaegen, Michel; Korkiakoski, Visa A.; Keller, Christoph U.; Doelman, Niek Bibcode: 2012SPIE.8447E..6UA Altcode: In this paper we give a new wavefront estimation technique that overcomes the main disadvantages of the phase diversity (PD) algorithms, namely the large computational complexity and the fact that the solutions can get stuck in a local minima. Our approach gives a good starting point for an iterative algorithm based on solving a linear system, but it can also be used as a new wavefront estimation method. The method is based on the Born approximation of the wavefront for small phase aberrations which leads to a quadratic point-spread function (PSF), and it requires two diversity images. First we take the differences between the focal plane image and each of the two diversity images, and then we eliminate the constant object, element-wise, from the two equations. The result is an overdetermined set of linear equations for which we give three solutions using linear least squares (LS), truncated total least squares (TTLS) and bounded data uncertainty (BDU). The last two approaches are suited when considering measurements affected by noise. Simulation results show that the estimation is faster than conventional PD algorithms. Title: The Majorana Demonstrator: A Search for Neutrinoless Double-beta Decay of Germanium-76 Authors: Wilkerson, J. F.; Aguayo, E.; Avignone, F. T., Iii; Back, H. O.; Barabash, A. S.; Beene, J. R.; Bergevin, M.; Bertrand, F. E.; Boswell, M.; Brudanin, V.; Busch, M.; Chan, Y. -D.; Christofferson, C. D.; Collar, J. I.; Combs, D. C.; Cooper, R. J.; Detwiler, J. A.; Doe, P. J.; Efremenko, Yu; Egorov, V.; Ejiri, H.; Elliott, S. R.; Esterline, J.; Fast, J. E.; Fields, N.; Finnerty, P.; Fraenkle, F. M.; Gehman, V. M.; Giovanetti, G. K.; Green, M. P.; Guiseppe, V. E.; Gusey, K.; Hallin, A. L.; Hazama, R.; Henning, R.; Hoppe, E. W.; Horton, M.; Howard, S.; Howe, M. A.; Johnson, R. A.; Keeter, K. J.; Keller, C.; Kidd, M. F.; Knecht, A.; Kochetov, O.; Konovalov, S. I.; Kouzes, R. T.; LaFerriere, B. D.; LaRoque, B. H.; Leon, J.; Leviner, L. E.; Loach, J. C.; MacMullin, S.; Marino, M. G.; Martin, R. D.; Mei, D. -M.; Merriman, J. H.; Miller, M. L.; Mizouni, L.; Nomachi, M.; Orrell, J. L.; Overman, N. R.; Phillips, D. G., Ii; Poon, A. W. P.; Perumpilly, G.; Prior, G.; Radford, D. C.; Rielage, K.; Robertson, R. G. H.; Ronquest, M. C.; Schubert, A. G.; Shima, T.; Shirchenko, M.; Snavely, K. J.; Steele, D.; Strain, J.; Thomas, K.; Timkin, V.; Tornow, W.; Vanyushin, I.; Varner, R. L.; Vetter, K.; Vorren, K.; Yakushev, E.; Young, A. R.; Yu, C. -H.; Yumatov, V.; Zhang, C. Bibcode: 2012JPhCS.375d2010W Altcode: The observation of neutrino less double-beta decay would show that neutrinos are Majorana particles and provide information on neutrino mass. Attaining sensitivities for neutrino masses in the inverted hierarchy region requires large, tonne scale detectors with extremely low backgrounds, at the level of 10-3 counts keV-1 t-1 y-1 or lower in the region of the signal. The MAJORANA collaboration is constructing the DEMONSTRATOR, an array consisting of 40 kg of p-type point contact germanium detectors, at least half of which will be enriched to 86% in 76Ge. The primary aim is to show the feasibility for a future tonne scale measurement. With a sub-keV energy threshold, the array should also be able to search for light WIMP dark matter. This paper presents a brief update on the status of constructing the DEMONSTRATOR including an electroforming facility that is now operating underground at the Sanford Underground Research Facility. Title: Experimental validation of optimization concepts for focal-plane image processing with adaptive optics Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek; Fraanje, Rufus; Andrei, Raluca; Verhaegen, Michel Bibcode: 2012SPIE.8447E..5ZK Altcode: We show experimental results demonstrating the feasibility of an extremely fast sequential phase-diversity (SPD) algorithm for point sources. The algorithm can be implemented on a typical adaptive optics (AO) system to improve the wavefront reconstruction beyond the capabilities of a wavefront sensor by using the information from the imaging camera. The algorithm is based on a small-phase approximation enabling fast numerical implementation, and it finds the optimal wavefront correction by iteratively updating the deformable mirror. Our experiments were made at an AO-setup with a 37 actuator membrane mirror, and the results show that the algorithm finds an optimal image quality in 5-10 iterations, when the initial wavefront errors are typical non-common path aberrations having a magnitude of 1-1.5 rad rms. The results are in excellent agreement with corresponding numerical simulations. Title: Dark matter sensitivities of the Majorana Demonstrator Authors: Giovanetti, G. K.; Aguayo, E.; Avignone, F. T., Iii; Back, H. O.; Barabash, A. S.; Beene, J. R.; Bergevin, M.; Bertrand, F. E.; Boswell, M.; Brudanin, V.; Busch, M.; Chan, Y. -D.; Christofferson, C. D.; Collar, J. I.; Combs, D. C.; Cooper, R. J.; Detwiler, J. A.; Doe, P. J.; Efremenko, Yu; Egorov, V.; Ejiri, H.; Elliott, S. R.; Esterline, J.; Fast, J. E.; Fields, N.; Finnerty, P.; Fraenkle, F. M.; Gehman, V. M.; Green, M. P.; Guiseppe, V. E.; Gusey, K.; Hallin, A. L.; Hazama, R.; Henning, R.; Hoppe, E. W.; Horton, M.; Howard, S.; Howe, M. A.; Johnson, R. A.; Keeter, K. J.; Keller, C.; Kidd, M. F.; Knecht, A.; Kochetov, O.; Konovalov, S. I.; Kouzes, R. T.; LaFerriere, B. D.; LaRoque, B. H.; Leon, J.; Leviner, L. E.; Loach, J. C.; MacMullin, S.; Marino, M. G.; Martin, R. D.; Mei, D. -M.; Merriman, J. H.; Miller, M. L.; Mizouni, L.; Nomachi, M.; Orrell, J. L.; Overman, N. R.; Phillips, D. G., Ii; Poon, A. W. P.; Perumpilly, G.; Prior, G.; Radford, D. C.; Rielage, K.; Robertson, R. G. H.; Ronquest, M. C.; Schubert, A. G.; Shima, T.; Shirchenko, M.; Snavely, K. J.; Steele, D.; Strain, J.; Thomas, K.; Timkin, V.; Tornow, W.; Vanyushin, I.; Varner, R. L.; Vetter, K.; Vorren, K.; Wilkerson, J. F.; Yakushev, E.; Young, A. R.; Yu, C. -H.; Yumatov, V.; Zhang, C. Bibcode: 2012JPhCS.375a2014G Altcode: The Majorana Demonstrator is an array of natural and enriched high purity germanium detectors that will search for the neutrinoless double-beta decay of Germanium-76 and perform a search for weakly interacting massive particles with masses below 10 GeV. To reach the background rate goal in the neutrinoless double-beta decay region of interest of 4 counts/keV/t/y, the DEMONSTRATOR will utilize a number of background reduction strategies, including a time-correlated event cut for 68Ge that requires a sub-keV energy threshold. This low energy threshold allows the DEMONSTRATOR to extend its physics reach to include a search for light WIMPs. We will discuss the detector systems and data analysis techniques required to achieve sub-keV thresholds as well as present the projected dark matter sensitivities of the Majorana Demonstrator. Title: Extremely fast focal-plane wavefront sensing for extreme adaptive optics Authors: Keller, Christoph U.; Korkiakoski, Visa; Doelman, Niek; Fraanje, Rufus; Andrei, Raluca; Verhaegen, Michel Bibcode: 2012SPIE.8447E..21K Altcode: 2012arXiv1207.3273K We present a promising approach to the extremely fast sensing and correction of small wavefront errors in adaptive optics systems. As our algorithm's computational complexity is roughly proportional to the number of actuators, it is particularly suitable to systems with 10,000 to 100,000 actuators. Our approach is based on sequential phase diversity and simple relations between the point-spread function and the wavefront error in the case of small aberrations. The particular choice of phase diversity, introduced by the deformable mirror itself, minimizes the wavefront error as well as the computational complexity. The method is well suited for high­ contrast astronomical imaging of point sources such as the direct detection and characterization of exoplanets around stars, and it works even in the presence of a coronagraph that suppresses the diffraction pattern. The accompanying paper in these proceedings by Korkiakoski et al. describes the performance of the algorithm using numerical simulations and laboratory tests. Title: FOAM: the modular adaptive optics framework Authors: van Werkhoven, T. I. M.; Homs, L.; Sliepen, G.; Rodenhuis, M.; Keller, C. U. Bibcode: 2012SPIE.8447E..2VV Altcode: Control software for adaptive optics systems is mostly custom built and very specific in nature. We have developed FOAM, a modular adaptive optics framework for controlling and simulating adaptive optics systems in various environments. Portability is provided both for different control hardware and adaptive optics setups. To achieve this, FOAM is written in C++ and runs on standard CPUs. Furthermore we use standard Unix libraries and compilation procedures and implemented a hardware abstraction layer in FOAM. We have successfully implemented FOAM on the adaptive optics system of ExPo - a high-contrast imaging polarimeter developed at our institute - in the lab and will test it on-sky late June 2012. We also plan to implement FOAM on adaptive optics systems for microscopy and solar adaptive optics. FOAM is available* under the GNU GPL license and is free to be used by anyone. Title: Bilinear solution to the phase diversity problem for extended objects based on the Born approximation Authors: Andrei, Raluca M.; Fraanje, Rufus; Verhaegen, Michel; Korkiakoski, Visa A.; Keller, Christoph U.; Doelman, Niek Bibcode: 2012SPIE.8447E..6TA Altcode: We propose a new approach for the joint estimation of aberration parameters and unknown object from diversity images with applications in imaging systems with extended objects as astronomical ground-based observations or solar telescopes. The motivation behind our idea is to decrease the computational complexity of the conventional phase diversity (PD) algorithm and avoid the convergence to local minima due to the use of nonlinear estimation algorithms. Our approach is able to give a good starting point for an iterative algorithm or it can be used as a new wavefront estimation method. When the wavefront aberrations are small, the wavefront can be approximated with a linear term which leads to a quadratic point-spread function (PSF) in the aberration parameters. The presented approach involves recording two or more diversity images and, based on the before mentioned approximation estimates the aberration parameters and the object by solving a system of bilinear equations, which is obtained by subtracting from each diversity image the focal plane image. Moreover, using the quadratic PSFs gives improved performance to the conventional PD algorithm through the fact that the gradients of the PSFs have simple analytical formulas. Title: Simulation of automotive EMC emission test procedures based on cable bundle measurements Authors: Gonser, M.; Keller, C.; Hansen, J.; Khillkevich, V.; Radchenko, A.; Pommerenke, D.; Weigel, R. Bibcode: 2012imsd.conf59432G Altcode: No abstract at ADS Title: HARPS spectropolarimetry of classical T Tauri stars Authors: Johns-Krull, C. M.; Valenti, J. A.; Jeffers, S. V.; Piskunov, N. E.; Kochukhov, O.; Keller, C.; Snik, F.; Rodenhuis, M.; Makaganiuk, V.; Stempels, H. Bibcode: 2012AIPC.1429...43J Altcode: We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 A and 6678 A. The He I lines in both these CTTS contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large scale magnetospheric accretion flow. We detect strong polarization in the narrow component of both the He I emission lines in both stars. We observe a maximum implied field strength of 5.8 +/- 0.3 kG in the 5876 A˚ line of GQ Lup, the highest field strength measured to date in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, unlike what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative the formation region of the narrow component of the He I lines. Title: SPEX2Earth, a novel spectropolarimeter for remote sensing of aerosols and clouds Authors: Smit, J. M.; Rietjens, J. H. H.; Hasekamp, O.; Stam, D. M.; Snik, F.; van Harten, G.; Keller, C. U.; van der Togt, O.; Verlaan, A. L.; Moddemeijer, K.; Beijersbergen, M.; Voors, R.; Wielinga, K.; Vollmuller, B. -J. Bibcode: 2012EGUGA..1414166S Altcode: Multi-angle spectro-polarimetry is the tool for the remote detection and characterization of aerosol and clouds in the Earth's atmosphere. Using a novel technique to measure polarization, we have developed a 30 kg instrument design to simultaneously measure the intensity and state of linear polarization of scattered sunlight, from 400 to 800 nm and 1200 to 1600 nm, for 30 viewing directions, each with a 30° swath. Aerosols affect the climate directly by scattering and absorption of solar radiation, and by scattering, absorption, and emission of thermal radiation. Aerosols also affect the climate by changing the macro- and microphysical properties of clouds (the so-called indirect and semi-direct effects). Estimates of aerosol effects on the climate are hampered by insufficient knowledge of aerosol properties (size distribution, shape, and single scattering albedo) at a global scale. From several studies we know that these properties can only be determined with sufficient accuracy and unambiguously with satellite instruments that measure both intensity and polarization at multiple wavelengths and multiple viewing angles1,2.Polarization measurements must have a high accuracy, typically better than 0.1%. Achieving global coverage requires a large instantaneous field of view. Developing an instrument that combines all of these specifications can be considered as the most important challenge in polarimetric aerosol remote sensing. SPEX2Earth is such an instrument. It has been derived from the prototype spectropolarimeter SPEX (Spectro-polarimeter for Planetary Exploration), that was originally developed for a Mars orbiter. Possible target platforms for SPEX2Earth are the International Space Station, or a low-Earth orbit platform. SPEX2Earth uses a novel technique for its radiance and polarization measurements: through a series of carefully selected birefringent crystals, the radiance of scattered sunlight is spectrally modulated3. The modulation amplitude and phase are proportional to the degree and angle of linear polarization respectively. Two modulated spectra are produced per ground pixel, with a 180° degree phase shift between their modulations. The sum of the two spectra yields a modulation-free high resolution radiance spectrum of the scattered sunlight. The birefringent crystals determine the modulation frequency and thereby the resolution of the polarization spectrum. The technique is entirely passive, i.e. the polarization modulation is established without moving parts or active components.. SPEX2Earth's novel polarimetric technique allows for achieving the extremely high polarimetric accuracy (~0.001 in linear polarization) needed to derive properties of aerosol (size, shape, refractive index, optical thickness, single scattering albedo) and clouds (droplet size, number concentration, optical thickness, phase, top/base height, cloud cover) with sufficient accuracy for climate research. With its relatively high spectral resolution, SPEX2Earth resolves the O2-A absorption band, which is important for deriving aerosol and cloud height. The viewing angles sample the scattering phase functions of aerosol and cloud particles, resolving characteristic angular features, and allowing to distinguish different types of particles. We will present the SPEX2Earth instrument, outline its spectral modulation principle and discuss its advantages compared with traditional polarimetric techniques. Expected performances are discussed, and recent performance results of the SPEX prototype are presented. Title: Multiwavelength imaging polarimetry of Venus at various phase angles Authors: Einarsen, L. J.; Rodenhuis, M.; Snik, F.; Keller, C. U.; Stam, D. M.; de Kok, R. J.; Bianda, M.; Ramelli, R. Bibcode: 2012EGUGA..14.8670E Altcode: Venus is the only planet with an atmosphere that we can observe from the ground at a large range of phase angles. Therefore it constitutes an important benchmark for direct observations of exoplanets, which will soon become available. Moreover, polarimetric observations at various phase angles and wavelengths provide a unique way to characterize any (exo-)planetary atmosphere. For instance, the famous study by Hansen & Hovenier (1974) which combines disk-integrated polarimetric observations and modeling has unambiguously shown that Venus' upper atmosphere consists of sulphuric acid droplets of ~1 um in diameter. We present new spatially resolved observations of Venus using the imaging polarimeters ExPo at the William Herschel Telescope and ZIMPOL at the IRSOL telescope. These observations are taken in narrow-band filters from 364--648 nm, and span phase angles from 10--49 degrees. We find that the degree of polarization varies strongly with wavelength and phase angle, as generally predicted by the model by Hansen & Hovenier. However, the polarization behaviour near the equator differs considerably from that at the poles, hinting at different atmospheric compositions and/or stratifications. In the intensity images we detect a significant shift of the location of maximum intensity with wavelength. These observations allow us to refine the model by Hansen & Hovenier, and we present the preliminary results of our efforts to do so. Title: Observing the Earth as an exoplanet Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Keller, C. U.; Sparks, W. B.; Bagnulo, S. Bibcode: 2012EGUGA..1410571K Altcode: Observations of Solar System planets, including the Earth, have shown the power of polarimetry for the characterization of planetary atmospheres and surfaces, and its ability to break degeneracies in retrievals from flux observations only and is thus essential for the full characterization of atmospheres and surfaces of (exo-) planets. With the discoveries of the first rocky exoplanets, the quest for Earth-like exoplanets and signs of their habitability has started. Since exoplanet observations will yield a signal that is integrated over the illuminated and visible part of the planet's disk, the main challenge for the interpretation of future exoplanet observations in terms of habitability will be disentangling the contributions from the different surface types and clouds. Numerical codes have been developed to model the spectral signals of oceans, continents, atmospheric gases, aerosols and clouds, but neither these codes nor retrieval algorithms can be validated by lack of disk-integrated observations of the Earth at a range of phase angles and wavelengths. We present LOUPE (Lunar Observatory for Unresolved Polarimetry of the Earth) as an instrument for a lunar lander. LOUPE will measure the disk-integrated flux and state of polarization of sunlight that is reflected by the Earth. LOUPE will offer a unique opportunity to observe the Earth as if it were an exoplanet. Thanks to the characteristics of the Moon's orbit around our planet, such a lunar observatory will witness the daily rotation of the Earth, with various surface types rotating in and out of view. During a month, it will also see the Earth through all phase angles, ranging from a full Earth to a new or almost new Earth, just as we can expect for observations of an exoplanet (depending on its orbital inclination angle). Finally, during the year, seasonal variations will become apparent. Such observations cannot be obtained by integrating spatially resolved observations by Earth remote-sensing satellites, nor by so-called Earthshine measurements, which capture sunlight that has first been reflected by the Earth and then by the lunar surface, because these can only be done when the moon is seen at large phase angles (thus when a large fraction of the lunar nightside and hence a large fraction of the Earth dayside are in view). Apart from a description of the LOUPE instrument, we will show numerically simulated flux and polarization spectra of Earth-like exoplanets to 1. illustrate the spectral and temporal variations that we can expect to observe from the moon, 2. point out the information that could be retrieved from such observations Title: Planetary science: In search of biosignatures Authors: Keller, Christoph U.; Stam, Daphne M. Bibcode: 2012Natur.483...38K Altcode: An analysis of the intensity and polarization of sunlight reflected by Earth reveals signatures of life on our planet. What prospects are there for using similar measurements to find life on planets outside the Solar System? Planetary scientists offer some answers. See Letter p.64 Title: Magnetism, chemical spots, and stratification in the HgMn star ϕ Phoenicis Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2012A&A...539A.142M Altcode: 2011arXiv1111.6065M Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.
Aims: In this study, we investigate the possible presence of a magnetic field in ϕ Phe (HD 11753) and reconstruct surface distribution of chemical elements that show variability in spectral lines. We also test a hypothesis that a magnetic field is concentrated in chemical spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.
Methods: Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructed surface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elements was calculated with the DDAFit program.
Results: Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field. For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric layers.
Conclusions: Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in ϕ Phe. This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the spot evolution time-scale reported for ϕ Phe.

Based on observations collected at the European Southern Observatory, Chile (ESO programme 084.D-0338). Figures 9-12 are available in electronic form at http://www.aanda.org Title: Direct imaging of a massive dust cloud around R Coronae Borealis Authors: Jeffers, S. V.; Min, M.; Waters, L. B. F. M.; Canovas, H.; Rodenhuis, M.; de Juan Ovelar, M.; Chies-Santos, A. L.; Keller, C. U. Bibcode: 2012A&A...539A..56J Altcode: 2012arXiv1203.1265J We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star's circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately 1.3'' or 2000 AU from the star. We find that to obtain a decrease in the stellar light of 9 mag and with 30% of the light being reemitted at infrared wavelengths (from R CrB's SED) the grains in R CrB's circumstellar environment must have a very low albedo of approximately 0.07%. We show that the properties of the dust clouds formed around R CrB are best fitted using a combination of two distinct populations of grains size. The first are the extremely small 5 nm grains, formed in the low density continuous wind, and the second population of large grains (~0.14 μm) which are found in the ejected dust clouds. The observed scattering cloud, not only contains such large grains, but is exceptionally massive compared to the average cloud.

Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Title: The effects of disk and dust structure on observed polarimetric images of protoplanetary disks Authors: Min, M.; Canovas, H.; Mulders, G. D.; Keller, C. U. Bibcode: 2012A&A...537A..75M Altcode: 2011arXiv1111.4348M Context. Imaging polarimetry is a powerful tool for imaging faint circumstellar material. It is a rapidly developing field with great promise for diagnostics of both the large-scale structures and the small-scale details of the scattering particles.
Aims: For a correct analysis of observations we need to fully understand the effects of dust particle parameters, as well as the effects of the telescope, atmospheric seeing, and assumptions about the data reduction and processing of the observed signal. Here we study the major effects of dust particle structure, size-dependent grain settling, and instrumental properties.
Methods: We performed radiative transfer modeling using different dust particle models and disk structures. To study the influence of seeing and telescope diffraction we ran the models through an instrument simulator for the ExPo dual-beam imaging polarimeter mounted at the 4.2 m William Herschel Telescope (WHT).
Results: Particle shape and size have a strong influence on the brightness and detectability of the disks. In the simulated observations, the central resolution element also contains contributions from the inner regions of the protoplanetary disk besides the unpolarized central star. This causes the central resolution element to be polarized, making simple corrections for instrumental polarization difficult. This effect strongly depends on the spatial resolution, so adaptive optics systems are needed for proper polarization calibration.
Conclusions: We find that the commonly employed homogeneous sphere model gives results that differ significantly from more realistic models. For a proper analysis of the wealth of data available now or in the near future, one must properly take the effects of particle types and disk structure into account. The observed signal depends strongly on the properties of these more realistic models, thus providing a potentially powerful diagnostic. We conclude that it is important to correctly understand telescope depolarization and calibration effects for a correct interpretation of the degree of polarization.

Appendix A is available in electronic form at http://www.aanda.org Title: Signatures of Water Clouds on Exoplanets: Numerical Simulations. Authors: Karalidi, T.; Stam, D. M.; Keller, C. U. Bibcode: 2011ASPC..450..101K Altcode: Clouds are of crucial importance for a planetary climate, because they store atmospheric volatiles, and because they scatter and absorb incident starlight and absorb, emit and scatter thermal radiation. Consequently, the detection and characterization of clouds on a planet can provide us with a wealth of information on the conditions on the surface. Here, we present numerically simulated flux and polarization spectra, from 0.3 μm to 1.0 μm, of starlight reflected by Earth-like exoplanets that are covered by horizontally homogeneous water clouds, for different cloud altitudes and particle sizes. Our results show that the degree of polarization P is sensitive to the particle size, in particular at phase angles between 30° and ∼50° and around 90°, and to the cloud top altitude, in particular at wavelengths between 0.35 μm and 0.7 μm. The information in P should be easier to retrieve than that in F. Title: Innovative Imaging of Young Stars: First Light ExPo Observations Authors: Jeffers, S. V.; Canovas, H.; Keller, C. U.; Min, M.; Rodenhuis, M. Bibcode: 2011ASPC..448...15J Altcode: 2011csss...16...15J We have developed an innovative imaging polariemter, ExPo, that excels in the imaging of the circumstellar environments of young stars. The basic physics that ExPo exploits is that starlight reflected from a star's circumstellar environment becomes linearly polarised, making it easily separable from unpolarised starlight. Our preliminary results, from the William Herschel Telescope in La Palma, show that ExPo has successfully detected several known protoplanetary disks out to a much larger distance and at a finer resolution than previously observed. ExPo has also made a significant number of new detections of protoplanetary disks and stellar outflows. We use innovative data analysis techniques, related to speckle interferometry, to detect the innermost parts of the disk to much closer than any other techniques operating at visible wavelengths. In this paper I present highlights of ExPo's first light observations. Title: New Insights into Stellar Magnetism from the Spectropolarimetry in All Four Stokes Parameters Authors: Kochukhov, O.; Snik, F.; Piskunov, N.; Jeffers, S. V.; Keller, C. U.; Makaganiuk, V.; Valenti, J. A.; Johns-Krull, C. M.; Rodenhuis, M.; Stempels, H. C. Bibcode: 2011ASPC..448..245K Altcode: 2011csss...16..245K Development of high-resolution spectropolarimetry has stimulated a major progress in our understanding of the magnetism and activity of late-type stars. During the last decade magnetic fields were discovered and mapped for various types of active stars using spectropolarimetric methods. However, these observations and modeling attempts are inherently incomplete since they are based on the interpretation of the stellar circular polarization alone. Taking advantage of the recently commissioned HARPS polarimeter, we obtained the first systematic observations of cool active stars in all four Stokes parameters. Here we report detection of the magnetically induced linear polarization in the RS CVn binary HR 1099 and phase-resolved full Stokes vector observations of varepsilon Eri. For the latter star we measured the field strength with the precision of ∼0.1 G over a complete rotation cycle and reconstructed the global field topology with the help of magnetic Doppler imaging. Our observations of the inactive solar-like star α Cen A indicate the absence of the global field stronger than 0.2 G. Title: Spectropolarimeter for planetary exploration (SPEX): performance measurements with a prototype Authors: Voors, Robert; Moon, Scott G.; Hannemann, Sandro; Rietjens, Jeroen H. H.; van Harten, Gerard; Snik, Frans; Smit, Martijn; Stam, Daphne M.; Keller, Christoph U.; Laan, Erik C.; Verlaan, Adrianus L.; Vliegenthart, Willem A.; ter Horst, Rik; Navarro, Ramón; Wielenga, Klaas Bibcode: 2011SPIE.8176E..0DV Altcode: 2011SPIE.8176E...9V SPEX (Spectropolarimeter for Planetary Exploration) was developed in close cooperation between scientific institutes and space technological industries in the Netherlands. It is used for measuring microphysical properties of aerosols and cloud particles in planetary atmospheres. SPEX utilizes a number of novel ideas. The key feature is that full linear spectropolarimetry can be performed without the use of moving parts, using an instrument of approximately 1 liter in volume. This is done by encoding the degree and angle of linear polarization (DoLP and AoLP) of the incoming light in a sinusoidal modulation of the intensity spectrum. Based on this principle, and after gaining experience from breadboard measurements using the same principle, a fully functional prototype was constructed. The functionality and the performance of the prototype were shown by extensive testing. The simulated results and the laboratory measurements show striking agreement. SPEX would be a valuable addition to any mission that aims to study the composition and structure of planetary atmospheres, for example, missions to Mars, Venus, Jupiter, Saturn and Titan. In addition, on an Earth-orbiting satellite, SPEX could give unique information on particles in our own atmosphere. Title: Data Reduction Approach for the Extreme Polarimeter Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Keller, C. U. Bibcode: 2011ASPC..449...79C Altcode: ExPo (Extreme Polarimeter) is an imaging polarimeter that we are building at Utrecht University, The Netherlands. It will detect polarized light from circumstellar disks and extrasolar planets, initially at the 4.2 m WHT and later at other telescopes. We have developed a data reduction approach that minimizes the influence of instrumental and atmospherical effects by using a partially transmitting coronagraph focal-plane mask. The approach has been tested with a laboratory simulator and an ExPo prototype. Title: Simulating Polarized Light from Exoplanets Authors: Jeffers, S. V.; Miesen, N.; Rodenhuis, M.; Keller, C. U.; Canovas, H. Bibcode: 2011ASPC..449..391J Altcode: In Utrecht we are building an imaging polarimeter, ExPo (Extreme Polarimeter), to image circumstellar disk and characterize extra-solar planets. To test and calibrate ExPo, we have built a laboratory-based simulator that mimicks a star with a Jupiter-like exoplanet as seen by the 4.2 m William-Herschel Telescope. The star and planet are simulated using two single-mode fibres in close proximity that are fed with a broadband arc lamp. The unpolarized star has a flux of 1011 photons s-1, to simulate a mv=0 star, and the planet is partially linearly polarized, with a flux of as little as 102 photons s-1 to simulate reflected star light with a contrast ratio of as much as 10-9. The telescope is simulated with two lenses, and seeing can be included with a rotating glass plate covered with hairspray, while dispersion is approximated with a wedge prism. These are the first realistic laboratory simulations of imaging polarimetry for exoplanet detection and characterization. Title: Design and Prototype Results of the ExPo Imaging Polarimeter Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S.; Keller, C. Bibcode: 2011ASPC..449...33R Altcode: We present the design and prototype laboratory results of ExPo, an imaging polarimeter for the study of circumstellar disks and possibly exoplanet detection currently under development at the University of Utrecht. The instrument is designed to achieve a contrast ratio of 10-9 between the unpolarized starlight and the polarized source. First light is scheduled for the second half of 2008 at the 4.2 m William Herschel telescope at La Palma. The instrument is based on the dual beam-exchange technique, simultaneously imaging the two orthogonal polarization states. It employs a ferro-electric liquid crystal retarder and a single electron-multiplying camera for fast modulation of the polarization. The instrument operates in the visible and has a field of view of (20″ × 20″). Title: M&m's: an error budget and performance simulator code for polarimetric systems Authors: de Juan Ovelar, Maria; Snik, Frans; Keller, Christoph U. Bibcode: 2011SPIE.8160E..0CD Altcode: 2011SPIE.8160E...8D; 2012arXiv1207.4241O Although different approaches to model a polarimeter's accuracy have been described before, a complete error budgeting tool for polarimetric systems has not been yet developed. Based on the framework introduced by Keller & Snik, in 2009, we have developed the M&m's code as a first attempt to obtain a generic tool to model the performance and accuracy of a given polarimeter, including all the potential error contributions and their dependencies on physical parameters. The main goal of the code is to provide insight on the combined influence of many polarization errors on the polarimetric accuracy of any polarimetric instrument. In this work we present the mathematics and physics based on which the code is developed as well as its general structure and operational scheme. Discussion of the advantages of the M&m's approach to error budgeting and polarimetric performance simulation is carried out and a brief outlook of further development of the code is also given. Title: Prototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurements Authors: van Harten, Gerard; Snik, Frans; Rietjens, Jeroen H. H.; Smit, J. Martijn; de Boer, Jozua; Diamantopoulou, Renia; Hasekamp, Otto P.; Stam, Daphne M.; Keller, Christoph U.; Laan, Erik C.; Verlaan, Ad L.; Vliegenthart, Willem A.; ter Horst, Rik; Navarro, Ramón; Wielinga, Klaas; Hannemann, Sandro; Moon, Scott G.; Voors, Robert Bibcode: 2011SPIE.8160E..0ZV Altcode: 2011SPIE.8160E..28V We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (~ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype has been developed and calibrated, showing excellent agreement with end-to-end performance simulations. Calibration tests show that the precision of the polarization measurements is at least 2 • 10-4. We performed multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring network. Title: The ZIMPOL high contrast imaging polarimeter for SPHERE: sub-system test results Authors: Roelfsema, Ronald; Gisler, Daniel; Pragt, Johan; Schmid, Hans Martin; Bazzon, Andreas; Dominik, Carsten; Baruffolo, Andrea; Beuzit, Jean-Luc; Charton, Julien; Dohlen, Kjetil; Downing, Mark; Elswijk, Eddy; Feldt, Markus; de Haan, Menno; Hubin, Norbert; Kasper, Markus; Keller, Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov, Alexey; Puget, Pascal; Rochat, Sylvain; Salasnich, Bernardo; Steiner, Peter; Thalmann, Christian; Waters, Rens; Wildi, François Bibcode: 2011SPIE.8151E..0NR Altcode: 2011SPIE.8151E..21R SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. The instrument will search for direct light from old planets with orbital periods of several months to several years as we know them from our solar system. These are planets which are in or close to the habitable zone. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. The search for reflected light from extra-solar planets is very demanding because the signal decreases rapidly with the orbital separation. For a Jupiter-sized object and a separation of 1 AU the planet/star contrast to be achieved is on the order of 10-8 for a successful detection. This is much more demanding than the direct imaging of young self-luminous planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2012. ZIMPOL is currently in the subsystem testing phase. We describe the results of verification and performance testing done at the NOVA-ASTRON lab. We will give an overview of the system noise performance, the polarimetric accuracy and the high contrast testing. For the high contrast testing we will describe the impact of crucial system parameters on the contrast performance. SPHERE is an instrument designed and built by a consortium consisting of IPAG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO. Title: No magnetic field in the spotted HgMn star μ Leporis Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2011A&A...534L..13K Altcode: 2011arXiv1110.0829K Context. Chemically peculiar stars of the mercury-manganese (HgMn) type represent a new class of spotted late-B stars, in which evolving surface chemical inhomogeneities are apparently unrelated to the presence of strong magnetic fields but are produced by some hitherto unknown astrophysical mechanism.
Aims: The goal of this study is to perform a detailed line profile variability analysis and carry out a sensitive magnetic field search for one of the brightest HgMn stars - μ Lep.
Methods: We acquired a set of very high-quality intensity and polarization spectra of μ Lep with the HARPSpol polarimeter. These data were analyzed with the multiline technique of least-squares deconvolution in order to extract information on the magnetic field and line profile variability.
Results: Our spectra show very weak but definite variability in the lines of Sc, all Fe-peak elements represented in the spectrum of μ Lep, as well as Y, Sr, and Hg. Variability might also be present in the lines of Si and Mg. Anomalous profile shapes of Ti ii and Y ii lines suggest a dominant axisymmetric distribution of these elements. At the same time, we found no evidence of the magnetic field in μ Lep, with the 3σ upper limit of only 3 G for the mean longitudinal magnetic field. This is the most stringent upper limit on the possible magnetic field derived for a spotted HgMn star.
Conclusions: The very weak variability detected for many elements in the spectrum μ Lep suggests that low-contrast chemical inhomogeneities may be common in HgMn stars and that they have not been recognized until now due to the limited precision of previous spectroscopic observations and a lack of time-series data. The null result of the magnetic field search reinforces the conclusion that formation of chemical spots in HgMn stars is not magnetically driven.

Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 086.D-0240). Title: Fast horizontal flows in a quiet sun MHD simulation and their spectroscopic signatures Authors: Vitas, N.; Fischer, C. E.; Vögler, A.; Keller, C. U. Bibcode: 2011A&A...532A.110V Altcode: Numerical simulations of solar surface convection have predicted the existence of supersonic horizontal flows in the photospheric granulation. Recently, the detection of such flows in data from the Hinode satellite was reported. We study supersonic granular flows in detail to understand their signatures in spectral lines and to test the observational detection method used to identify these flows in the Hinode observations. We perform time-dependent 3D radiative MHD numerical simulations and synthesize the Fe i 6302 Å spectral lines at the resolution of the Hinode data for different viewing angles covering the center-limb variation. There is very large variation in the detailed shape of the emergent line profiles depending on the viewing angle and the particular flow properties and orientation. At the full simulation resolution the supersonic flows can even produce distinct satellite lines. After smearing to the Hinode resolution sufficient signature of supersonic motion remains. Our analysis shows that the detection method used to analyze the Hinode data is indeed applicable. However, the detection is very sensitive to ad hoc parameter choices and can also misidentify supersonic flows. Title: The search for magnetic fields in mercury-manganese stars Authors: Makaganiuk, Vitalii; Kochukhov, Oleg; Piskunov, Nikolai; Jeffers, Sandra V.; Johns-Krull, Christopher M.; Keller, Christoph U.; Rodenhuis, Michiel; Snik, Frans; Stempels, Henricus C.; Valenti, Jeff A. Bibcode: 2011IAUS..272..202M Altcode: Mercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars. Title: Data-reduction techniques for high-contrast imaging polarimetry. Applications to ExPo Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Min, M.; Keller, C. U. Bibcode: 2011A&A...531A.102C Altcode: 2011arXiv1105.2961C Context. Imaging polarimetry is a powerful tool for detecting and characterizing exoplanets and circumstellar environments. Polarimetry allows a separation of the light coming from an unpolarized source such as a star and the polarized source such as a planet or a protoplanetary disk. Future facilities like SPHERE at the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detect exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeter that can currently reach contrast ratios of 105, enough to characterize circumstellar environments.
Aims: We present the data-reduction steps for a dual-beam imaging polarimeter that can reach contrast ratios of 105.
Methods: The data obtained with ExPo at the William Herschel Telescope (WHT) are analyzed. Instrumental artifacts and noise sources are discussed for an unpolarized star and for a protoplanetary disk (AB Aurigae).
Results: The combination of fast modulation and dual-beam techniques allows us to minimize instrumental artifacts. A proper data processing and alignment of the images is fundamental when dealing with high contrasts. Imaging polarimetry proves to be a powerful method to resolve circumstellar environments even without a coronagraph mask or an adaptive optics system. Title: Spectral and polarimetric characterization of gazeous and telluric planets with SEE COAST Authors: Boccaletti, A.; Baudoz, P.; Mawet, D.; Schneider, J.; Tinetti, G.; Galicher, R.; Stam, D.; Cavarroc, C.; Hough, J.; Doel, P.; Pinfield, D.; Keller, C. -U.; Beuzit, J. -L.; Udry, S.; Ferrari, A.; Martin, E.; Ménard, F.; Sein, E. Bibcode: 2011EPJWC..1607002B Altcode: SEE COAST stands for Super Earth Explorer - Coronagraphic Off-Axis Space Telescope. The concept was initially proposed to ESA for Cosmic Vision. None of the direct detection exoplanet proposals were selected in 2007 and we are now pursuing our efforts to consolidate the astrophysical program and the technical developments for the next call for proposal. The prime objective of SEE COAST is to contribute to the understanding of the formation and evolution of planetary systems. Exploring the diversity of these objects is therefore the main driver to define the instrumentation. In the next decade the improvement of radial velocity instruments and obviously temporal coverage will provide us with a large numbers of long period giants as well as telluric planets, namely Super Earths. Obtaining the spectral and polarimetric signatures of these objects in the visible range to measure atmospheric parameters (molecular composition, clouds, soils, …) will be unique and with important scientific returns. A space mission complementary to near IR instruments like SPHERE, GPI, JWST and later ELTs for the full characterization of giants and Super Earths is a first secure step towards the longer term goal that is the characterization of telluric planets with mass and atmosphere comparable to that of the Earth. An overview of the astrophysical motivation and the trade-off that lead to a simple integrated concept of a space-based high contrast imaging instrument are given here. Title: Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2011A&A...529A.160M Altcode: 2011arXiv1102.4661M Context. According to our current understanding, a subclass of the upper main-sequence chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were recently discovered on their surfaces. At the same time, no global magnetic fields stronger than 1-100 G are detected by systematic studies.
Aims: The goals of our study are to search for a magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar surfaces of both components.
Methods: Our analysis is based on high-quality spectropolarimetric time-series observations obtained during 10 consecutive nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search we employed a least-squares deconvolution (LSD). We used spectral disentangling to measure radial velocities and study the line profile variability. Chemical spot geometry was reconstructed using multi-line Doppler imaging.
Results: We report a non-detection of magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field. Circular polarization profiles also do not indicate any signatures of complex surface magnetic fields. For a simple dipolar field configuration we estimated an upper limit of the polar field strength to be 60-70 G. For the HgMn component we found variability in spectral lines of Ti, Ba, Y, and Sr with the rotational period equal to the orbital one. The surface maps of these elements reconstructed with the Doppler imaging technique show a relative underabundance on the hemisphere facing the secondary component. The contrast of chemical inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.

Based on observations collected at the European Southern Observatory, Chile (ESO program 084.D-0338). Title: First Detection of Linear Polarization in the Line Profiles of Active Cool Stars Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Snik, F.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Valenti, J. A. Bibcode: 2011ApJ...732L..19K Altcode: 2011arXiv1103.6028K The application of high-resolution spectropolarimetry has led to major progress in understanding the magnetism and activity of late-type stars. During the last decade, magnetic fields have been discovered and mapped for many types of active cool stars using spectropolarimetric data. However, these observations and modeling attempts are fundamentally incomplete since they are based on the interpretation of the circular polarization alone. Taking advantage of the newly built HARPS polarimeter, we have obtained the first systematic observations of several cool active stars in all four Stokes parameters. Here we report the detection of magnetically induced linear polarization for the primary component of the very active RS CVn binary HR 1099 and for the moderately active K dwarf ɛ Eri. For both stars the amplitude of linear polarization signatures is measured to be ~10-4 of the unpolarized continuum, which is approximately a factor of 10 lower than for circular polarization. This is the first detection of the linear polarization in line profiles of cool active stars. Our observations of the inactive solar-like star α Cen A show neither circular nor linear polarization above the level of ~10-5, indicating the absence of a net longitudinal magnetic field stronger than 0.2 G.

Based on observations obtained at the European Southern Observatory (ESO programs 083.D-1000(A) and 084.D-0338(A)). Title: The Polarization Optics for the European Solar Telescope Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.; Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.; Snik, F.; Socas-Navarro, H. Bibcode: 2011ASPC..437..329B Altcode: EST, the European Solar Telescope, is a 4-m class solar telescope, which will be located at the Canary Islands. It is currently in the conceptual design phase as a European funded project. In order to fulfill the stringent requirements for polarimetric sensitivity and accuracy, the polarimetry has been included in the design work from the very beginning. The overall philosophy has been to use a combination of techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters, differential measurement schemes, fast modulation and demodulation, and accurate calibration, and at the same time not giving up flexibility. The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and non-varying in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system in which eigenvectors can travel undisturbed through the system. Title: The HARPS Polarimeter Authors: Snik, F.; Kochukhov, O.; Piskunov, N.; Rodenhuis, M.; Jeffers, S.; Keller, C.; Dolgopolov, A.; Stempels, E.; Makaganiuk, V.; Valenti, J.; Johns-Krull, C. Bibcode: 2011ASPC..437..237S Altcode: 2010arXiv1010.0397S We recently commissioned the polarimetric upgrade of the HARPS spectrograph at ESO's 3.6-m telescope at La Silla, Chile. The HARPS polarimeter is capable of full Stokes spectropolarimetry with large sensitivity and accuracy, taking advantage of the large spectral resolution and stability of HARPS. In this paper we present the instrument design and its polarimetric performance. The first HARPSpol observations show that it can attain a polarimetric sensitivity of ∼10-5 (after addition of many lines) and that no significant instrumental polarization effects are present. Title: HARPSpol — The New Polarimetric Mode for HARPS Authors: Piskunov, N.; Snik, F.; Dolgopolov, A.; Kochukhov, O.; Rodenhuis, M.; Valenti, J.; Jeffers, S.; Makaganiuk, V.; Johns-Krull, C.; Stempels, E.; Keller, C. Bibcode: 2011Msngr.143....7P Altcode: The HARPS spectrograph can now perform a full polarisation analysis of spectra. It has been equipped with a polarimetric unit, HARPSpol, which was jointly designed and produced by Uppsala, Utrecht and Rice Universities and by the STScI. Here we present the new instrument, demonstrate its polarisation capabilities and show the first scientific results. Title: The search for magnetic fields in mercury-manganese stars Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels, H. C.; Valenti, J. A. Bibcode: 2011A&A...525A..97M Altcode: 2010arXiv1010.3931M Context. A subclass of the upper main-sequence chemically peculiar stars, mercury-manganese (HgMn) stars were traditionally considered to be non-magnetic, showing no evidence of variability in their spectral line profiles. However, discoveries of chemical inhomogeneities on their surfaces imply that this assumption should be investigated. In particular, spectroscopic time-series of AR Aur, α And, and five other HgMn stars indicate the presence of chemical spots. At the same time, no signatures of global magnetic fields have been detected.
Aims: We attempt to understand the physical mechanism that causes the formation of chemical spots in HgMn stars and gain insight into the potential magnetic field properties at their surfaces; we performed a highly sensitive search for magnetic fields for a large set of HgMn stars.
Methods: With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6 m-telescope, we obtained high-quality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines to ensure significantly greater sensitivity to the presence of magnetic fields, including very weak fields.
Results: For the 47 individual objects studied, including six components of SB2 systems, we do not detect any magnetic fields at greater than the 3σ level. The lack of detection in the circular polarization profiles indicates that if strong fields are present on these stars, they must have complex surface topologies. For simple global fields, our detection limits imply upper limits to the fields present of 2-10 Gauss in the best cases.
Conclusions: We conclude that HgMn stars lack large-scale magnetic fields, which is typical of spotted magnetic Ap stars, of sufficient strength to form and sustain the chemical spots observed on HgMn stars. Our study confirms that in addition to magnetically altered atomic diffusion, there exists another differentiation mechanism operating in the atmospheres of late-B main sequence stars that can produce compositional inhomogeneities on their surfaces.

Based on observations collected at the European Southern Observatory, Chile (ESO programs 083.D-1000, 084.D-0338, 085.D-0296).Figure 5 is only available in electronic form at http://www.aanda.org Title: Imaging polarimetry of circumstellar environments with the Extreme Polarimeter Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Min, M.; Keller, C. U. Bibcode: 2010lyot.confE..20R Altcode: Three successful observation campaigns have been conducted with the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. A contrast ratio between the central star and the circumstellar source of 10-5 can be achieved with polarimetry, with a Lyot coronograph capable of increasing this contrast by several orders of magnitude. The instrument currently operates without an adaptive optics system. An Adaptive Optics system under development for ExPo is expected to increase the contrast further. The polarimeter uses the dual-beam exchange technique, in which the two orthogonal polarisation states are imaged simultaneously after which a polarisation modulator is used to swap the polarisation states of the two beams before the next image is taken. The imaging polarimetry technique developed with ExPo will be used in the polarimetry arm of the EPICS exoplanet characterisation instrument proposed for the E-ELT. Here we present the results from the first observation campaigns, highlighting observations of protoplanetary disks around several young stars. Systematic effects that limit the polarimetric sensitivity, and the strategies we employ to overcome them, are discussed in detail. In particular, the advantages of the dual-beam exchange polarimetry method are demonstrated. Title: EPOL: the exoplanet polarimeter for EPICS at the E-ELT Authors: Snik, F.; Keller, C.; Ovelar, M. J.; Rodenhuis, M.; Korkiakoski, V.; Venema, L.; Jager, R.; Rigal, F.; Hanenburg, H.; Roelfsema, R.; Schmidt, H. M.; Verinaud, C.; Kasper, M.; Martinez, P.; Yaitskova, N. Bibcode: 2010lyot.confE..82S Altcode: EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging polarimetry to differentiate between linearly polarized light from exoplanets and unpolarized, scattered starlight and to characterize properties of exoplanet atmospheres and surfaces that cannot be determined from intensity observations alone. EPOL consists of a coronagraph and a dual-beam polarimeter with a liquid-crystal retarder to exchange the polarization of the two beams. The polarimetry thereby increases the contrast between star and exoplanet by 3 to 5 orders of magnitude over what the extreme adaptive optics and the EPOL coronagraph alone can achieve. EPOL operates between 600 and 900 nm, can select more specific wavelength bands with filters and aims at having an integral field unit to obtain linearly polarized spectra of known exoplanets. We present the conceptual design of EPOL along with an analysis of its performance. Title: Imaging polarimetry of protoplanetary disks: feasibility and usability Authors: Min, M.; Jeffers, S. V.; Rodenhuis, M.; Canovas, H.; Buenzli, E.; Keller, C. U.; Waters, L. B. F. M.; Dominik, C. Bibcode: 2010lyot.confE..34M Altcode: Imaging polarimetry is one of the most promising tools to map the structure of faint protoplanetary disks. In this contribution we discuss the feasibility of imaging polarimetry of protoplanetary disks and the usability to answer the scientific questions in the field. From the theoretical side we do this by simulations of disks of various geometries and dust properties. We model the expected signal and detailed predictions for current and upcoming imaging polarimeters. This way we can address the question what the diagnostic value of polarimetry is for the structure of the disk and the characteristics of the grains in it. We compare extremely fluffy aggregated grains and compact homogeneous grains and show that their expected signal is significantly different. In combination with infrared/mm observations this could allow us to obtain grain properties in addition to mapping of the disk geometry. From the observational side we address the issues by discussing some of the early results from the Extreme Polarimeter (ExPo). ExPo is a sensitive imaging polarimeter designed to be a pathfinding instrument for the large imaging polarimetry projects planned for the VLT and the ELT. Already it proves to be a pioneering instrument in the field of imaging polarimetry of circumstellar matter. Title: Observations of solar scattering polarization at high spatial resolution Authors: Snik, F.; de Wijn, A. G.; Ichimoto, K.; Fischer, C. E.; Keller, C. U.; Lites, B. W. Bibcode: 2010A&A...519A..18S Altcode: 2010arXiv1005.5042S Context. The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain.
Aims: We aim to measure the difference in scattering polarization between granules and intergranules. We present the respective center-to-limb variations, which may serve as input for future models.
Methods: We perform full Stokes filter polarimetry at different solar limb positions with the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents the first scattering polarization observations with sufficient spatial resolution to discern the granulation. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We extend the instrumental polarization calibration routine to the observing wavelength, and correct for various systematic effects.
Results: The scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules. We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8±3.0)% less scattering polarization for 0.2 < μ ≤ 0.3, although systematic effects cannot be completely excluded.
Conclusions: These observations constrain MHD models in combination with (polarized) radiative transfer in terms of CN band line formation, radiation anisotropy, and magnetic fields. Title: The ZIMPOL high-contrast imaging polarimeter for SPHERE: design, manufacturing, and testing Authors: Roelfsema, Ronald; Schmid, Hans Martin; Pragt, Johannes; Gisler, Daniel; Waters, Rens; Bazzon, Andreas; Baruffolo, Andrea; Beuzit, Jean-Luc; Boccaletti, Anthony; Charton, Julien; Cumani, Claudio; Dohlen, Kjetil; Downing, Mark; Elswijk, Eddy; Feldt, Markus; Groothuis, Charlotte; de Haan, Menno; Hanenburg, Hiddo; Hubin, Norbert; Joos, Franco; Kasper, Markus; Keller, Christoph; Kragt, Jan; Lizon, Jean-Louis; Mouillet, David; Pavlov, Aleksej; Rigal, Florence; Rochat, Sylvain; Salasnich, Bernardo; Steiner, Peter; Thalmann, Christian; Venema, Lars; Wildi, François Bibcode: 2010SPIE.7735E..4BR Altcode: 2010SPIE.7735E.144R ZIMPOL is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2011. ZIMPOL is currently in the manufacturing, integration and testing phase. We describe the optical, polarimetric, mechanical, thermal and electronic design as well as the design trade offs. Specifically emphasized is the optical quality of the key performance component: the Ferro-electric Liquid Crystal polarization modulator (FLC). Furthermore, we describe the ZIMPOL test setup and the first test results on the achieved polarimetric sensitivity and accuracy. These results will give first indications for the expected overall high contrast system performance. SPHERE is an instrument designed and built by a consortium consisting of LAOG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO. Title: SPEX: the spectropolarimeter for planetary exploration Authors: Snik, Frans; Rietjens, Jeroen H. H.; van Harten, Gerard; Stam, Daphne M.; Keller, Christoph U.; Smit, J. Martijn; Laan, Erik C.; Verlaan, Ad L.; Ter Horst, Rik; Navarro, Ramón; Wielinga, Klaas; Moon, Scott G.; Voors, Robert Bibcode: 2010SPIE.7731E..1BS Altcode: 2010SPIE.7731E..34S SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact instrument for spectropolarimetry, and in particular for detecting and characterizing aerosols in planetary atmospheres. With its ~1-liter volume it is capable of full linear spectropolarimetry, without moving parts. The degree and angle of linear polarization of the incoming light is encoded in a sinusoidal modulation of the intensity spectrum by an achromatic quarter-wave retarder, an athermal multiple-order retarder and a polarizing beam-splitter in the entrance pupil. A single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization. Polarimetry has proven to be an excellent tool to study microphysical properties (size, shape, composition) of atmospheric particles. Such information is essential to better understand the weather and climate of a planet. The current design of SPEX is tailored to study Martian dust and ice clouds from an orbiting platform: a compact module with 9 entrance pupils to simultaneously measure intensity spectra from 400 to 800 nm, in different directions along the flight direction (including two limb viewing directions). This way, both the intensity and polarization scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying over it. We describe the optical and mechanical design of SPEX, and present performance simulations and initial breadboard measurements. Several flight opportunities exist for SPEX throughout the solar system: in orbit around Mars, Jupiter and its moons, Saturn and Titan, and the Earth. Title: EPICS: direct imaging of exoplanets with the E-ELT Authors: Kasper, Markus; Beuzit, Jean-Luc; Verinaud, Christophe; Gratton, Raffaele G.; Kerber, Florian; Yaitskova, Natalia; Boccaletti, Anthony; Thatte, Niranjan; Schmid, Hans Martin; Keller, Christoph; Baudoz, Pierre; Abe, Lyu; Aller-Carpentier, Emmanuel; Antichi, Jacopo; Bonavita, Mariangela; Dohlen, Kjetil; Fedrigo, Enrico; Hanenburg, Hiddo; Hubin, Norbert; Jager, Rieks; Korkiakoski, Visa; Martinez, Patrice; Mesa, Dino; Preis, Olivier; Rabou, Patrick; Roelfsema, Ronald; Salter, Graeme; Tecza, Mathias; Venema, Lars Bibcode: 2010SPIE.7735E..2EK Altcode: 2010SPIE.7735E..81K Presently, dedicated instruments at large telescopes (SPHERE for the VLT, GPI for Gemini) are about to discover and explore self-luminous giant planets by direct imaging and spectroscopy. The next generation of 30m-40m ground-based telescopes, the Extremely Large Telescopes (ELTs), have the potential to dramatically enlarge the discovery space towards older giant planets seen in reflected light and ultimately even a small number of rocky planets. EPICS is a proposed instrument for the European ELT, dedicated to the detection and characterization of Exoplanets by direct imaging, spectroscopy and polarimetry. ESO completed a phase-A study for EPICS with a large European consortium which - by simulations and demonstration experiments - investigated state-of-the-art diffraction and speckle suppression techniques to deliver highest contrasts. The paper presents the instrument concept and analysis as well as its main innovations and science capabilities. EPICS is capable of discovering hundreds of giant planets, and dozens of lower mass planets down to the rocky planets domain. Title: EPOL: the exoplanet polarimeter for EPICS at the E-ELT Authors: Keller, Christoph U.; Schmid, Hans Martin; Venema, Lars B.; Hanenburg, Hiddo; Jager, Rieks; Kasper, Markus; Martinez, Patrice; Rigal, Florence; Rodenhuis, Michiel; Roelfsema, Ronald; Snik, Frans; Verinaud, Christophe; Yaitskova, Natalia Bibcode: 2010SPIE.7735E..6GK Altcode: 2010SPIE.7735E.212K EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging polarimetry to differentiate between linearly polarized light from exoplanets and unpolarized, scattered starlight and to characterize properties of exoplanet atmospheres and surfaces that cannot be determined from intensity observations alone. EPOL consists of a coronagraph and a dual-beam polarimeter with a liquid-crystal retarder to exchange the polarization of the two beams. The polarimetry thereby increases the contrast between star and exoplanet by 3 to 5 orders of magnitude over what the extreme adaptive optics and the EPOL coronagraph alone can achieve. EPOL operates between 600 and 900 nm, can select more specific wavelength bands with filters and aims at having an integral field unit to obtain linearly polarized spectra of known exoplanets. We present the conceptual design of EPOL along with an analysis of its performance. Title: The polarization optics for the European Solar Telescope (EST) Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.; Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.; Snik, F.; Socas-Navarro, H. Bibcode: 2010SPIE.7735E..6IB Altcode: 2010SPIE.7735E.214B EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST will be located at the Canary Islands and aims at observations with the best possible spectral, spatial and temporal resolution and best polarimetric performance, of the solar photosphere and chromosphere, using a suite of instruments that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over many scale heights, and ranging from λ=350 until 2300 nm. In order to be able to fulfill the stringent requirements for polarimetric sensitivity and accuracy, from the very beginning the polarimetry has been included in the design work. The overall philosophy has been to use a combination of techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters, differential measurement schemes, fast modulation and demodulation, and accurate calibration. The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and nonvarying in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system in which eigenvectors can travel undisturbed through the system. Title: Prediction of underground argon content for dark matter experiments Authors: Mei, D. -M.; Yin, Z. -B.; Spaans, J.; Koppang, M.; Hime, A.; Keller, C.; Gehman, V. M. Bibcode: 2010PhRvC..81e5802M Altcode: 2009arXiv0912.5368M In this paper, we demonstrate the use of physical models to evaluate the production of Ar39 and Ar40 underground. Considering both cosmogenic Ar39 production and radiogenic Ar40 production in situ and from external sources, we can derive the ratio of Ar39 to Ar40 in underground sources. We show for the first time that the Ar39 production underground is dominated by stopping negative muon capture on K39 and (α,n) induced subsequent K39(n,p)Ar39 reactions. The production of Ar39 is shown as a function of depth. We demonstrate that argon depleted in Ar39 can be obtained only if the depth of the underground resources is greater than 500 m.w.e. below the surface. Stopping negative muon capture on K39 dominates over radiogenic production at depths of less than 2000 m.w.e., and that production by muon-induced neutrons is subdominant at any depth. The depletion factor depends strongly on both radioactivity level and potassium content in the rock. We measure the radioactivity concentration and potassium concentration in the rock for a potential site of an underground argon source in South Dakota. Depending on the probability of Ar39 and Ar40 produced underground being dissolved in the water, the upper limit of the concentration of Ar39 in the underground water at this site is estimated to be in a range of a factor of 1.6 to 155 less than the Ar39 concentration in the atmosphere. The calculation tools presented in this paper are also critical to the dating method with Ar39. Title: Inversions of High-Cadence SOLIS-VSM Stokes Observations Authors: Fischer, C. E.; Keller, C. U.; Snik, F. Bibcode: 2010ASSP...19..515F Altcode: 2010mcia.conf..515F We have processed full-Stokes observations made with the SOLIS-VSM using Fe I 630.15 and Fe I 630.25 nm. The data have high spectral and temporal resolution, moderate spatial resolution, and large polarimetric sensitivity and accuracy. We use the code LILIA, an LTE inversion code written by Socas-Navarro (2001), in order to invert the data in vector magnetic fields. The 180-degree ambiguity in magnetic field orientation is solved by using the Non-Potential Field Calculation (NPFC) method of Georgoulis (2005). The output product are maps of the fullmagnetic field vector at the photospheric level, as illustrated in Fig. 1. We performed such inversions for observations of active region NOAA 10808 taken during an X-class flare in September 2005. Details of the data processing and the first results are given in the proceedings of the Fifth Solar PolarizationWorkshop (ASP Conf. Ser., in press). Title: EPICS, the exoplanet imager for the E-ELT Authors: Kasper, M.; Beuzit, J. -L.; Verinaud, C.; Baudoz, P.; Boccaletti, A.; Gratton, R.; Keller, C.; Kerber, F.; Schmid, H. M.; Thatte, N.; Venema, L.; Yaitskova, N. Bibcode: 2010aoel.confE2009K Altcode: Very soon, dedicated instruments developments at large telescopes (SPHERE for the VLT, GPI for Gemini) are about to discover and explore self-luminous giant planets by direct imaging and spectroscopy in significant numbers. The next generation of 30m-40m ground-based telescopes, the Extremely Large Telescopes (ELTs), have the potential to dramatically enlarge the discovery space towards older giant planets seen in reflected light and ultimately even a small number of rocky planets. EPICS is a proposed instrument for the European ELT, dedicated to the detection and characterization of expolanets by direct imaging and spectroscopy. EPICS is currently mid-way through a phase-A study carried out by a large European consortium which - by simulations and demonstration experiments - will investigate state-of-the-art diffraction and speckle suppression techniques to deliver highest contrasts. The final result of the study early 2010 will be a conceptual design and a development plan for the instrument. We will present the EPICS concept including the performance analysis and first results from prototyping experiments and discuss the main challenges and science capabilities of EPICS. Title: Two Ways of Improving Stokes Inversions Authors: Becher, H. -M.; Fischer, C.; Keller, C. Bibcode: 2009ASPC..415..160B Altcode: Performing Stokes inversion on single pixels can be difficult due to seeing and instrumental smearing. We present two different techniques to improve Stokes inversions. The first method, REGIONS, is a code that finds pixels that are likely to contain signal from the same magnetic element. The second method, mCLEAN, reconstructs a magnetogram if the point spread function (PSF) of the instrument and seeing is known. Title: Statistics of Convective Collapse Events in the Photosphere and Chromosphere Observed with the HINODE SOT Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.; Keller, C. U. Bibcode: 2009ASPC..415..127F Altcode: Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we observed 49 such events and present a statistical analysis of convective collapse events. Our data sets consist of high resolution time series of polarimetric spectral scans of two iron lines formed in the lower photosphere and filter images in Mg I b2 and Ca II H. We were thus able to study the implication of convective collapse events on the high photospheric and the chromospheric layers. The physical parameters from the full Stokes profiles were obtained with the MERLIN Milne-Eddington inversion code. For each of the 49 events we determined the duration, maximum photospheric downflow, and field strength increase. We found event durations of about 10 minutes and field strengths of up to 1.65 kG. Title: Statistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOT Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.; Keller, C. U. Bibcode: 2009A&A...504..583F Altcode: 2009arXiv0906.2308F Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008, ApJ, 677, L145) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we observed 49 such events and present a statistical analysis of convective collapse events. Our data sets consist of high resolution time series of polarimetric spectral scans of two iron lines formed in the lower photosphere and filter images in Mg I b{2} and Ca II H, spectral lines that are formed in the high photosphere and the lower chromosphere, respectively. We were thus able to study the implication of convective collapse events on the high photospheric and the chromospheric layers. We found that in all cases, the event was accompanied by a continuum bright point and nearly always by a brightening in the Ca II H images. The magnesium dopplergram exhibits a strong downflow in about three quarters of the events that took place within the field of view of the magnesium dopplergram. The physical parameters from the full Stokes profiles were obtained with the MERLIN Milne-Eddington inversion code. For each of the 49 events we determined the duration, maximum photospheric downflow, field strength increase and size. We found event durations of about 10 min, magnetic element radii of about 0.43 arcsec and 0.35 arcsec, before and after the event, respectively, and field strengths of up to 1.65 kG. Title: The case for spectropolarimetry with SPEX on EJSM Authors: Stam, D. M.; Smit, J. M.; Snik, F.; Keller, C. U. Bibcode: 2009epsc.conf..536S Altcode: No abstract at ADS Title: Polarimetric Measurements of Protoplanetary Disks with ExPo Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Keller, C. U. Bibcode: 2009AIPC.1158..381C Altcode: Polarimetry is a powerful tool for detecting and characterizing exoplanets and protoplanetary disks as light scattered from circumstellar material is linearly polarized. We present the first light results of ExPo (Extreme Polarimeter) [1], a sensitive imaging polarimeter developed at Utrecht University that works in the visible part of the spectrum. Our first light observations at the 4.2-meter William Herschel Telescope (WHT) show that ExPo can reach the high-contrast ratios that are necessary to observe protoplanetary disks. We present images of the protoplanetary disks around the Herbig Ae star AB Aurigae and the T Tauri star SU Aurigae. Our results show the power of polarimetry for future projects e.g. the ZIMPOL arm of SPHERE, and the EPOL part of EPICS. Title: An IFU for diffraction-limited 3D spectroscopic imaging: laboratory and on-site tests Authors: Ren, Deqing; Keller, Christoph; Plymate, Claude Bibcode: 2009SPIE.7438E..18R Altcode: 2009SPIE.7438E..35R We have developed a state-of-the-art image slicer Integral Field Unit (IFU) for the McMath-Pierce Solar Telescope (McMP) located at Kitt Peak National Solar Observatory. The IFU will be used for high-resolution 3-dimensional spectroscopy and polarimetry over a small field of view that is well corrected by adaptive optics. It consists of 19 effective slices that correspond to a field of view of 6.27"x 7". The IFU delivers a 152" long slit to an existing spectrograph producing diffraction-limited 3-dimensional spectroscopy. The 3-D instrument is being used for highspatial and high-temporal resolution imaging of the Sun, which is crucial for the magnetic field and spectroscopic studies of 2-dimensional solar fine structures. We discuss the instrument construction, laboratory test and on-site trial observations with the McMP. Title: A Brief History of the Second Solar Spectrum Authors: Keller, C. U. Bibcode: 2009ASPC..405...29K Altcode: The development of the ZIMPOL principle at ETH Zurich led to a dramatic increase in our observational knowledge of the solar scattering polarization spectrum, now known as the Second Solar Spectrum. This contribution summarizes the historical developments of this field with a special focus on the observational events between 1994 and 1996 when many interesting things happened that cannot be found in the literature. Title: Bioaccumulated manganese and nickel in endemic plants from New Caledonian mining soils Authors: Pouschat, P.; Rose, J.; Alliot, I.; Dominici, C.; Keller, C.; Olivi, L.; Rabier, J.; Ambrosi, J. -P. Bibcode: 2009GeCAS..73R1049P Altcode: No abstract at ADS Title: Polarimetry from the Ground Up Authors: Keller, C. U.; Snik, F. Bibcode: 2009ASPC..405..371K Altcode: 2008arXiv0809.2772K Ground-based solar polarimetry has made great progress over the last decade. Nevertheless, polarimetry is still an afterthought in most telescope and instrument designs, and most polarimeters are designed based on experience and rules of thumb rather than using more formal systems engineering approaches as is common in standard optical design efforts. Here we present the first steps in creating a set of systems engineering approaches to the design of polarimeters that makes sure that the final telescope-instrument-polarimeter system is more than the sum of its parts. Title: The Prototype of the Small Synoptic Second Solar Spectrum Telescope (S^5T) Authors: Snik, F.; Melich, R.; Keller, C. U. Bibcode: 2009ASPC..405..383S Altcode: 2009arXiv0903.2730S We present the design and the prototype of the Small Synoptic Second Solar Spectrum Telescope (S^5T), which can autonomously measure scattering polarization signals on a daily basis with large sensitivity and accuracy. Its data will be used to investigate the nature of weak, turbulent magnetic fields through the Hanle effect in many lines. Also the relation between those fields and the global solar dynamo can be revealed by spanning the observations over a significant fraction of a solar cycle. The compact instrument concept is enabled by a radial polarization converter that allows for ``one-shot'' polarimetry over the entire limb of the Sun. A polarimetric sensitivity of ∼10-5 is achieved by minimizing the instrumental polarization and by FLC modulation in combination with a fast line-scan camera in the fiber-fed spectrograph. The first prototype results successfully show the feasibility of the concept. Title: Vector Magnetic Field Inversions of High Cadence SOLIS-VSM Data Authors: Fischer, C. E.; Keller, C. U.; Snik, F. Bibcode: 2009ASPC..405..311F Altcode: We have processed full Stokes observations from the SOLIS VSM in the photospheric lines Fe I 630.15 nm and 630.25 nm. The data sets have high spectral and temporal resolution, moderate spatial resolution, and large polarimetric sensitivity and accuracy. We used the LILIA, an LTE code written by \citet{fischer_Navarro2001} to invert the data. We also applied the non-potential magnetic field calculation method of \citet{fischer_Manolis2005} in order to resolve the 180 degree ambiguity. The output are maps of the full magnetic field vector at the photospheric level. Here we present the first inversions of the active region NOAA 10808 during an X-class flare, which occurred on 13 September 2005. Title: SOLIS Vector Spectromagnetograph: Status and Science Authors: Henney, C. J.; Keller, C. U.; Harvey, J. W.; Georgoulis, M. K.; Hadder, N. L.; Norton, A. A.; Raouafi, N. -E.; Toussaint, R. M. Bibcode: 2009ASPC..405...47H Altcode: 2008arXiv0801.0013H The Vector Spectromagnetograph (VSM) instrument has recorded photospheric and chromospheric magnetograms daily since August 2003. Full-disk photospheric vector magnetograms are observed at least weekly and, since November 2006, area-scans of active regions daily. Quick-look vector magnetic images, plus X3D and FITS formated files, are now publicly available daily. In the near future, Milne-Eddington inversion parameter data will also be available and a typical observing day will include three full-disk photospheric vector magnetograms. Besides full-disk observations, the VSM is capable of high temporal cadence area-scans of both the photosphere and chromosphere. Carrington rotation and daily synoptic maps are also available from the photospheric magnetograms and coronal hole estimate images. Title: Polarization Properties of Real Aluminum Mirrors, I. Influence of the Aluminum Oxide Layer Authors: van Harten, G.; Snik, F.; Keller, C. U. Bibcode: 2009PASP..121..377V Altcode: 2009arXiv0903.2740V In polarimetry, it is important to characterize the polarization properties of the instrument itself to disentangle real astrophysical signals from instrumental effects. This article deals with the accurate measurement and modeling of the polarization properties of real aluminum mirrors, as used in astronomical telescopes. Main goals are the characterization of the aluminum oxide layer thickness at different times after evaporation, and its influence on the polarization properties of the mirror. The full polarization properties of an aluminum mirror are measured with Mueller matrix ellipsometry at different incidence angles and wavelengths. The best fit of theoretical Mueller matrices to all measurements simultaneously is obtained by taking into account a model of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller matrix measurements of a mirror are obtained with an absolute accuracy of ∼1% after calibration. The determined layer thicknesses indicate logarithmic growth in the first few hours after evaporation, but stability at a value of 4.12 ± 0.08 nm in the long term. Although the aluminum oxide layer is established to be thin, it is necessary to consider it to accurately describe the mirror’s polarization properties. Title: Super earth explorer: a coronagraphic off-axis space telescope Authors: Schneider, J.; Boccaletti, A.; Mawet, D.; Baudoz, P.; Beuzit, J. -L.; Doyon, R.; Marley, M.; Stam, D.; Tinetti, G.; Traub, W.; Trauger, J.; Aylward, A.; Cho, J. Y. -K.; Keller, C. -U.; Udry, S.; SEE-COAST Team Bibcode: 2009ExA....23..357S Altcode: 2008arXiv0811.3908S The Super-Earth Explorer is an Off-Axis Space Telescope (SEE-COAST) designed for high contrast imaging. Its scientific objective is to make the physico-chemical characterization of exoplanets possibly down to 2 Earth radii. For that purpose it will analyze the spectral and polarimetric properties of the parent starlight reflected by the planets, in the wavelength range 400-1,250 nm. Title: An analytical model to demonstrate the reliability of reconstructed `active longitudes'. Authors: Jeffers, S. V.; Keller, C. U. Bibcode: 2009AIPC.1094..664J Altcode: 2009csss...15..664J Photometric light curves show apparent spot concentrations separated by 180 degrees in longitude that are commonly referred to as `active longitudes'. These spot concentrations have been observed to change in strength resulting in the `flip-flop' effect. We use a simple analytical model to calculate the light curve of a star with an arbitrary spot pattern to show that `active longitudes' are a likely consequence of the limited information content contained in a light curve. We also show that the same effects apply to heavily spotted stars. Title: Characterization of Extra-solar Planets with Direct-Imaging Techniques Authors: Tinetti, Giovanna; Cash, Webster; Glassman, Tiffany; Keller, C.; Oakley, Phil; Snik, F.; Stam, Daphne; Turnbull, Margret Bibcode: 2009astro2010S.296T Altcode: No abstract at ADS Title: Polarimetry of Mars with SPEX, an Innovative Spectropolarimeter Authors: Stam, D. M.; Laan, E.; Snik, F.; Karalidi, T.; Keller, C.; Ter Horst, R.; Navarro, R.; Aas, C.; de Vries, J.; Oomen, G.; Hoogeveen, R. Bibcode: 2008LPICo1447.9078S Altcode: We present SPEX, an innovative, compact, and robust spectropolarimeter that measures fluxes and polarization of sunlight reflected by Mars from 400 to 800 nm. With simulations we'll show how with SPEX atmospheric dust and the surface can be studied. Title: A New Era in Solar Thermal-IR Astronomy: the NSO Array Camera (NAC) on the McMath-Pierce Telescope Authors: Ayres, T.; Penn, M.; Plymate, C.; Keller, C. Bibcode: 2008ESPM...12.2.74A Altcode: The U.S. National Solar Observatory Array Camera (NAC) is a cryogenically cooled 1Kx1K InSb ``Aladdin" array that recently became operational at the McMath-Pierce facility on Kitt Peak, a high dry site in the southwest U.S. (Arizona). The new camera is similar to those already incorporated into instruments on nighttime telescopes, and has unprecedented sensitivity, low noise, and excellent cosmetics compared with the Amber Engineering (AE) device it replaces. (The latter was scavenged from a commercial surveillance camera in the 1990's: only 256X256 format, high noise, and annoying flatfield structure). The NAC focal plane is maintained at 30 K by a mechanical closed-cycle helium cooler, dispensing with the cumbersome pumped--solid-N2 40 K system used previously with the AE camera. The NAC linearity has been verified for exposures as short as 1 ms, although latency in the data recording holds the maximum frame rate to about 8 Hz (in "streaming mode"). The camera is run in tandem with the Infrared Adaptive Optics (IRAO) system. Utilizing a 37-actuator deformable mirror, IRAO can--under moderate seeing conditions--correct the telescope image to the diffraction limit longward of 2.3 mu (if a suitable high contrast target is available: the IR granulation has proven too bland to reliably track). IRAO also provides fine control over the solar image for spatial scanning in long-slit mode with the 14 m vertical "Main" spectrograph (MS). A 1'X1' area scan, with 0.5" steps orthogonal to the slit direction, requires less than half a minute, much shorter than p-mode and granulation evolution time scales. A recent engineering test run, in April 2008, utilized NAC/IRAO/MS to capture the fundamental (4.6 mu) and first-overtone (2.3 mu) rovibrational bands of CO, including maps of quiet regions, drift scans along the equatorial limbs (to measure the off-limb molecular emissions), and imaging of a fortuitous small sunspot pair, a final gasp, perhaps, of Cycle 23. Future work with the NAC will emphasize pathfinding toward the next generation of IR imaging spectrometers for the Advanced Technology Solar Telescope, whose 4 m aperture finally will bring sorely needed high spatial resolution to daytime infrared astronomy. In the meantime, the NAC is available to qualified solar physicists from around the world to conduct forefront research in the 1-5 mu region, on the venerable--but infrared friendly--McMath-Pierce telescope. Title: The upgrade of HARPS to a full-Stokes high-resolution spectropolarimeter Authors: Snik, Frans; Jeffers, Sandra; Keller, Christoph; Piskunov, Nikolai; Kochukhov, Oleg; Valenti, Jeff; Johns-Krull, Christopher Bibcode: 2008SPIE.7014E..0OS Altcode: 2008SPIE.7014E..22S We present the design of a compact module that converts the HARPS instrument at the 3.6-m telescope at La Silla to a full-Stokes high-resolution spectropolarimeter. The polarimeter will replace the obsolete Iodine cell inside the HARPS Cassegrain adapter. Utilizing the two fibers going into the spectrograph, two dual-beam systems can be positioned in the beam: one with a rotating superachromatic quarter-wave plate for circular polarimetry and one with a rotating superachromatic half-wave plate for linear polarimetry. A large polarimetric precision is ensured by the beam-exchange technique and a minimal amount of instrumental polarization. The polarimeter, in combination with the ultra-precise HARPS spectrograph, enables unprecedented observations of stellar magnetic fields and circumstellar material without compromising the successful planet-finding program. Title: SPHERE ZIMPOL: overview and performance simulation Authors: Thalmann, Christian; Schmid, Hans M.; Boccaletti, Anthony; Mouillet, David; Dohlen, Kjetil; Roelfsema, Ronald; Carbillet, Marcel; Gisler, Daniel; Beuzit, Jean-Luc; Feldt, Markus; Gratton, Raffaele; Joos, Franco; Keller, Christoph U.; Kragt, Jan, II; Pragt, Johan H.; Puget, Pascal; Rigal, Florence; Snik, Frans; Waters, Rens; Wildi, François Bibcode: 2008SPIE.7014E..3FT Altcode: 2008SPIE.7014E.112T The ESO planet finder instrument SPHERE will search for the polarimetric signature of the reflected light from extrasolar planets, using a VLT telescope, an extreme AO system (SAXO), a stellar coronagraph, and an imaging polarimeter (ZIMPOL). We present the design concept of the ZIMPOL instrument, a single-beam polarimeter that achieves very high polarimetric accuracy using fast polarization modulation and demodulating CCD detectors. Furthermore, we describe comprehensive performance simulations made with the CAOS problem-solving environment. We conclude that direct detection of Jupiter-sized planets in close orbit around the brightest nearby stars is achievable with imaging polarimetry, signal-switching calibration, and angular differential imaging. Title: The Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeter Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Keller, C. U. Bibcode: 2008SPIE.7014E..6TR Altcode: 2008SPIE.7014E.227R The Extreme Polarimeter (ExPo) is approaching its first deployment at the 4.2 m William Herschel Telescope at La Palma. This imaging polarimeter, developed at the Astronomical Institute of Utrecht University, aims to study circumstellar material at a contrast ratio with the central star of 10-9. Working at visible wavelengths, it will provide an inner working angle down to 0.5 arcsec and a field of view of 20 arcsec diameter. ExPo employs a dual beam-exchange technique based on polarimeter designs for solar studies. A partially transmitting coronagraph mask placed in the first focus reduces the light of the star. The beam is modulated using three ferro-electric liquid crystals in a Pancharatnam configuration, then split in a polarizing beamsplitter. Both beams are re-imaged onto the same Electron-Multiplying CCD camera. We present the design of the ExPo instrument, highlighting the elements that are critical to the polarimetric performance. Some prototype laboratory experiments demonstrating the instrument concept are discussed. These have been performed using our realistic exoplanet laboratory simulator. Title: Design of a laboratory simulator to test exoplanet imaging polarimetry Authors: Jeffers, S. V.; Miesen, N.; Rodenhuis, M.; Keller, C. U. Bibcode: 2008SPIE.7014E..7BJ Altcode: 2008SPIE.7014E.239J Research on extrasolar planets is one of the most rapidly advancing fields of astrophysics. In just over a decade since the discovery of the first extra-solar planet orbiting around 51 Pegasi, 289 extrasolar planets have been discovered. This breakthrough is the result of the development of a wide range of new observational techniques and facilities for the detection and characterisation of extrasolar planets. In Utrecht we are building the Extreme Polarimeter (ExPo) to image extra-solar planets and circumstellar environments using polarimetry at contrast ratio of 10-9. To test and calibrate ExPo, we have built a laboratory-based simulator that mimics a star with a Jupiter-like exoplanet as seen by the 4.2m William Herschel Telescope. The star and planet are simulated using two single-mode fibres in close proximity that are fed with a broadband arc lamp with a contrast ratio down to 10-9. The planet is partially linearly polarized. The telescope is simulated with two lenses, and seeing can be included with a rotating glass plate covered with hairspray. In this paper we present the scientific requirements and the simulator design. Title: SPEX: an in-orbit spectropolarimeter for planetary exploration Authors: Snik, Frans; Karalidi, Theodora; Keller, Christoph; Laan, Erik; ter Horst, Rik; Navarro, Ramon; Stam, Daphne; Aas, Christina; de Vries, Johan; Oomen, Gijs; Hoogeveen, Ruud Bibcode: 2008SPIE.7010E..15S Altcode: 2008SPIE.7010E..35S SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact remote-sensing instrument for detecting and characterizing aerosols. With its 1-liter volume it is capable of full linear spectropolarimetry, without moving parts. High precision polarimetry is performed through encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the intensity spectrum. This is achieved by using an achromatic quarter-wave retarder, an athermal multiple-order retarder and a polarizing beamsplitter behind each entrance pupil. Measuring a single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization. Polarimetry has proven to be an excellent tool to study microphysical properties (size, shape, composition) of atmospheric particles. Such information is essential to better understand the weather and climate of a planet. Although SPEX can be used to study any planetary atmosphere, including the Earth's, the current design of SPEX is tailored to study Martian dust and ice clouds from an orbiting platform: a compact module with 9 entrance pupils to simultaneously measure intensity spectra from 350 to 800 nm, in different directions along the flight direction (including two limb viewing directions). This way, both the intensity and polarization scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying over it. In the absence of significant amounts of dust and clouds, the surface properties can be studied. SPEX provides synergy with instruments on rovers and landers, as it provides a global view of spatial and temporal variations of the planet. Title: New Observations of the Magnetic Vector Field across the Solar Disk Authors: Keller, C. U.; Harvey, J. W.; Henney, C. J. Bibcode: 2008ASPC..384..166K Altcode: 2008csss...14..166K Full disk solar magnetograms have been available for more than three decades. However, those maps only show the line-of-sight magnetic flux. The physical quantity we really want to know is the magnetic field vector along with the filling factor, i.e. the fractional area of the resolution element that is occupied by the magnetic field. Since August 2003, the SOLIS Vector-SpectroMagnetograph has been recording the photospheric magnetic field vector across the full solar disk with high sensitivity and resolution. Some of the data are now becoming available for beta testing, and the first science results are emerging. Here we provide a brief introduction to the instrument and its data products and then present some of the data from the first three years of operation. Title: Science Goals of the Extreme Polarimeter (ExPo) Authors: Jeffers, S. V.; Keller, C. U.; Rodenhuis, M.; Miesen, N. Bibcode: 2007lyot.confE..42J Altcode: To advance our understanding of the formation, evolution and structure of extra-solar planetary systems we are building a high-precision imaging polarimeter (ExPo). ExPo will initially be located at the 4.2m William Herschel Telescope on La Palma. We will use polarimetric techniques similar to those developed for high-precision solar polarimetry to reach a sensitivity of 10^-5, to polarimetrically image and characterize planets and protoplanetary debris discs. I will present a review of the proposed data analysis techniques and science goals that will be achievable using the significant improvement in polarimetric imaging capabilities. Title: Design Options for the Extreme Polarimeter (ExPo) Authors: Rodenhuis, M.; Keller, C. U. Bibcode: 2007lyot.confQ..43R Altcode: The Extreme Polarimeter, ExPo, is being developed for the detailed study of circumstellar disks and exoplanet characterization at the 4.2-m William Herschel Telescope at La Palma. This imaging polarimeter is designed to measure linear polarization at the 10-5 level around bright stars at distances outward of about 0.5 arcsec. We will discuss the design options for this instrument as well as the advantages and disadvantages of specific components such as the Atmospheric Dispersion Compensator, the polarising beamsplitting element, the coronograph mask, and the polarisation modulator based on either ferroelectric or nematic liquid crystals. The merits and disadvantages of each of these will be discussed along with the impact of a certain choice on the overall instrument performance. Finally, an analysis of several off-the-shelf scientific cameras for the actual image recording will be presented. Title: Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere Authors: Harvey, J. W.; Branston, D.; Henney, C. J.; Keller, C. U.; SOLIS Team; GONG Team Bibcode: 2007AAS...21010001H Altcode: 2007BAAS...39S.227H The photospheric magnetic field outside of active regions and the network has a ubiquitous and dynamic line-of-sight component that strengthens away from disk center as expected for a nearly horizontal orientation. This component shows a striking time variation with an average temporal rms of 1.7 G at 3 arcsec resolution. In our moderate resolution observations the nearly horizontal component has a frequency variation power law exponent of -1.4 below 1.5 mHz and is spatially patchy on scales up to 15 arcsec.

The field may be a manifestation of changing magnetic connections between eruptions and evolution of small magnetic flux elements in response to convective motions. It shows no detectable latitude or longitude variations. Preliminary observations show no evidence of this field in the 854.2 nm chromosphere. Title: Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere Authors: Harvey, J. W.; Branston, D.; Henney, C. J.; Keller, C. U.; SOLIS and GONG Teams Bibcode: 2007ApJ...659L.177H Altcode: 2007astro.ph..2415H The photospheric magnetic field outside of active regions and the network has a ubiquitous and dynamic line-of-sight component that strengthens from disk center to limb as expected for a nearly horizontal orientation. This component shows a striking time variation with an average temporal rms near the limb of 1.7 G at ~3" resolution. In our moderate-resolution observations the nearly horizontal component has a frequency variation power-law exponent of -1.4 below 1.5 mHz and is spatially patchy on scales up to ~15". The field may be a manifestation of changing magnetic connections between eruptions and evolution of small magnetic flux elements in response to convective motions. It shows no detectable latitude or longitude variations. Title: The Ba II 4554 / Hβ Imaging Polarimeter for the Dutch Open Telescope Authors: Snik, F.; Bettonvil, F. C. M.; Jägers, A. P. L.; Hammerschlag, R. H.; Rutten, R. J.; Keller, C. U. Bibcode: 2006ASPC..358..205S Altcode: In order to expand the high-resolution, multi-wavelength imaging capabilities of the Dutch Open Telescope (DOT), an additional polarimetric channel based on a 80 mÅ tunable Lyot filter for Ba II 4554 and Hβ has been designed and constructed. The large atomic mass and the resulting steep line wings, make Ba II 4554 particularly suitable for the creation of photospheric Dopplergrams and Stokes-V magnetograms. The line also yields a significant degree of linear (scattering) polarization for observations near the limb of the Sun, which is modified by both horizontal and vertical weak-field topologies through the Hanle effect and hyperfine-structure level crossing. The polarimeter is based on liquid crystal variable retarders (LCVRs) as polarization modulators in combination with the Lyot filter's entrance polarizer. The tunability of the LCVRs is exploited to enable specific wavelength calibration, selection of the reference frame of linear polarization, and optimization of instrumental polarization cross-talk, which for the DOT is constant in time. With the future Ba II 4554 photospheric magnetograms, we expect to be able to discern magnetic structures of about 150 km with field strengths down to 100 G, and that Hanle-type observations can be performed at a resolution of about 1 arcsec. The range of applicability of Hβ imaging polarimetry has to be explored after installation. Title: SOLIS-VSM Solar Vector Magnetograms Authors: Henney, C. J.; Keller, C. U.; Harvey, J. W. Bibcode: 2006ASPC..358...92H Altcode: 2006astro.ph.12584H The Vector SpectroMagnetograph (VSM) instrument has recorded full-disk photospheric vector magnetograms weekly since August 2003 as part of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project. After the full deployment of the VSM data processing system, a typical observing day will include three Fe I 630.2 nm full-disk photospheric vector magnetograms, one full-disk photospheric and three Ca II 854.2 nm chromospheric longitudinal magnetograms, along with three He I 1083 nm spectroheliograms. The photospheric vector magnetograms will be available over the Internet in two stages: first, as a quick-look product within minutes of data acquisition, and then as a Milne-Eddington inversion product within a day of each observation. Title: High-Resolution Surface Observations and What They Can Tell MHD Simulations Authors: Keller, C. U. Bibcode: 2006ASPC..354....3K Altcode: I review selected state-of-the art observations of the solar photosphere at the highest possible spatial resolution with a special emphasis on properties that can directly be compared with current MHD simulations. Observations of the solar surface have made big advances during the last decade thanks to new telescopes and new data processing approaches. I will provide an overview of the latest observations and discuss parameters such as the rms contrast and other statistical parameters that can be directly related to state-of-the-art MHD simulations. Title: Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and Their Isotopes Authors: Ayres, Thomas R.; Plymate, Claude; Keller, Christoph U. Bibcode: 2006ApJS..165..618A Altcode: 2006astro.ph..6153A A solar photospheric ``thermal profiling'' analysis is presented, exploiting the infrared (2.3-4.6 μm) rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at higher altitudes. The oxygen abundance was self-consistently determined from weak CO absorptions (for C/O≡0.5). Our analysis was meant to complement recent studies based on three-dimensional (3D) convection models, which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly 2, although in fact our conclusions do not support such a revision. Based on various considerations, an ɛO=700+/-100 ppm (parts per million relative to hydrogen) is recommended; the large uncertainty reflects the model sensitivity of CO. New solar isotopic ratios also are reported: 12C/13C=80+/-1, 16O/17O=1700+/-220, and 16O/18O=440+/-6-all significantly lower than terrestrial. CO synthesis experiments utilizing a stripped down version of the 3D model-which has large temperature fluctuations in the middle photosphere, possibly inconsistent with CO ``movies'' from the Infrared Imaging Spectrometer (IRIS), and a steeper mean temperature gradient than matches visible continuum center-limb measurements-point to a lower oxygen abundance (~500 ppm) and isotopic ratios closer to terrestrial. A low oxygen abundance from CO-and other molecules like OH-thus hinges on the reality of the theoretically predicted midphotospheric convective properties. Title: Design of a polarimeter for extrasolar planetary systems characterization Authors: Keller, Christoph U. Bibcode: 2006SPIE.6269E..0TK Altcode: 2006SPIE.6269E..26K Unpolarized light from the central star that is reflected by exoplanets, protoplanetary disks, and debris disks becomes partially polarized by the reflection process. Imaging polarimetry is therefore the ideal way to discriminate between the polarized light from circumstellar environments and the unpolarized light from the nearby central star. A sensitivity of 10 -5 (fraction of polarized intensity to the total intensity) must be achieved to detect exoplanets; 10 -4 is sufficient for disks. Based on extensive experience in precision polarimetry of the Sun, the newly formed experimental astrophysics group at Utrecht University, The Netherlands, will design, build, and use a high-precision imaging polarimeter for use at the 4.2-meter William Herschel Telescope. Since systematic errors typically limit conventional imaging polarimeters to about 10 -3, laboratory setups and theoretical models will be used to understand and then minimize and/or calibrate systematic errors. Published catalogues of exoplanets and stars that harbor disks will guide extensive observations with this new polarimeter. The effort will focus on retrieving fundamental properties of circumstellar environments that cannot be obtained with other observational approaches. Title: Development of an IFU for diffraction-limited 3D spectropolarimetry Authors: Ren, Deqing; Keller, Christoph; Plymate, Claude Bibcode: 2006SPIE.6269E..5ZR Altcode: 2006SPIE.6269E.191R Ground-based telescopes can achieve diffraction-limited images when equipped with adaptive optics (AO). A major limitation of AO is the small field of view, which is due to the limited isoplanatic patch size. Nevertheless, conventional long-slit spectrographs cannot sample the entire AO-corrected field of view in a single exposure. However, equipped with a modern, large detector array, the Integral Field Unit (IFU) technique will allow a 3-dimensional (3-D) data cube to be recorded simultaneously over the entire AO corrected field of view, with a conventional long-slit spectrographs. We are building a state-of-the-art image slicer IFU for the National Solar Observatory's (NSO) McMath-Pierce Solar Telescope (McMP). This will be the first time that an advanced image slicer IFU is used for 3-D spectroscopy and polarimetry at a solar telescope. The IFU consists of 25 slices that will sample a 6.25" x 8" AO corrected field of view simultaneously, and produces a 200" long slit for diffraction-limited 3-D spectroscopy and polarimetry. This IFU 3-D technique will provide the most high spatial, high temporal resolution with high throughput for solar spectroscopy and polarimetry. This is critical for state-of-the-art spectral diagnosis of solar velocity and magnetic fields. We discuss the design, construction, and testing of this new IFU. Title: Mapping sodium distribution in the exosphere of Mercury with tip-tilt image stabilization Authors: Potter, A. E.; Plymate, C.; Keller, C.; Killen, R. M.; Morgan, T. H. Bibcode: 2006AdSpR..38..599P Altcode: Mapping the sodium emission about Mercury is a difficult observational problem, since Mercury is seen either against the bright daytime sky, or against a dark sky at very high air masses. The distribution of sodium emission over the surface of Mercury is non-uniform, and changes over time. These effects give clues to the processes that produce the sodium and control its distribution, so that improved mapping of sodium emissions over the Mercury surface will help clarify their relative importance. We have adapted an image stabilizer utilizing a piezoelectric driven tip-tilt correction mirror for daytime spectral imaging of Mercury. The image stabilizer, which was originally developed for solar observations at the McMath Pierce solar telescope, results in a noticeable improvement in spatial resolution of our Mercury sodium images. In this paper we give initial results from use of the tip-tilt image stabilizer for observations of Mercury’s sodium exosphere. Further systematic observations and improvements are planned for the image stabilizer system, as well as experimental observations with a low-order adaptive optics system incorporating a commercially available 37-actuator deformable mirror. Title: C3Po --- The Ultimate Focal Plane Array for Imaging Polarimetry from 200 nm to 20 μm Authors: Keller, C. U. Bibcode: 2005ASPC..343...33K Altcode: C^3Po is a novel array detector concept that is optimized for highly sensitive and precise differential imaging. Chopping between two or more independent image states (such as four linearly independent polarization states) can be performed at speeds in the kHz domain to provide virtually simultaneous images without the need to read out the array at kHz frame rates. This allows the technology to be applied to large arrays with precise, slow readouts. All independent image planes are observed with the same physical pixel on the detector, which renders normalized differences between image planes insensitive to the gain of individual pixels. The detector concept has 100% geometrical fill factor and a quantum efficiency approaching unity. The technology can be applied to silicon to cover the 200--1100 nm wavelength range, and to infrared-sensitive materials such as HgCdTe or InSb for the 1-20 μm wavelength range. The detector concept has a wide range of potential applications outside of astronomy. Title: High precision polarimetry with the Advanced Technology Solar Telescope Authors: Socas-Navarro, Hector; Elmore, David F.; Keller, Christoph U.; Seagraves, Paul H.; Streander, Kim V.; Card, Gregory L.; Warner, Mark; Kuhn, Jeffrey R.; Mickey, Donald L. Bibcode: 2005SPIE.5901...52S Altcode: This paper addresses the issue of calibrating the Advanced Technology Solar Telescope for high-precision polarimetry, in particular of the optical train above the Gregorian station (where suitable calibration optics will be placed). Conventional techniques would not be adequate for this telescope given its large aperture. Here we explore two different methods that are currently being considered by the design team. The first one is the "sub-aperture" method, which uses small calibration optics above the primary mirror to calibrate a small sub-aperture of the system. This calibration is then extended to the full aperture by means of actual observations. The second method is based on analyzing the polarization observed in a spectral line with a peculiar Zeeman pattern, such as the FeII 614.9 nm line, which does not produce any intrinsic linear polarization. Numerical simulations are presented that show the robustness of both techniques and their respective advantages and disadvantages are discussed. Title: Reduction of SOLIS/Vector Spectromagnetograph He I 1083 nm Observations Authors: Jones, H. P.; Malanushenko, O. V.; Harvey, J. W.; Henney, C. J.; Keller, C. U. Bibcode: 2005AGUSMSP51A..02J Altcode: The SOLIS/Vector Spectromagnetograph (VSM) now continues the three-decade record of full-disk He I 1083 nm spectroheliograms from NSO/Kitt Peak (see poster by Henney et al.). Although the sensitivity of the VSM interim cameras at 1083 nm is far greater than that of previous NSO/KP instruments, the observations are compromised by strong interference fringes produced within the detectors. We discuss in some detail the extent to which this difficulty is overcome in the simple algorithm used to produce the synoptic observations and explore methods for removing remaining fringe effects for true imaging spectroscopy. These techniques include Fourier and Wavelet filtering, the flat-fielding algorithm used for earlier NASA/NSO Spectromagnetograph (SPM) observations, and physical modeling of the detector fringe pattern. Title: The Solar Oxygen Abundance, and the Rare Isotopes of C and O, Derived from Infrared Spectra of Carbon Monoxide Authors: Ayres, T. R.; Plymate, C.; Keller, C.; Kurucz, R. L. Bibcode: 2005AGUSMSP41B..09A Altcode: A detailed abundance analysis is presented for solar oxygen based on the ΔV=1 fundamental (4.6~μm) and ΔV=2 (2.3~μm) first-overtone rovibrational bands of carbon monoxide observed above the Earth's atmosphere at very high spectral resolution and high signal-to-noise by the Shuttle-borne ATMOS Fourier transform spectrometer (FTS). Additional observations to define the reference photospheric thermal structure were taken of the CO fundamental bands in an atmospheric window at 2145~cm-1 (4.6~μm) using the 1~m FTS of the McMath-Pierce telescope at Kitt Peak and a fast tip/tilt image stabilization system. The latter allowed measurements at the extreme limb where the highly slanted rays probe into the outer layers of the photosphere. High spatial resolution "movies" of weak CO lines at disk center taken under excellent seeing conditions with the Infrared Imaging Spectrometer (IRIS), also on the McMath-Pierce telescope, further constrained thermal and velocity fluctuations in the layers in which the abundance-sensitive CO lines form. This work is meant to complement a series of recent studies which have revised the previously recommended solar oxygen abundance downward by nearly a factor of two; although in fact our conclusions do not support such a revision. The oxygen abundance recovered in the present work is 700±70~ppm (parts per million relative to hydrogen) compared with the proposed downward revision to 460±60~ppm, and the recommended value of 650±100~ppm of a decade ago. In our analysis, a fixed C/O ratio of 0.5, derived in independent work, was assumed; so the associated carbon abundance is 350~ppm. New accurate values for the solar abundance ratios of the rare isotopes of C and O also are reported: 12C/13C= 70, 16O/17O= 400, and 16O/18O= 2000. All three ratios are lower than terrestrial or meteoritic values (indicating higher isotopic abundances). We find no evidence in the ATMOS3 spectra for measurable 14C16O lines. Title: Are You Being Served? - Solar Data Management at the National Solar Observatory Authors: Hill, F.; Suarez-Sola, I.; Harvey, J. W.; Henney, C. J.; Keller, C.; Wampler, S. Bibcode: 2005AGUSMSH43B..04H Altcode: Solar physics is a data driven field, where no progress can be made without multi-wavelength and multi-cadence information about the sun's behavior. The National Solar Observatory (NSO) has provided data to the research community for over 30 years and thus has considerable experience in the areas of data processing, archiving, and distribution. This presentation will describe the development of NSO's data services, outline what lessons have been learned along the way, and attempt to forecast the future of data management at NSO. Title: First-Light Instrumentation for the Advanced Technology Solar Telescope Authors: Rimmele, T.; Balasubramaniam, K.; Berger, T.; Elmore, D.; Gary, A.; Keller, C.; Kuhn, J.; Lin, H.; Mickey, D.; Pevtsov, A.; Robinson, B.; Sigwarth, M.; Soccas-Navarro, H. Bibcode: 2005AGUSMSP34A..03R Altcode: The 4m Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at the Nasmyth focus. For example, instruments for observing the faint corona preferably will be mounted at Nasmyth where maximum throughput is achieved. In addition, the Nasmyth focus has minimum telescope polarization and minimum stray light. We give an overview of the initial set of first generation instruments: the Visible-Light Broadband Imager (VLBI), the Visible Spectro-Polarimeter (ViSP), the Near-IR Spectro-Polarimeter (NIRSP), which includes a coronal module, and the Visible Tunable Filter. We also discuss the unique and efficient approach to the ATST instrumentation, which builds on the use of common components such as detector systems, polarimetry packages and various opto-mechanical components. For example, the science requirement for polarimetric sensitivity (10-5 relative to intensity) and accuracy (5'10-4 relative to intensity) place strong constraints on the polarization analysis and calibration units. Consequently, these systems are provided at the facility level, rather than making it part of the requirement for each instrument. Title: SOLIS-VSM Synoptic Products Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U. Bibcode: 2005AGUSMSP51A..04H Altcode: Since August 2003, the Vector Spectromagnetograph (VSM) instrument has recorded full-disk photospheric and chromospheric magnetograms along with He I 1083 nm spectroheliograms daily as part of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project. In addition, the VSM measured full-disk photospheric vector magnetograms weekly. After the full deployment of the VSM data processing system, a typical observing day will include three full-disk photospheric vector magnetograms. Products from observations for the past year and half are expected to become available during the year 2005, these include Dopplergrams, equivalent width and line depth images. Besides individual full-disk images, several derived products are also or soon will be available via the SOLIS web site. These products include integrated full-disk magnetogram and helium spectroheliogram time series. Additionally, Carrington rotation and daily synoptic maps are now available from the photospheric magnetograms. Daily coronal hole estimate images using VSM data, along with the creation of solar wind source maps will also be available. The VSM synoptic products are available on the NSO-SOLIS web site cooperatively by NSF/NSO and NASA/GSFC. The National Solar Observatory is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. Title: Adaptive Optics Observations of the Sun Authors: Keller, Christoph U. Bibcode: 2005sao..conf..119K Altcode: Modern solar observations are performed close to the diffraction limit of the existing solar telescopes since important, fundamental processes on the Sun occur on scales of a tenth of an arc-second and smaller. Spatial resolution is therefore paramount in solar observations. Even though solar telescopes have relatively small apertures as compared to modern night-time telescopes, their spatial resolution is still limited by seeing, which is worse during the day as compared to at night. Adaptive optics has thus become an important addition to almost all large solar telescopes. While the first solar adaptive optics system was among the first to be used at astronomical telescopes, only in the past few years have such systems become available for regular science use. Publications of solar science obtained with adaptive optics are therefore still rare, but their number is rapidly increasing. Here I discuss the existing solar adaptive optics systems and summarize some of the science they have produced to date. Title: Helicity and the SOLIS Vector-Spectromagnetograph Authors: Keller, Christoph U.; Harvey, John W.; Henney, Carl. J.; Jones, Harrison P. Bibcode: 2005HiA....13..126K Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that greatly improve ground-based synoptic solar observations. The main instrument the Vector Spectro-Magnetograph (VSM) is a compact high-throughput vector-polarimeter that measures the magnetic field strength and direction over the full solar disk within 15 minutes. Helicity can be directly calculated from the full-disk vector field data provided by the Stokes inversion of the observed polarized line profiles. We present an anlysis of the difficulties and problems that one faces when interpreting the the helicity derived from VSM data. Title: Radio Observations of the Quiet Sun Authors: Keller, Christoph U.; Krucker, Säam Bibcode: 2004ASSL..314..287K Altcode: While radio observations of the Sun have mostly focused on active region phenomena, they also contribute unique data to our knowledge of the quiet Sun, in particular through accurate measurements of the temperature as a function of height in the atmosphere and through the measurement of nonthermal emissions from chromospheric and coronal heating events. Here we review observations of the quiet Sun using radio telescopes and discuss current science problems that will be addressed with future facilities such as the Frequency Agile Solar Radiotelescope (FASR). Title: Instrumentation for the Advanced Technology Solar Telescope Authors: Rimmele, Thomas R.; Hubbard, Robert P.; Balasubramaniam, K. S.; Berger, Tom; Elmore, David; Gary, G. Allen; Jennings, Don; Keller, Christoph; Kuhn, Jeff; Lin, Haosheng; Mickey, Don; Moretto, Gilberto; Socas-Navarro, Hector; Stenflo, Jan O.; Wang, Haimin Bibcode: 2004SPIE.5492..944R Altcode: The 4-m aperture Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at Nasmyth or a small Gregorian area. For example, instruments for observing the faint corona preferably will be mounted at Nasmyth focus where maximum throughput is achieved. In addition, the Nasmyth focus has minimum telescope polarization and minimum stray light. We describe the set of first generation instruments, which include a Visible-Light Broadband Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters, Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter & Spectrometer and a UV-Polarimeter. We also discuss unique and efficient approaches to the ATST instrumentation, which builds on the use of common components such as detector systems, polarimetry packages and various opto-mechanical components. Title: Solar and Space Weather Radiophysics - Current Status and Future Developments Authors: Gary, D. E.; Keller, C. U. Bibcode: 2004ASSL..314.....G Altcode: 2004sswr.book.....G The book explores what can be learned about the Sun and interplanetary space using present-day and future radio observations and techniques. The emphasis is on interpretation of radio data with high spatial and spectral resolution, motivated by the planned construction of a new, powerful, solar-dedicated radio array called the Frequency Agile Solar Radiotelescope (FASR). The book is unique in exploring a broad frequency range, which corresponds to heights ranging from the low solar atmosphere out to the Earth. The book contains a thorough review of the entire field of solar and Space Weather radio research; gives background information suitable for advanced undergraduates, graduates, and researchers in solar and Space Weather research and related fields; and looks at what new results may be expected in the next two decades with FASR and other new instruments now under development. The individual chapters are written by international experts in each topic, and although each chapter may be read as a stand-alone article, the ordering of the chapters and the topical development makes the book readable from beginning to end, to provide an excellent understanding of the field as a whole. Title: Evaluation of Seeing-Induced Cross Talk in Tip-Tilt-Corrected Solar Polarimetry Authors: Judge, Philip G.; Elmore, David F.; Lites, Bruce W.; Keller, Christoph U.; Rimmele, Thomas Bibcode: 2004ApOpt..43.3817J Altcode: We reanalyze the effects of atmosphere-induced image motions on the measurement of solar polarized light using a formalism developed by Lites. Our reanalysis is prompted by the advent of adaptive optics (AO) systems that reduce image motion and higher-order aberrations, by the availability of liquid crystals as modulation devices, and by the need to understand how best to design polarimeters for future telescopes such as the Advanced Technology Solar Telescope. In this first attempt to understand the major issues, we analyze the influence of residual image motion (tip-tilt) corrections of operational AO systems on the cross talk between Stokes parameters and present results for several polarization analysis schemes. Higher-order wave-front corrections are left for future research. We also restrict our discussion to the solar photosphere, which limits several important parameters of interest, using some recent magnetoconvection simulations. Title: Improved Spatial Resolution of the Sodium Distribution on Mercury Authors: Potter, A. E.; Plymate, C.; Keller, C.; Killen, R. M.; Morgan, T. H. Bibcode: 2004AAS...204.6407P Altcode: 2004BAAS...36R.780P The distribution of sodium emission over the surface of Mercury is non-uniform, and changes over time. These non-uniformities and time-dependent changes give clues to the processes that produce the sodium. However, observations of the distribution of sodium on Mercury are hampered by the bad seeing that accompanies the need to observe either in daytime, or at extremely large air masses in twilight. In an effort to improve this situation, we have developed an image stabilizer utilizing a piezoelectric driven tip-tilt correction mirror for daytime spectral imaging of Mercury. The system is a modification of one that was originally developed for solar observations at the McMath-Pierce solar telescope. Use of image stabilization results in a noticeable improvement in spatial resolution of our Mercury sodium images. A series of sodium images taken over an eight-day period show changes in which an emission peak in high southern latitudes disappears, and is replaced by an emission peak at high northern latitudes. Further systematic observations and improvements are planned for the image stabilizer system, as well as experimental observations with a low-order adaptive optics system incorporating a commercially available 37-actuator deformable mirror.

This work was supported by the NASA Planetary Astronomy Program. Title: High-accuracy Multiangle Spectropolarimetric Imaging Concept for Aerosol Remote Sensing from Space Authors: Diner, D. J.; Chipman, R. A.; Cairns, B.; Foo, L. D.; Keller, C. U.; Macenka, S. A.; Bruce, C. F. Bibcode: 2004AGUSMGC51A..07D Altcode: Satellite remote sensing has a key role in measuring the distribution, radiative impact, and regional and global spatial context of tropospheric aerosols. A synergistic combination of multispectral, multiangle, and polarimetric approaches would improve the accuracies of aerosol optical depth and particle property characterizations compared to what is achievable using each method by itself. In this paper we discuss the science benefits and technical feasibility of combining key attributes of multiple aerosol remote sensing instruments into a single instrument package. The features of the conceptual instrument are: spectral coverage from the near-UV to the shortwave infrared; global coverage within a few days; intensity and polarimetric imaging simultaneously at multiple view angles; kilometer to sub-kilometer spatial resolution; and measurement of the degree of linear polarization in one visible and one shortwave-infrared spectral band, i.e., a subset of the full spectral complement, with an uncertainty of 0.5% or less. The polarimetric accuracy is the driving requirement of the instrument design, and is stipulated in order to achieve uncertainty goals in optical depth (0.01) and single scattering albedo (0.03) that appear difficult to reach given the current state-of-the-art of the calibration of intensity-only measurements. Bispectral polarimetry is invoked to enable size-resolved retrievals of particle real refractive index. After examining many approaches and technologies for imaging polarimetry, we conclude that ultrafast time-multiplexing is the best option for meeting the instrument performance requirements. The approach is based upon innovative advances in high-precision imaging polarimetry developed for ground-based solar astronomy. Rapid modulation of the linear polarization Stokes components Q and U, coupled with synchronous demodulation in a charge-caching focal plane, provides two essential benefits: (1) the same detector is used to measure the relative proportions of Q or U to the total intensity, thus circumventing inaccuracies introduced by detector gain changes or uncertainties in flight, and (2) rapid interlacing of the measurements at sub-pixel scale insulates against false polarization signals as the spacecraft flies over a spatially varying scene. Technology advances needed to implement this approach are identified. Title: On the Origin of Solar Faculae Authors: Keller, C. U.; Schüssler, M.; Vögler, A.; Zakharov, V. Bibcode: 2004ApJ...607L..59K Altcode: Solar faculae appear as bright small features close to the solar limb. Recent high-resolution images show these brightenings in unprecedented detail. Our analysis of numerical MHD simulations reproduces the observed small-scale features. The simulations reveal that faculae originate from a thin layer within granules just below largely transparent magnetic flux concentrations. This is basically the ``bright wall'' model of Spruit. The dark, narrow lanes often associated with faculae occur at the opposite side of the magnetic flux concentration and are due to an extended layer with lower-than-average temperature. Title: SOLIS-VSM Longitudinal Magnetogram Calibration Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U.; Jones, H. P. Bibcode: 2004AAS...204.3702H Altcode: 2004BAAS...36..709H The VSM (Vector SpectroMagnetograph) instrument started recording daily full-disk magnetograms during August, 2003 at a temporary site in Tucson, Arizona. Along with the other instruments that constitute the SOLIS (Synoptic Optical Long-term Investigations of the Sun) project, the VSM will be relocated to Kitt Peak in April, 2004. The VSM instrument provides a unique record of solar full-disk vector magnetograms along with the high sensitivity photospheric and chromospheric longitudinal magnetograms. In addition, the VSM takes daily full-disk He I 1083 nm equivalent width observations. These parameter-grams are constructed from individual scans in declination of the projected solar image on the entrance slit with a relatively large field angle of over a half of a degree in the spectrograph. The calibration of the VSM longitudinal magnetic observations is reviewed, along with a comparison between the VSM mean field, sun-as-a-star, measurements with those by the Wilcox Solar Observatory. In addition, the making of synoptic maps derived from the VSM magnetograms is discussed, including the filling of data gaps in polar regions and the use of accompanying weight maps. VSM magnetograms and synoptic maps are available via the Internet at: http://solis.nso.edu/. This research was supported in part by the Office of Naval Research Grant N00014-91-J-1040. The National Solar Observatory is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. Title: Early Results from SOLIS Authors: Harvey, J.; Giampapa, M.; Henney, C.; Keller, C.; Jones, H. Bibcode: 2004AAS...204.3701H Altcode: 2004BAAS...36..708H SOLIS (Synoptic Optical Long-Term Investigations of the Sun) is a project that is replacing antiquated synoptic observing equipment at the National Solar Observatory. SOLIS consists of a suite of three instruments on an equatorial mount that will be installed on Kitt Peak in April 2004. The major SOLIS instrument is a vector spectromagnetograph (VSM) that maps magnetic fields across the full solar disk using a slit spectrograph and one arc sec pixels. Limited daily observations started at a temporary site in August, 2003 and include line-of-sight component magnetograms in the photosphere and chromosphere and, for the first time, full-disk vector magnetograms. At a medium scan speed ( ∼ 10 minutes for the full disk) noise is less than 1 Mx/cm2. This low noise, combined with negligible instrumental polarization and well resolved spectral line profiles, yields moderate resolution magnetograms of unprecedented quality. Observations show magnetic flux nearly everywhere in the photosphere from the disk center to the solar limb. Weak, intranetwork fields are now routinely observed and show a tendency to be of opposite polarity to the stronger surrounding fields. Diffuse fields surround decaying active regions and appear to be distinct from canopy fields. Vector magnetograms easily show the radial orientation of network fields, and the diffuse component surrounding decaying active regions. Near the disk center, the transverse magnetic fields of network elements change on a time scale of minutes. Detailed quantitative calibration of the observations is in progress. Good results have been obtained from the other SOLIS instruments: a full-disk filter imager at several narrow wavelengths and a double-pass grating spectrograph that provides high-accuracy line spectra of integrated sunlight. SOLIS data are freely available via the Internet and users are invited to submit observing time requests for special observations. The National Solar Observatory is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. Additional support for the development of SOLIS from NASA and ONR is gratefully acknowledged. Title: Innovative Information Technology for Space Weather Research Authors: Wang, H.; Qu, M.; Shih, F.; Denker, C.; Gerbessiotis, A.; Lofdahl, M.; Rees, D.; Keller, C. Bibcode: 2004AAS...204.5209W Altcode: 2004BAAS...36..755W Solar activity is closely related to the near earth environment -- summarized descriptively as space weather. Changes in space weather have adverse effect on many aspects of life and systems on earth and in space. Real-time, high-quality data and data processing would be a key element to forecast space weather promptly and accurately. Recently, we obtained a funding from US National Science Foundation to apply innovative information technology for space weather prediction.

(1) We use the technologies of image processing and pattern recognition, such as image morphology segmentation, Support Vector Machines (SVMs), and neural networks to detect and characterize three important solar activities in real-time: filament eruptions, flares, and emerging flux regions (EFRs). Combining the real time detection with the recent statistical study on the relationship among filament eruptions, flares, EFRs, coronal mass ejections (CMEs), and geomagnetic storms, we are establishing real time report of solar events and automatic forecasting of earth directed CMEs and subsequent geomagnetic storms.

(2) We combine state-of-art parallel computing techniques with phase diverse speckle imaging techniques, to yield near real-time diffraction limited images with a cadence of approximately 10 sec. We utilize the multiplicity of parallel paradigms to optimize the calculation of phase diverse speckle imaging to improve calculation speed. With such data, we can monitor flare producing active regions continuously and carry out targeted studies of the evolution and flows in flare producing active regions.

(3) We are developing Web based software tools to post our processed data, events and forecasting in real time, and to be integrated with current solar activity and space weather prediction Web pages at BBSO. This will also be a part of Virtual Solar Observatory (VSO) being developed by the solar physics community.

This research is supported by NSF ITR program. Title: Evershed Flow of CO at Different Depths in a Sunspot Penumbra. Authors: Clark, T. Alan; Plymate, C.; Bergman, M. W.; Keller, C. U. Bibcode: 2004AAS...204.3720C Altcode: 2004BAAS...36..712C CO lines of different strengths near 4.7 microns have been used to observe Evershed flow of molecular gas in the penumbra of a large sunspot near to the solar limb. An adaptive optics system and IR camera on the main spectrograph of the McMath-Pierce Solar Telescope produced a sequence of diffraction-limited spectral-spatial images to 0.8 arc-second resolution as the telescope was scanned across this sunspot. Dopplergrams constructed from this sequence for several of these CO lines have been used to outline the characteristics of this molecular flow as a function of depth within the penumbra.

Particularly noteworthy in these Dopplergrams is the change in pattern of high-speed outflow, from a radial direction deep in the penumbra from weak-line images, matching the structure in the continuum penumbral image, to a spiral pattern more closely resembling the appearance of penumbral structure at chromospheric heights. Typical weak-line flow speeds were a few km/s, significantly lower than those measured recently by Penn et al.,(2003) for weak CH lines at shorter wavelengths, which are postulated to originate in dark fibrils deeper in the penumbra. Strong CO-line images show lower flow speeds. Small regions of narrow inverse Evershed flow channels appear in these latter images near to the outer penumbral boundary. For all lines, the speed of the Evershed flow reaches a maximum just before this boundary and all flow appears to cease at or just beyond it.

Future Evershed flow investigations will include observations of other potentially useful spectral features within this spectral range such as weak OH lines, in addition to several atomic FeI and SiI lines whose high Zeeman sensitivity make them suitable for simultaneous penumbral magnetic field mapping.

Penn, M.J, et al., Ap.J. 590, L119, 2003. Title: Measurement Scale of the SOLIS Vector Spectromagnetograph Authors: Jones, H. P.; Harvey, J. W.; Henney, C. J.; Keller, C. U.; Malanushenko, O. M. Bibcode: 2004AAS...204.3703J Altcode: 2004BAAS...36Q.709J Longitudinal magnetograms obtained with the SOLIS Vector Spectromagnetograph (VSM) during a cross-calibration period are compared with similar data from the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope as well as with SOHO/MDI and GONG magnetograms. The VSM began observations at the University of Arizona agricultural test site and collaborative observations were obtained with both the VSM and SPM from 2003 Aug 05 through 2003 Sep 21 when the SPM was officially retired. The VSM replaces the SPM and continues the 30-year NSO/Kitt Peak synoptic magnetogram record. Magnetograms are compared by equating histograms and, for selected examples, by pixel-by-pixel comparison of coregistered images. The VSM was not corrected for polarization crosstalk and was operated without fast guiding. Solar activity was at best moderate during this period. Over the range of observed fields, the VSM magnetograms show greatly improved sensitivity but are otherwise virtually identical with "raw" SPM magnetograms. GONG magnetograms are also closely comparable with the SPM while MDI flux values tend to be stronger by a factor of 1.2 - 1.4. Dependence of the results on seeing will be discussed. Partial funding for this work was provided through Solar and Heliospheric Research Supporting Research and Technology grants from NASA's Office of Space Sciences. Title: Advanced Technology Solar Telescope: a progress report Authors: Oschmann, Jim; Dalrymple, Nathan; Warner, Mark; Price, Ron; Hill, Frank; Hubbard, Rob; Rimmele, Thomas R.; Keller, Christoph U.; Keil, Stephen Bibcode: 2004SPIE.5171..160O Altcode: The 4m ATST will be the most powerful solar telescope in the world, providing a unique scientific tool to study the Sun and other astronomical objects. The design and development phase for the Advance Technology Solar Telescope (ATST) is progressing. The conceptual design review (CoDR) for the ATST is scheduled for August 2003. We present a brief description of the science requirements of ATST, and remind the reader of some of the technical challenges of building a 4-m solar telescope. We will discuss some of the design strategies that will allow us to achieve the required performance specifications, present conceptual designs for the ATST, and summarize the results of trades we have made on our path to the CoDR. The thermal impacts to local, self-induced seeing with respect to some of our system level trades that have been completed will be discussed. Title: High-speed cameras and solar observing Authors: Harvey, John; Keller, Christoph U.; Cole, Lonnie; Tucker, Roy; Jaksha, David Bibcode: 2004SPIE.5171..258H Altcode: The National Solar Observatory operates two facilities with demanding needs for rapid image collection (i.e. > television frame rates). The first is GONG, a global network of six identical small telescopes devoted to nearly continuous observations of the sun's surface vibrations in order to study its internal properties by helioseismology. The second, SOLIS, is a suite of three instruments that collects images and spectra of the sun needed to study the behavior of solar activity on time scales of minutes to decades. Five different types of cameras are installed in these instruments. High speed, high sensitivity, large dynamic range, and good photometric performance are key factors for cameras used to make measurements of subtle solar signals that pass through the noisy terrestrial atmosphere. A camera that combines all these characteristics is elusive. The combination of high speed and good photometric performance, when observing small intensity changes, is particularly hard to get in practice. High speed in large format CCD and hybrid FPA cameras is achieved by dividing the array into multiple channels that are read simultaneously. An unwanted result of this technique is cross talk between signal channels. It is of order 1 percent in the case of Silicon Mountain Design 1M60_20 cameras (1k x 1k, 60 fps) and Rockwell Scientific Company HyViSI-1024 cameras (1k x 256, 92 fps). Cross talk (and also successive-frame image retention) are particularly hard to deal with since they may exhibit non-linear characteristics that depend on illumination light level. We describe these and other phenomena, attempts to mitigate the effects, and results from solar observations. Title: Charge caching CMOS detector for polarimetry (C3Po) Authors: Keller, Christoph U. Bibcode: 2004SPIE.5171..239K Altcode: C3Po is a concept for a novel array detector concept that is optimized for highly sensitive and precise differential imaging such as needed for astrophysical polarimetry. Chopping between two or more independent image states (such as four linearly independent polarization states) can be performed at speeds in the kHz domain to provide virtually simultaneous images without the need to read out the array at kHz frame rates. This allows the technology to be applied to large arrays with precise, slow readouts. All independent image planes are observed with the same physical pixel on the detector, which renders normalized differences between image planes insensitive to the gain of individual pixels. The detector concept has 100% geometrical fill factor and a quantum efficiency approaching unity. The technology can be applied to silicon to cover the 200-1100 nm wavelength range, and to infrared-sensitive materials such as HgCdTe or InSb for the 1-20 μm wavelength range. While the detector concept has a wide range of potential applications outside of astronomy, we focus here on its application to polarimetric observations of the Sun. Title: Mapping sodium distribution on the surface of Mercury Authors: Potter, A.; Plymate, C.; Keller, C.; Killen, R.; Morgan, T. Bibcode: 2004cosp...35.1293P Altcode: 2004cosp.meet.1293P The distribution of sodium emission over the surface of Mercury is non-uniform, and changes over time. These non-uniformities and time-dependent changes give clues to the processes that produce sodium. Photon-stimulated desorption may be the major sodium source process on Mercury, so the sodium density might be expected to maximize at the subsolar point. Another source process may be sputtering by solar particles that impact the surface at high latitudes in the polar cusps, producing sodium emissions at these locations. Variations in the geographic distribution of sodium-rich minerals might produce localized emissions that do not change position with time. Mapping the sodium emission is a difficult observational problem, since Mercury is seen either against the bright daytime sky, or against a dark sky at very high air masses. We have adapted an image stabilizer utilizing a piezoelectric driven tip-tilt correction mirror for daytime spectral imaging of Mercury. The image stabilizer was originally developed for solar observations at the McMath-Pierce solar telescope. Use of image stabilization results in a noticeable improvement in spatial resolution of our Mercury sodium images. A series of sodium images taken over an eight-day period show changes in which an emission peak in high southern latitudes disappears, and is replaced by an emission peak at high northern latitudes. Further systematic observations and improvements are planned for the image stabilizer system, as well as experimental observations with a low-order adaptive optics system incorporating a commercially available 37-actuator deformable mirror. Title: First Results from SOLIS Authors: Harvey, J.; Giampapa, M.; Henney, C.; Jones, H.; Keller, C. Bibcode: 2003AGUFMSH42B0545H Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a project to replace antiquated synoptic observing equipment at the National Solar Observatory. SOLIS includes a suite of three instruments on an equatorial mount originally installed at a site in Tucson and will be moved to Kitt Peak before the end of 2003. The major SOLIS instrument is a vector spectromagnetograph that maps magnetic fields across the full solar disk using a slit spectrograph and one arc sec pixels. Daily observations include several line-of-sight component magnetograms in the photosphere and chromosphere and, for the first time, full-disk vector magnetograms. At a medium scan speed ( ∼10 minutes for the full disk) noise is less than 1 Mx/cm2. This low noise, combined with negligible instrumental polarization and well resolved spectral line profiles, yields moderate resolution magnetograms of unprecedented quality. Sample observations show magnetic flux nearly everywhere in the photosphere from the disk center to the solar limb. The flux is organized in large scale patterns that heretofore had been visible only in strong flux elements or after substantial spatial smearing. Good results have been obtained from the other SOLIS instruments: a full-disk filter imager at several narrow wavelengths and a grating spectrograph that provides high-accuracy line spectra of integrated sunlight. SOLIS data are freely available via the Internet and users may submit observing time requests for special observations. The National Solar Observatory is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. Additional support for the development of SOLIS from NASA and ONR is gratefully acknowledged. Title: Linear Polarization Measurements of Chromospheric Emission Lines Authors: Sheeley, N. R., Jr.; Keller, C. U. Bibcode: 2003ApJ...594.1085S Altcode: We have used the Zurich Imaging Stokes Polarimeter (ZIMPOL I) with the McMath-Pierce 1.5 m main telescope on Kitt Peak to obtain linear polarization measurements of the off-limb chromosphere with a sensitivity better than 1×10-5. We found that the off-disk observations require a combination of good seeing (to show the emission lines) and a clean heliostat (to avoid contamination by scattered light from the Sun's disk). When these conditions were met, we obtained the following principal results:1. Sometimes self-reversed emission lines of neutral and singly ionized metals showed linear polarization caused by the transverse Zeeman effect or by instrumental cross talk from the longitudinal Zeeman effect in chromospheric magnetic fields. Otherwise, these lines tended to depolarize the scattered continuum radiation by amounts that ranged up to 0.2%.2. Lines previously known to show scattering polarization just inside the limb (such as the Na I λ5889 D2 and the He I λ5876 D3 lines) showed even more polarization above the Sun's limb, with values approaching 0.7%.3. The O I triplet at λ7772, λ7774, and λ7775 showed a range of polarizations. The λ7775 line, whose maximum intrinsic polarizability, Pmax, is less than 1%, revealed mainly Zeeman contributions from chromospheric magnetic fields. However, the more sensitive λ7772 (Pmax=19%) and λ7774 (Pmax=29%) lines had relatively strong scattering polarizations of approximately 0.3% in addition to their Zeeman polarizations. At times of good seeing, the polarization spectra resolve into fine structures that seem to be chromospheric spicules. Title: Calibration and Data Analysis for the SOLIS-VSM Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U.; Jones, H. P.; SOLIS Team Bibcode: 2003SPD....34.0303H Altcode: 2003BAAS...35..808H The Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project will provide a unique 25-year record of synoptic solar observations with daily full-disk photospheric vector and high sensitivity longitudinal magnetograms. In addition, the VSM will produce daily full-disk chromospheric longitudinal magnetograms along with various He I 1083 nm parameter images. The VSM is being prepared for installation at a temporary site in Tucson for a comparison period with observations from the Kitt Peak Vacuum Telescope (KPVT). After the cross-calibration period, the VSM will replace the KPVT spectromagnetograph at Kitt Peak. We outline the various VSM data products, with emphasis on the calibration and data reduction efforts.

The data transmission and storage resources are such that the reduction of VSM data will be performed at the observing site on Kitt Peak. Reduced data products will be transmitted via a DS3 link from Kitt Peak to the National Solar Observatory's digital archive in Tucson. During a typical observing day, three full-disk photospheric vector magnetograms will be available over the Internet in two stages: first, as a ``quick-look'' product within 10 minutes of data acquisition, and then as a full Milne-Eddington (ME) inversion product within 12 hours of each observation. The quick-look parameters will include estimates of the magnetic field strength, azimuth and inclination based on Auer, Heasley, House (1977, Solar Physics 55, p. 47). The high-precision vector products will be determined with the High Altitude Observatory ME inversion technique implemented by Skumanich and Lites (1987, ApJ, 322, p.473). The flexible design of the VSM data handling system can incorporate future improvements under consideration (e.g., principal component analysis).

This research was supported in part by the Office of Naval Research Grant N00014-91-J-1040. Title: Advanced Technology Solar Telescope - Approach to a Four-meter Diffraction Limited Solar Telescope Authors: Keil, S.; Rimmele, T.; Oschmann, J.; Warner, M.; Dalrymple, N.; Hubbard, R.; Price, R.; Goodrich, B.; Keller, C.; ATST Team Bibcode: 2003SPD....34.2019K Altcode: 2003BAAS...35..847K The Advanced Technology Solar Telescope (ATST) is intended to be the next major step in ground based solar observatories. The ATST will provide a laboratory for ultra high resolution, polarimetric measurements of all layers of the solar atmosphere. Currently the project is preparing a conceptual design to fulfill this mission, including plans for the design, development, construction and operation of this facility. Given the nearly three-fold increase in aperture size over the largest existing solar facilities, our approach combines techniques from the newest solar facilities with lessons from recent nighttime telescope designs. This approach insures the ATST will meet the scientific goals that include diffraction-limited performance in the optical for high spatial resolution solar observations and very low scattered light to advance coronal observation capabilities. The current telescope design incorporates the latest active optics techniques, fast focal ratios for the primary optics, an open design for ventilation of locally produced seeing, an un-obscured off-axis pupil and a very high order adaptive optics system built into the telescope from the beginning. Examples of some of the current design concepts for the telescope structure, optics, thermal management, scattered light control, upgrade paths to multi-conjugate adaptive optics, software and facilities to support future potential upgrades and instrumentation are given along with some of the key challenges that lie ahead.

The National Solar Observatory is sponsored and supported by the National Science Foundation. Title: The SOLIS Vector-Spectromagnetograph (VSM) Authors: Keller, C. U.; SOLIS Team Bibcode: 2003SPD....34.2023K Altcode: 2003BAAS...35..848K SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that will greatly improve ground-based synoptic solar observations. The main instrument, the Vector Spectro-Magnetograph (VSM), is a compact, high-throughput vector-polarimeter with an active secondary mirror, an actively controlled grating spectrograph, and two high-speed cameras based on silicon-on-CMOS-multiplexer hybrid focal plane arrays. It measures the photospheric magnetic field strength and direction over the full solar disk within 15 minutes using two iron lines at 630 nm. It can also record the longitudinal magnetic flux in the chromosphere and coronal proxies derived from the HeI 1083 nm spectral line.

The VSM has just been assembled and is expected to see first light at a temporary site in Tucson before the end of May. We will present an overview of the instrument, describe its performance, and discuss first results.

SOLIS has been supported by a specific grant from NSF, NSO base funding, NASA/GSFC, and the ONR. The project has also received significant technical assistance from HAO and the Solar and Astrophysics Lab of Lockheed-Martin. Title: Site testing issues for the frequency agile solar radiotelescope (FASR) Authors: Gary, Dale E.; Keller, Christoph U. Bibcode: 2003SPIE.4853..523G Altcode: The Frequency Agile Solar Radiotelescope (FASR) will be a broadband synthesis-imaging array with 3-km or larger baselines, operating over a broad frequency range of 0.1-30 GHz. The instrument demands a site with low levels of Radio Frequency Interference (RFI) over this entire band. The site also must be large enough to accommodate the expected size of the array configuration and ideally would provide room to grow with future upgrades. The site must have a benign environment in which at least 100 separate elements will operate with little degradation and weather-related downtime. Several sites in the U.S. are being considered. We discuss what criteria are being used to assess the sites, and give some initial results of testing some of the sites. Title: Solar polarimetry close to the diffraction limit Authors: Keller, Christoph U. Bibcode: 2003SPIE.4843..100K Altcode: Much progress has been made during the last years in obtaining polarimetric observations of the Sun close to the diffraction limit. Here I summarize the problems one encounters when observing close to the diffraction limit of a large solar telescope, review techniques, present examples of recent observations, and discuss the need for further developments of instruments and image reconstruction techniques. Title: PEPSI spectro-polarimeter for the LBT Authors: Strassmeier, Klaus G.; Hofmann, Axel; Woche, Manfred F.; Rice, John B.; Keller, Christoph U.; Piskunov, N. E.; Pallavicini, Roberto Bibcode: 2003SPIE.4843..180S Altcode: PEPSI (Postham Echelle Polarimetric and Spectroscopic Instrument) is to use the unique feature of the LBT and its powerful double mirror configuration to provide high and extremely high spectral resolution full-Stokes four-vector spectra in the wavelength range 450-1100nm. For the given aperture of 8.4m in single mirror mode and 11.8m in double mirror mode, and at a spectral resolution of 40,000-300,000 as designed for the fiber-fed Echelle spectrograph, a polarimetric accuracy between 10-4 and 10-2 can be reached for targets with visual magnitudes of up to 17th magnitude. A polarimetric accuracy better than 10-4 can only be reached for either targets brighter than approximately 10th magnitude together wiht a substantial trade-off wiht the spectral resolution or with spectrum deconvolution techniques. At 10-2, however, we will be able to observe the brightest AGNs down to 17th magnitude. Title: SOLIS: an innovative suite of synoptic instruments Authors: Keller, Christoph U.; Harvey, John W.; Giampapa, Mark S. Bibcode: 2003SPIE.4853..194K Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments under construction that will greatly improve ground-based synoptic solar observations. The Vector Spectromagnetograph (VSM) is a compact, high-throughput vector-polarimeter with an active secondary mirror, an actively controlled grating spectrograph, and two high-speed cameras with silicon-on-CMOS-multiplexer hybrid focal plane arrays. It will measure the magnetic field strength and direction over the full solar disk within 15 minutes. The Full-Disk Patrol (FDP) takes full-disk solar intensity and Doppler images in various spectral lines and in the continuum at a high cadence through liquid-crystal tuned birefringent filters. The Integrated Sunlight Spectrometer (ISS) uses a fiber-fed spectrograph to measure minute changes of the Sun-as-a-star in many spectral lines. A high degree of automation and remote control provides fast user access to data and flexible interaction with the data-collection process. SOLIS is currently in the final assembly phase and will become operational early in 2003. Title: Design and development of the Advanced Technology Solar Telescope (ATST) Authors: Keil, Stephen L.; Rimmele, Thomas; Keller, Christoph U.; Hill, Frank; Radick, Richard R.; Oschmann, Jacobus M.; Warner, Mark; Dalrymple, Nathan E.; Briggs, John; Hegwer, Steven L.; Ren, Dauxing Bibcode: 2003SPIE.4853..240K Altcode: High-resolution studies of the Sun's magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun, and what mechanisms trigger flares and coronal mass ejections. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scales fundamental to these processes. The 4m aperture Advanced Technology Solar Telescope (ATST) will be a unique scientific tool, with excellent angular resolution, a large wavelength range, and low scattered light. With its integrated adaptive optics, the ATST will achieve a spatial resolution nearly 10 times better than any existing solar telescope. Building a large aperture telescope for viewing the sun presents many challenges, some of the more difficult being: · Heat control and rejection · Contamination and scattered light control · Control of telescope and instrument polarization · Site selection This talk will present a short summary of the scientific questions driving the ATST design, the design challenges faced by the ATST, and the current status of the developing design and siting considerations Title: Low-cost solar adaptive optics in the infrared Authors: Keller, Christoph U.; Plymate, Claude; Ammons, S. M. Bibcode: 2003SPIE.4853..351K Altcode: We have developed a low-cost adaptive optics system for solar observations in the infrared between 1 and 28 μm with the 1.5-m McMath-Pierce solar telescope. The 37-actuator membrane mirror and a fast tip-tilt mirror are controlled by a PC running Linux RedHat 7.1 that analyzes images from a 256 by 256 pixel, 1 kHz frame rate CCD camera. The total hardware cost is less than $25,000, and the system provides diffraction-limited performance under median seeing conditions above 2.3 μm. The single Pentium III processor provides enough computing power to analyze the 200 subapertures of the Shack-Hartmann wavefront sensor in real time. We describe the hardware and software implementations and show results from the first tests at the telescope. Title: Technical challenges of the Advanced Technology Solar Telescope Authors: Rimmele, Thomas R.; Keil, Stephen L.; Keller, Christoph U.; Hill, Frank; Briggs, John; Dalrymple, Nathan E.; Goodrich, Bret D.; Hegwer, Steven L.; Hubbard, Rob; Oschmann, Jacobus M.; Radick, Richard R.; Ren, Deqing; Wagner, Jeremy; Wampler, Stephen; Warner, Mark Bibcode: 2003SPIE.4837...94R Altcode: The 4m Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world, providing a unique scientific tool to study the Sun and possibly other astronomical objects, such as solar system planets. We briefly summarize the science drivers and observational requirements of ATST. The main focus of this paper is on the many technical challenges involved in designing a large aperture solar telescope. The ATST project has entered the design and development phase. Development of a 4-m solar telescope presents many technical challenges. Most existing high-resolution solar telescopes are designed as vacuum telescopes to avoid internal seeing caused by the solar heat load. The large aperture drives the ATST to an open-air design, similar to night-time telescope designs, and makes thermal control of optics and telescope structure a paramount consideration. A heat stop must reject most of the energy (13 kW) at prime focus without introducing internal seeing. To achieve diffraction-limited observations at visible and infrared wavelengths, ATST will have a high order (order 1000 DoF) adaptive optics system using solar granulation as the wavefront sensing target. Coronal observations require occulting in prime focus, a Lyot stop and contamination control of the primary. An initial set of instruments will be designed as integral part of the telescope. First telescope design and instrument concepts will be presented. Title: Design and development of the Advanced Technology Solar Telescope Authors: Keil, S.; Rimmele, T.; Keller, C.; ATST Team Bibcode: 2003AN....324..303K Altcode: Led by the National Solar Observatory, plans have been made to design and to develop the Advanced Technology Solar Telescope (ATST). The ATST will be a 4-m general-purpose solar telescope equipped with adaptive optics and versatile post-focus instrumentation. Its main aim will be to achieve an angular resolution of 0.03 arcsec (20 km on the solar surface). The project and the telescope design are briefly described. Title: Polarimetry with the ATST (Invited review) Authors: Keller, C. U. Bibcode: 2003ASPC..286...31K Altcode: 2003ctmf.conf...31K No abstract at ADS Title: Solar observing facilities Authors: Fleck, B.; Keller, C. U. Bibcode: 2003dysu.book..403F Altcode: An overview is given of current and planned ground-based solar telescopes and instruments, balloon-borne and suborbital solar telescopes, and solar and heliospheric space missions. These observing facilities operate in all areas of solar physics, from the solar interior to interplanetary space and from regimes of high energy to observations requiring high resolution. The next generation of solar telescopes and instruments promise us the ability to investigate solar processes on their fundamental scales, whether sub-arc second or global in nature. Title: Science Objectives and Technical Challenges of the Advanced Technology Solar Telescope (Invited review) Authors: Rimmele, T.; Keil, S. L.; Keller, C.; Hill, F.; Penn, M.; Goodrich, B.; Hegwer, S.; Hubbard, R.; Oschmann, J.; Warner, M.; Dalrymple, N.; Radick, R.; Atst Team Bibcode: 2003ASPC..286....3R Altcode: 2003ctmf.conf....3R No abstract at ADS Title: Polarimetry with the Advanced Technology Solar Telescope Authors: Keller, C. U. Bibcode: 2003ASPC..307...23K Altcode: No abstract at ADS Title: The SOLIS Vector-Spectromagnetograph Authors: Keller, C. U.; Harvey, J. W.; Solis Team Bibcode: 2003ASPC..307...13K Altcode: No abstract at ADS Title: Helicity and the SOLIS Vector-Spectromagnetograph Authors: Keller, Christoph U.; Harvey, John W.; Henney, Carl J.; Jones, Harrison P. Bibcode: 2003IAUJD...3E..18K Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that greatly improve ground-based synoptic solar observations. The main instrument the Vector Spectro-Magnetograph (VSM) is a compact high-throughput vector-polarimeter that measures the magnetic field strength and direction over the full solar disk within 15 minutes. Helicity can be directly calculated from the full-disk vector field data provided by the Stokes inversion of the observed polarized line profiles. We present an anlysis of the difficulties and problems that one faces when interpreting the the helicity derived from VSM data. Title: Data analysis for the SOLIS Vector Spectromagnetograph Authors: Jones, Harrison P.; Harvey, John W.; Henney, Carl J.; Hill, Frank; Keller, Christoph U. Bibcode: 2002ESASP.505...15J Altcode: 2002solm.conf...15J; 2002IAUCo.188...15J The National Solar Observatory's SOLIS Vector Spectromagnetograph, which will produce three or more full-disk maps of the Sun's photospheric vector magnetic field every day for at least one solar magnetic cycle, is in the final stages of assembly. Initial observations, including cross-calibration with the current NASA/NSO spectromagnetograph (SPM), will soon be carried out at a test site in Tucson. This paper discusses data analysis techniques for reducing the raw data, calculation of line-of-sight magnetograms, and both quick-look and high-precision inference of vector fields from Stokes spectral profiles. Existing SPM algorithms, suitably modified to accommodate the cameras, scanning pattern, and polarization calibration optics for the VSM, will be used to "clean" the raw data and to process line-of-sight magnetograms. A recent version of the High Altitude Observatory Milne-Eddington (HAO-ME) inversion code will be used for high-precision vector fields. Title: Spectroscopic Detection of the 3.934 Micron Line of Si IX in the Solar Corona Authors: Judge, P. G.; Tomczyk, S.; Livingston, W. C.; Keller, C. U.; Penn, M. J. Bibcode: 2002ApJ...576L.157J Altcode: We report the detection of the 2s22p23P1-->0 line of Si IX using the McMath-Pierce telescope on Kitt Peak. Observations were made of the solar disk and at various heights above the limb between 2002 May 13 and 17, under nonideal sky conditions, using the 13.5 m vertical spectrograph and an InSb single-diode detector. We report a new rest wavelength for the line and discuss its potential use as a diagnostic of coronal magnetic fields using ground-based telescopes. Our observations give λrest=3.93434+/-0.00007 μm, consistent with earlier less accurate values, but it places the blue wing of the line under a strong telluric N2O line. In the active regions observed, the line's intensity is comparable to or larger than predicted in earlier work for the quiet Sun. Title: The Advanced Technology Solar Telescope Authors: Keller, C. U.; Rimmele, T. R.; Hill, F.; Keil, S. L.; Oschmann, J. M.; ATST Team Bibcode: 2002AN....323..294K Altcode: The Advanced Technology Solar Telescope is the largest solar optical facility currently under development. The National Solar Observatory and its partners have just started the design and development phase with first light being planned for late this decade. The 4-m telescope will provide an angular resolution down to 0.025 arcsec, a large photon flux for precise magnetic and velocity field measurements, and access to a broad set of diagnostics from 0.3 to 28 mu m. We summarize the currently envisioned scientific capabilities of the telescope and its suite of instruments along with a glimpse at some of the early concepts. Title: Spatial mapping of the Hanle and Zeeman effects on the Sun Authors: Stenflo, J. O.; Gandorfer, A.; Holzreuter, R.; Gisler, D.; Keller, C. U.; Bianda, M. Bibcode: 2002A&A...389..314S Altcode: Spatial mapping of the Hanle and Zeeman effects on the Sun has been done for the first time, through Stokes vector imaging with a narrow-band (0.2 Å) universal filter. It is shown how the polarization signatures of the Hanle and Zeeman effects can be cleanly distinguished from each other by comparing the Stokes images recorded at different, specially selected wavelengths within the Na I D2-D1 line system. Examples of the polarization signatures of sunspots, faculae, the supergranulation network, and large-scale canopy fields are shown. The most striking result of our observations is that the scattering polarization has an extremely intermittent structure rather than being a simple function of limb distance. These intermittent scattering polarization signals are cospatial with the facular and supergranulation network seen both in intensity and circular polarization. The observed pattern can be explained in terms of magnetic enhancement of the scattering polarization in the network and/or Hanle depolarization of the scattering polarization outside the network. Since however no magnetic fields are seen in circular polarization outside the network, the relative absence of linear scattering polarization there may be explained by Hanle depolarization only if the volume filling, depolarizing magnetic field has mixed polarities on a subarcsec scale that is not resolved. Title: Preliminary tests of a low-cost solar infrared adaptive optics system Authors: Ammons, S. M.; Keller, C. U. Bibcode: 2002AAS...200.5610A Altcode: 2002BAAS...34..736A Images produced by the National Solar Observatory's McMath-Pierce telescope on Kitt Peak, the largest solar telescope in the world, have been at the mercy of atmospheric turbulence for decades. Work is currently underway to install a low-cost adaptive optics system with the goal of correction in the infrared for a total hardware cost of \$25k. As a preliminary step, a slow AO system was constructed in the lab to demonstrate the feasibility of the low-cost approach. The design is a simple feedback loop that reads the wavefront shape with a Hartmann wavefront sensor and makes corrections through a micromachined membrane deformable mirror. A computer calculates the voltages to apply to the 37-actuator mirror based on the wavefront information. The system operates at 1 Hz and is able to correct a distorted laser wavefront within several cycles. This test paves the way to deploy a faster version of this system that runs at 500 Hz. Funded by NSF. Title: Stokes Inversion Techniques for the SOLIS-VSM Authors: Henney, C. J.; Keller, C. U.; Jones, H. P.; SOLIS Team Bibcode: 2002AAS...200.5514H Altcode: 2002BAAS...34Q.734H The Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project will begin operation in 2002 and provide a 25 year record of synoptic solar observations. The 50-cm aperture VSM will provide daily full-disk photospheric vector and high sensitivity longitudinal magnetograms. In addition, the VSM will produce daily full-disk chormospheric longitudinal magnetograms, along with 1083 nm equivalent width images. Current data transmission and storage resources are such that the reduction of VSM data will be performed at the observing site on Kitt Peak. Reduced data products will be transmitted via a DS3 link from Kitt Peak to the National Solar Observatory's digital archive in Tucson. During a typical observing day, three full-disk photospheric vector magnetograms will be available over the web in two stages: first, as a ``quick-look'' product within 10 minutes of data acquisition, and then as a full Milne-Eddington (ME) inversion product within 24 hours of each observation. The quick-look parameters will include estimates of the magnetic field strength, azimuth and inclination based on Auer, Heasley, House (1977, Solar Physics 55, p. 47). The high-precision vector products will be determined with the High Altitude Observatory ME inversion technique implemented by Skumanich and Lites (1987, ApJ, 322, p.473). The flexible design of the VSM data handling system can incorporate future improvements under consideration (e.g., principal component analysis). However, since the original Stokes profiles observed will not be archived, a retrospective reduction of VSM data will not be possible. Quick-look and full ME inversion results are compared using simulated VSM Stokes profile data based on Kitt Peak Vacuum Telescope magnetograms. This research was supported in part by the Office of Naval Research Grant N00014-91-J-1040. Title: Observational Signatures of a Solar Small-Scale Global Dynamo Authors: Keller, C. U.; Stein, R. F.; Nordlund, A. Bibcode: 2002AAS...200.8908K Altcode: 2002BAAS...34..792K There is ample theoretical and observational evidence for the existence of a dynamo operating in the solar convection zone that produces small-scale, weak magnetic fields. The next generation of solar telescopes such as the 4-m Advanced Technology Solar Telescope and the 1.5-m GREGOR will be able to provide observational data on these magnetic fields. In order to guide the development of instruments and observational procedures to investigate these small-scale magnetic fields, we have calculated polarized spectral line profiles from numerical simulations of a small-scale global dynamo and analyzed them as if they were actual observations of the Sun. The simulated observations include realistic noise, spatial smearing from a partially correcting AO system, and spectral smearing and scattered light from a spectrograph. We identify the unique signatures of these magnetic fields and relate them to the physical conditions in the numerical simulations. Title: The Advanced Technology Solar Telescope Authors: Rimmele, T. R.; Keil, S. L.; Keller, C. U.; Hill, F.; Oschmann, J. M.; Warner, M.; Dalrymple, N. E.; ATST Team Bibcode: 2002AAS...200.3408R Altcode: 2002BAAS...34..691R The 4m aperture Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world and a unique scientific tool to study the Sun and other astronomical objects, such as planets. The ATST will replace major existing national solar facilities at the end of this decade. The ATST project has entered the design and development phase. We present an overview of the ATST science drivers and discuss preliminary design concepts and technical challenges. The ATST science goals lead to the following general requirements for the ATST facility: - Diffraction limited angular resolution in the visible and infrared to study fundamental astrophysical processes with unprecedented resolution enabling verification of model predictions. - A high photon flux for accurate measurements of physical parameters throughout the solar atmosphere, such as magnetic strength and direction, temperature and velocity. - Access to a new diagnostics at relatively unexplored infrared wavelength. - Low scattered light to enable coronal observations. - Low instrumental polarization for accurate measurements of magnetic fields. Development of a 4m solar telescope presents many technical challenges. The large aperture drives the ATST to an open-air design and makes thermal control of optics and telescope structure a paramount consideration. To achieve diffraction-limited observations at visible and infrared wavelength ATST will have a high order solar adaptive optics system. Coronal observations require, occulting in prime focus, a Lyot stop and contamination control of the primary. An initial set of instruments will be designed as integral part of the telescope. Preliminary telescope and instrument concepts will be discussed. Title: Instrumentation for astrophysical spectropolarimetry Authors: Keller, Christoph U. Bibcode: 2002apsp.conf..303K Altcode: Astronomical spectropolarimetry is performed from the X-ray to the radio regimes of the electromagnetic spectrum. The following chapter deals with instruments and their components that are used in the wavelength range from 300 nm to 20 μm. After introducing the terminology and formalisms that are used in the context of astronomical spectropolarimeters, I discuss the most widely used optical components. These include crystal and sheet polarizers, fixed monochromatic and achromatic retarders, and variable retarders such as liquid crystals and photoelastic modulators. Since polarimetric measurements are often limited by systematic errors rather than statistical errors due to photon noise, I deal with these instrumentally induced errors in detail. Among these errors, I discuss instrumental polarization of various kinds and chromatic and angle of incidence errors of optical components. I close with a few examples of successful, modern night-time and solar spectropolarimeter. Title: The ionosphere of Titan Authors: Cravens, T.; Vann, J.; Clark, J.; Yu, J.; Keller, C. Bibcode: 2002cosp...34E.399C Altcode: 2002cosp.meetE.399C Titan has an atmosphere consisting mainly of molecular nitrogen and methane. Solar extreme ultraviolet and x-ray radiation and energetic electrons from SaturnSs magnetosphere interact with the upper atmosphere producing an ionosphere. We will review our current understanding of TitanSs ionosphere. We will emphasize recent models of the horizontal and vertical ion density structure. The ionospheric chemistry is quite complex and a large number of hydrocarbon ion species are present. The ion chemistry operating in the ionosphere will be reviewed. The ionosphere acts as an obstacle to the external magnetospheric flow and this will also be briefly discussed. Title: 1000 years of climate change Authors: Keller, C. Bibcode: 2002cosp...34E1791K Altcode: 2002cosp.meetE1791K Solar activity has been observed to vary on decadal and centennial time scales. Recent evidence (Bond, 2002) points to a major semi-periodic variation of approximately 1,500 yrs. For this reason, and because high resolution proxy records are limited to the past thousand years or so, assessing the role of the sun's variability on climate change over this time f ame has received much attention. A pressingr application of these assessments is the attempt to separate the role of the sun from that of various anthropogenic forcings in the past century and a half. This separation is complicated by the possible existence of natural variability other than solar, and by the fact that the time-dependence of solar and anthropogenic forcings is very similar over the past hundred years or so. It has been generally assumed that solar forcing is direct, i.e. changes in sun's irradiance. However, evidence has been put forth suggesting that there exist various additional indirect forcings that could be as large as or even exceed direct forcing (modulation of cosmic ray - induced cloudiness, UV- induced stratospheric ozone change s, or oscillator -driven changes in the Pacific Ocean). Were such forcings to be large, they could account for nearly all 20th Century warming, relegating anthropogenic effects to a minor role. Determination of climate change over the last thousand years offers perhaps the best way to assess the magnitude of total solar forcing, thus allowing its comparison with that of anthropogenic sources. Perhaps the best proxy records for climate variation in the past 1,000 yrs have been variations in temperat ure sensitive tree rings (Briffa and Osborne, 2002). A paucity of such records in the Southern Hemisphere has largely limited climate change determinations to the subtropical NH. Two problems with tree rings are that the rings respond to temperature differently with the age of the tree, and record largely the warm, growing season only. It appears that both these problems have been adequately solved although caution is warranted. A promising adjunct to tree rings is actual measurement of temperatures in boreholes. Inversion of such records gives low frequency temperatures that are potentially more accurate than any proxy- derived ones. All these records give a fairly consistent picture of at least one major warming and cooling extreme (Medieval Warming Period (MWP) and Little Ice Age (LIA). Many modeling efforts using direct solar forcing have been done. These typically employ proxy data (sunspot number and variations in Be-10 and C -14 calibrated by satellite observations) for changes in solar forcing, and give the same general picture-- that of a substantial warming 1,000 yrs ago (MWP) followed by cooling that was particularly marked in the late 17th and early 19th centuries (LIA). The resulting amplitude of temperature change between MWP and LIA agrees well with paleo-temperature reconstructions and suggests that solar forcing alone is inadequate to account for more than about half the 20th century warming (Lean et al 1995, Crowley and Lowry 2000). Since these quantitatively reproduce climate variations in the past 1000 years, the role of indirect solar forcing is inferred to be small but may be important (Lean and Rind 2001). Gerard Bond, Bernd Kromer, Juerg Beer, Raimund Muscheler, Michael N. Evans, William Showers, Sharon Hoffmann, Rusty Lotti-Bond, Irka Hajdas, and Georges Bonani, (2001) Persistent Solar Influence on North Atlantic Climate During the Holocene,Science 294: 2130-2136 Briffa and Osborne, (2002) Blowing Hot and Cold, Science 295, 2227-2228. Lean, J., Beer, J., and Bradley, R., (1995) Reconstruction of solar irradiance since 1610: Implications for climate change, Geophys. Res. Lett.., 22, 3195-3198. Crowley ,T., (2000) Causes of climate change over the past 1000 years, Science,289, 270- 277. Lean and Rind, (2001), Earth's Response to a Variable Sun, Science, 292, 234-236. Title: Polarization Modulation for Solar Vector-Polarimeters using Ferroelectric Liquid Crystals Authors: Pifer, R.; Keller, C. Bibcode: 2001AGUSM..SP51B05P Altcode: Polarization modulation for solar vector polarimetry can be performed in various ways. Ferroelectric liquid crystals (FLC) have the advantage of allowing modulation frequencies of up to several kHz, therefore avoiding spurious signals due to seeing. A new modulation scheme using two FLC modulators was designed for the SOLIS Vector-Spectromagnetograph (VSM). Here we present experimental confirmation of this modulation scheme from a laboratory setup using a fast CCD camera running at 300 frames per second. Furthermore, we also show measurements from a circular polarization modulator that operates at 854 nm but uses one FLC built for 630 nm. This provides a means to cover a large wavelength range with a single, chromatic FLC modulator. Title: The Advanced Technology Solar Telescope -- Science Goals and Instrument Description. Authors: Rimmele, T. R.; Keil, S. L.; Keller, C. U.; Hill, F. Bibcode: 2001AGUSM..SH31D08R Altcode: High-resolution studies of the Sun's magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun, and what mechanisms trigger flares and coronal mass ejections. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe physical scales fundamental to these processes. The 4m aperture ATST will be a unique scientific tool, with excellent angular resolution, a large wavelength range, and low scattered light. With its integrated adaptive optics, the ATST will achieve a spatial resolution nearly 10 times better than any existing solar telescope. The ATST will provide:

Unprecedented angular resolution of 0.03 arcsec in the visible and 0.08 arcsec at 1.6 microns to enable us to clearly resolve and study the fundamental astrophysical processes on their intrinsic scales and to verify model predictions. A high photon flux for accurate and precise measurements of physical parameters, such as magnetic field strength and direction, temperature and velocity, on the short time scales involved. Access to a broad set of diagnostics, from visible to thermal infrared wavelengths. Low scattered light observations and coronagraphic capabilities in the infrared, allowing measurements of coronal magnetic fields. The ATST has been highly ranked by the latest Decadal Survey of Astronomy and Astrophysics and the NAS/NRC study of ground-based solar astronomy. A large part of the solar community will participate in the design and development of the ATST. A strawman telescope design, design challenges and instrument concepts will be discussed. Examples of recent high resolution observations with adaptive optics, that demonstrate the potential of this new technology will be shown. Title: Influence of magnetic fields on the coherence effects in the Na I D1 and D2 lines Authors: Stenflo, J. O.; Gandorfer, A.; Wenzler, T.; Keller, C. U. Bibcode: 2001A&A...367.1033S Altcode: To clarify the physical nature of the enigmatic scattering polarization in the Na i D1 and D2 line cores we have explored their behavior with full Stokes vector polarimetry in regions with varying degree of magnetic activity near the solar limb. These observations represent the first time that ZIMPOL II, the second generation of our CCD based imaging polarimeter systems, has been used for a scientific program. With ZIMPOL II the four Stokes images can be demodulated and recorded with a single CCD sensor such that the resulting images of the fractional polarization Q/I, U/I, and V/I are entirely free from spurious features due to seeing or flat-field effects. The polarization in the cores of the lines, in particular in D2, exhibits dramatic and unexpected spatial variations in both Q/I and U/I, including polarization self-reversals of the D2 Q/I core peak. As the fluctuations in the Q, U, and V parameters appear to be relatively uncorrelated, we have parametrized the profiles and made scatter plots of the extracted parameters. Comparison with synthetic scatter plots based on different theoretical models suggests that the polarization signals in the cores of the D2 and D1 lines have different physical origins: While the D1 core is likely to be governed by ground-state atomic polarization, the D2 core is dominated by the alignment of the excited state and by effects of partial frequency redistribution. Title: The SOLIS Vector-Spectromagnetograph (VSM) Authors: Keller, C. U.; Solis Team Bibcode: 2001ASPC..236...16K Altcode: 2001aspt.conf...16K No abstract at ADS Title: High Resolution Polarimetry and the Need for a Large-Aperture Solar Telescope Authors: Keller, C. U. Bibcode: 2001ASPC..236..389K Altcode: 2001aspt.conf..389K No abstract at ADS Title: The Advanced Technology Solar Telescope Authors: Keil, S. L.; Rimmele, T. R.; Keller, C. U.; Atst Team Bibcode: 2001ASPC..236..597K Altcode: 2001aspt.conf..597K No abstract at ADS Title: Solar Chromospheric Magnetic Fields Authors: Keller, C. U. Bibcode: 2001ASPC..248...61K Altcode: 2001mfah.conf...61K No abstract at ADS Title: The Advanced Technology Solar Telescope: Science Goals and Instrument Description Authors: Rimmele, T.; Keller, C.; Keil, S.; Hill, F.; Atst Team Bibcode: 2001AGM....18S1006R Altcode: High-resolution studies of the Sun's magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. For example, the generation of magnetic fields through dynamo processes is still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scales fundamental to these processes. The 4m aperture ATST will be a unique scientific tool, which will provide unprecedented angular resolution, high photon flux, access to a broad set of diagnostics, from visible to thermal infrared wavelengths, and low scattered light observations and coronagraphic capabilities in the infrared. Development of a 4-m solar telescope presents several technical challenges. The large heat flux makes thermal control of optics and telescope structure a paramount consideration. To achieve diffraction-limited performance, a powerful solar adaptive optics system is required. Low scattered light is essential for observing the corona but also to accurately measure the physical properties of small structures in, for example, sunspots. Contamination control of the primary and secondary mirrors must therefore be addressed. An initial set of instruments will be designed as integral part of the telescope during the upcoming design and development phase. A strawman telescope design and instrument concepts will be discussed. Title: The Advanced Solar Telescope Authors: Keil, S. L.; Rimmele, T. R.; Keller, C.; Hill, F. Bibcode: 2000AAS...197.1710K Altcode: 2000BAAS...32.1433K The planned Advanced Technology Solar Telescope (ATST) will be a 4-m aperture general-purpose solar telescope with integrated adaptive optics and versatile post focus instrumentation. The ATST will achieve an angular resolution of 0.03 arcsec (20 km on the solar surface) in the visible, which is almost an order of magnitude better than what is achieved with current solar telescopes. This will make it possible to resolve the fundamental astrophysical hydrodynamic and magnetohydrodynamic processes and structures in the solar atmosphere such as the building blocks of solar magnetic fields that are believed to be responsible for solar irradiance variations and the heating of the outer solar atmosphere. The ATST will cover the wavelength range from 0.35 to 35 ?m and minimize scattered light. The initial set of post focus instruments will exploit the unique capabilities of the ATST to study magnetic fields at the highest spatial resolution in the visible and near-infrared parts of the spectrum. The ATST was highly recommended by the recent Decadal Study. A proposal for a four-year Design and Development phase has just been submitted to the NSF. Construction is expected to start in FY2005. The National Solar Observatory is operated by the Associated Universities for Research in Astronomy and is funded by the National Science Foundation under a cooperative agreement. Title: Results from a revisit to the K2V bright points Authors: Sivaraman, K. R.; Gupta, S. S.; Livingston, W. C.; Damé, L.; Kalkofen, W.; Keller, C. U.; Smartt, R.; Hasan, S. S. Bibcode: 2000A&A...363..279S Altcode: We have used pairs of temporally simultaneous CaII K-line spectroheliograms and magnetic area scans to search for spatial correlation between the CaII K2V bright points in the interior of the network and corresponding magnetic elements. We find that about 60% of the K2V bright points spatially coincide with magnetic elements of flux density > 4 Mx cm-2. About 25% of the K2V bright points with equally enhanced emission lie over bipole elements where the fields are > 4 Mx cm-2 for both polarity elements which merge and presumably cancel and result in low fields. The rest, 15%, of the bright points coincide with areas of fields < 4 Mx cm-2 which is the noise level set by us for the magnetic scans. When magnetic elements of opposite polarity merge and form bipoles, the associated K2V bright points show excess emission. Although such excess emission is a magnetic-field-driven phenomenon, the measured value of the field at the site of the bipole is typically low, and these cases would therefore be excluded in the count of coincidences of excess emission with excess magnetic fields. In our opinion, these cases of excess emission at the sites of the bipoles, as well as at the sites of fields > 4 Mx cm-2, are both instances of magnetic-field-related emissions. If the former are not taken into account as coincidences, the correlation will drop down and this might be interpreted as not an obvious correlation. Our present results, taking into account the low fields of merging bipoles, establish the association of K2V bright points with magnetic elements. Title: New Initiatives for Synoptic Observations Authors: Keller, C. U. Bibcode: 2000JApA...21..127K Altcode: No abstract at ADS Title: New Results from the Flare Genesis Experiment Authors: Rust, D. M.; Bernasconi, P. N.; Eaton, H. A.; Keller, C.; Murphy, G. A.; Schmieder, B. Bibcode: 2000SPD....31.0302R Altcode: 2000BAAS...32..834R From January 10 to 27, 2000, the Flare Genesis solar telescope observed the Sun while suspended from a balloon in the stratosphere above Antarctica. The goal of the mission was to acquire long time series of high-resolution images and vector magnetograms of the solar photosphere and chromosphere. Images were obtained in the magnetically sensitive Ca I line at 6122 Angstroms and at H-alpha (6563 Angstroms). The FGE data were obtained in the context of Max Millennium Observing Campaign #004, the objective of which was to study the ``Genesis of Solar Flares and Active Filaments/Sigmoids." Flare Genesis obtained about 26,000 usable images on the 8 targeted active regions. A preliminary examination reveals a good sequence on an emerging flux region and data on the M1 flare on January 22, as well as a number of sequences on active filaments. We will present the results of our first analysis efforts. Flare Genesis was supported by NASA grants NAG5-4955, NAG5-5139, and NAG5-8331 and by NSF grant OPP-9615073. The Air Force Office of Scientific Research and the Ballistic Missile Defense Organization supported early development of the Flare Genesis Experiment. Title: Evolution of small-scale magnetic fields from combined adaptive optics and phase-diverse speckle imaging Authors: Keller, C. U.; Rimmele, T. R.; Paxman, R. G.; Seldin, J. H.; Carrara, D.; Gleichman, K. Bibcode: 2000SPD....31.0301K Altcode: 2000BAAS...32..833K We have obtained movies of the photospheric magnetic field at a sustained resolution of 0.2 arcsec by combining the adaptive optics system at the Dunn Solar Telescope with the Zurich Imaging Polarimeter I (ZIMPOL) and processing the data with Phase-Diverse Speckle Imaging and speckle deconvolution. The adaptive optics was correcting the low-order aberrations with an update rate of about 1.5 kHz and fed a narrow-band channel through the Universal Birefringent Filter in the wing of the CaI 610.3 nm line and two white-light channels that were used to obtain one in-focus and one out-of-focus image for the phase-diversity processing, which removes the remaining aberrations. All three channels were equipped with ZIMPOL I cameras running simultaneously at 5 frames per second. The narrow-band intensity and magnetogram images were reconstructed using speckle deconvolution. This combined attack to obtain the best magnetogram movies of the solar surface was very successful and led to spectacular time sequences with a consistent spatial resolution of better than 0.2 arcsec. We will present the first scientific results on the evolution of the small-scale magnetic fields in an active region. This work was supported by the National Science Foundation. Title: Center-to-limb variation of the enigmatic Na bt I D_1 and D_2 polarization profiles Authors: Stenflo, J. O.; Gandorfer, A.; Keller, C. U. Bibcode: 2000A&A...355..781S Altcode: The remarkable polarization structure of the Na i D_1 and D_2 lines that is due to coherent scattering has remained an enigma, since it has not yet been possible to find an explanation that is consistent with both current understanding of quantum mechanics and the astrophysical properties of the Sun's atmosphere. To guide future theoretical efforts we have here explored the detailed center-to-limb variation of the linearly polarized profiles in non-magnetic regions. In particular we find that the unexplained narrow polarization peaks in the Doppler cores of the two lines become even more pronounced with respect to the relative profile shape as we move away from the limb towards the center of the solar disk. Title: Anomalous polarization effects due to coherent scattering on the Sun Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A. Bibcode: 2000A&A...355..789S Altcode: The richly structured linearly polarized spectrum that is produced by coherent scattering in the Sun's atmosphere contains a number of spectral features for which no explanation has been found within the standard scattering theory. According to this quantum-mechanical framework, the intrinsic polarizability of a given line should be determined by the total angular momentum quantum numbers of the atomic levels involved in the scattering transition (which may be resonant or fluorescent). Well defined polarization peaks have been observed in many lines, which according to these theoretical concepts should be intrinsically unpolarizable. A possible explanation for these anomalous spectral structures could be that the initial ground state of the scattering transition becomes polarized by an optical pumping process. However, such an explanation is contradicted by other observations, since it seems to require that much of the solar atmosphere must be filled with extremely weak magnetic fields (<~ 10 mG). We have searched through the whole visible solar spectrum for lines with the quantum numbers that should normally make them unpolarizable, and have carried out a systematic observing program for the most prominent of these lines. Here we report on the observed properties of the polarized line profiles of these lines and explain in what respect their behaviors are anomalous and cannot be understood within current conceptual frameworks. Title: 5,000 by 5,000 Spatial by 15,000 Spectral Resolution Elements: First Astronomical Observations with a Novel 3-D Detector Authors: Keller, C. U. Bibcode: 2000ASPC..195..495K Altcode: 2000iutd.conf..495K No abstract at ADS Title: Deconvolution of narrowband solar images using aberrations estimated from phase-diverse imagery Authors: Seldin, John H.; Paxman, Richard G.; Carrara, David A.; Keller, Christoph U.; Rimmele, Thomas R. Bibcode: 1999SPIE.3815..155S Altcode: Phase-Diverse Speckle (PDS) is a short-exposure data- collection and processing technique that blends phase- diversity and speckle-imaging concepts. PDS has been successfully used for solar astronomy to achieve near diffraction-limited resolution in ground-based imaging of solar granulation. Variants of PDS that involve narrow-band, spectroscopic, and polarimetric data provide more information observations. We present results from processing data collected with the 76-cm Richard B. Dunn Solar Telescope (DST) on Sacramento Peak, NM. Three-channel data sets consisting of a pair of phase-diverse images of the solar continuum and a narrow-band image were collected over spans of 15 - 20 minutes. Point-spread functions that are estimated from the PDS data are used in a multi-frame deconvolution algorithm to correct the narrow-band imagery. The data were processed into a number of time series. A rare, short-lived continuum bright point with a peak intensity at a factor of 2.1 above the mean intensity in the continuum was observed in one such sequence. The field of view spans multiple isoplanatic patches, and strategies for processing these large fields were developed. We will discuss these methods along with other techniques that were explored for accelerating the processing. Finally, we show the first PDS reconstruction of adaptive-optics (AO) compensated solar granulation taken at the DST. As expected, we find that these data are less aberrated and, thus, the use of AO in future experiments is planned. Title: Infrared lines as probes of solar magnetic features. XV. Evershed flow in cool, weak penumbral fields Authors: Rüedi, I.; Solanki, S. K.; Keller, C. U. Bibcode: 1999A&A...348L..37R Altcode: Observations of Ti I lines at 2.2 mu m show that the Evershed flow takes place in cool, almost horizontal channels with a low magnetic field strength (~ 500-900 G) that does not appear to change significantly across the penumbra. This property might allow an outward directed siphon flow to exist along such cool flux tubes. Title: Evolution of Protoplanetary Accretion Disks including Chemistry and Transport Processes Authors: Duschl, W. J.; Gail, H. -P.; Keller, C.; Tscharnuter, W. M. Bibcode: 1999AGAb...15...22D Altcode: 1999AGM....15..B14D According to our current understanding, planetary systems form in protostellar accretion disks. At late stages of the star formation process, conditions become favourable for the dust component in the disk to agglomerate first into km-sized planetesimals and by a complicated hierarchy of further growth process into planets. The simulation of the structure and evolution of protoplanetary accretion disks are a necessary prerequisite for understanding the formation of planetary systems. An important topic in this context is the study of the chemistry of gas and dust, the mineralogical and petrological properties of the dust, and of radial and vertical transport processes in the disc, since this determines the temporal evolution and radial distribution of the composition of the disk material, from which the planetary system bodies are formed. The construction of hydrodynamic evolution models including chemical and transport processes is part of a project within the SFB 359 "Reactive Flows, Diffusion, and Transport" at Heidelberg University. The present state of this project and the future plans are briefly discussed. As a special result, we discuss the chemistry and structure of a stationary disk model, which combines self-consistently chemical equilibrium calculations for the gas phase and the mineral assemblage in the disk with the complete set of equations for the radial and vertical disk structure in the 1+1 dimensional approximation. Convection and radiative transfer is included in the model calculation. We briefly discuss the mineral composition predicted by this model with (i) results of earlier condensation calculations, and (ii) the mineral assemblage observed for primitive meteorites. Title: Optimum Apodization for Speckle Imaging of Extended Sources Authors: Keller, C. Bibcode: 1999ASPC..183..342K Altcode: 1999hrsp.conf..342K No abstract at ADS Title: The Advanced Solar Telescope: I. Science Goals Authors: Keller, C. Bibcode: 1999ASPC..183..169K Altcode: 1999hrsp.conf..169K No abstract at ADS Title: Phase-Diversity Data Sets and Processing Strategies Authors: Paxman, R.; Seldin, J.; Keller, C. Bibcode: 1999ASPC..183..311P Altcode: 1999hrsp.conf..311P No abstract at ADS Title: Scattering polarization in the chromosphere Authors: Keller, C. U.; Sheeley, N. R., Jr. Bibcode: 1999ASSL..243...17K Altcode: 1999sopo.conf...17K No abstract at ADS Title: Infrared lines as probes of solar magnetic features. XIV. TI i and the cool components of sunspots Authors: Rueedi, I.; Solanki, S. K.; Keller, C. U.; Frutiger, C. Bibcode: 1998A&A...338.1089R Altcode: The first systematic observations of sunspot umbrae using the lines of the Ti Ii multiplet at 2.2 mu m are presented. Their diagnostic capabilities are investigated, developed and used to investigate the magnetic and velocity structure of a sunspot. These lines are most sensitive to cool plasma. In addition, they are extremely Zeeman sensitive. We find that a sunspot is composed of two distinct cool magnetic components. One of them is fairly vertical, has a large magnetic field strength and is associated with the central (umbral) part of the sunspot. The other component is strongest near the outer boundary of the spot (penumbra), is much more inclined, has a very low magnetic field strength and shows the signature of the Evershed effect. In contrast to the smooth transition of field strength from the darkest part of the umbra to the outer penumbral boundary usually visible in observations carried out in other spectral lines, the Ti Ii lines exhibit a sharp transition between the two magnetic components. Title: SOLIS: a modern facility for synoptic solar observations Authors: Keller, Christoph U. Bibcode: 1998SPIE.3352..732K Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. It will provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of data. A high degree of automation and remote control will provide faster user access to data and flexible interaction with the data- collection process. The instruments include a vector spectromagnetograph that will measure the magnetic field strength and direction over the full solar disk in 15 minutes, a full disk patrol delivering digital images in various spectral lines at a high cadence, and a Sun-as-a- star precision spectrometer to measure changes in many spectral lines. Title: SOLIS: A Modern Facility for Synoptic Solar Observations Authors: Keller, C. U. Bibcode: 1998STIN...9946076K Altcode: SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. It will provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of data. A high degree of automation and remote control will provide faster user access to data and flexible interaction with the data-collection process. The instruments include a vector spectromagnetograph that will measure the magnetic field strength and direction over the full solar disk in 15 minutes, a full disk patrol delivering digital images in various spectral lines at a high cadence, and a Sun-as-a-star precision spectrometer to measure changes in many spectral lines. Title: Complex magnetic fields in an active region Authors: Bernasconi, P. N.; Keller, C. U.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1998A&A...329..704B Altcode: High-resolution observations of the full Stokes vector in Fe\sc i spectral lines around 5250 Angstroms obtained at the Swedish Vacuum Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a complex active region reveal the presence of anomalously shaped Stokes profiles indicating the coexistence of at least two magnetic components within the same spatial resolution element. These Stokes profiles have been analyzed with an inversion code based on a 3-component atmospheric model with two magnetic and one field-free component. The fits to the observations in a magnetic region that resembles a small penumbra reveal the presence of a horizontal magnetic field component with an average field strength of /line{B}=840 G, a mean filling factor of /lineα=0.12, and an average temperature /line{T}=5400 K at log {tau_ {5000}}=-1.5 embedded in the main ``penumbral'' magnetic field that has /line{B}=1500 G, /lineα=0.56, and /line{T}=4900 K. The horizontal component exhibits a mean outflow of 2.7 km s(-1) which is mainly due to the Evershed flow. In a region where there are strong downflows up to 7 km s(-1) , we infer the possible presence of a shock front whose height changes along the slit. The height variation can be explained by a change of the gas pressure at the base of the photosphere below the shock front as proposed by Thomas & Montesinos (1991). Small plages with field strengths below 900 G have been observed in the vicinity of some pores. Finally, we present a puzzling field structure at the boundary between two adjacent pores. Ambiguous results suggest that although the inversion code is able to successfully invert even very complex Stokes profiles, we are far from a complete description of the field structure in complex magnetic regions. We warn that magnetograms and fits to data involving only a single magnetic component may hide the true complexity of the magnetic structure in at least some parts of active regions. Title: SOLIS --- A Modern Facility for Synoptic Solar Observations Authors: Keller, C. U.; Nso Staff Bibcode: 1998ASPC..154..636K Altcode: 1998csss...10..636K SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a proposed suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. The primary scientific goal is to provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. An operational goal is to produce real-time and near real-time data for forecasting space weather, and to augment the scientific yield from space missions such as SOHO and TRACE, and ground-based projects including RISE and GONG. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of data. A high degree of automation and remote control will provide faster user access to data and flexible interaction with the data-collection process. The instruments include a vector spectromagnetograph that will measure the magnetic field strength and direction over the full solar disk in 15 minutes, a full disk patrol delivering digital images in various spectral lines at a high cadence, a coronal emission line imager and photometer that will provide photometric and velocity images in at least five spectral lines, and a Sun-as-a-star precision spectrometer to measure changes in many spectral lines. Title: SOLIS Instrumentation Aspects Authors: Keller, C. U.; NSO Staff Bibcode: 1998ASPC..140..539K Altcode: 1998ssp..conf..539K No abstract at ADS Title: Population number inversion in disk-impinging-clumps Authors: Fiebig, D.; Els, S. G.; Keller, C. Bibcode: 1998AGAb...14...22F Altcode: 1998AGM....14..B27F The model of accretion disk-impinging-clumps is numerically investigated with respect to the occurrence of a population number inversion between the J_{K_{-1} {K_{+1}} = 616, 523 ortho-{H_2O} rotational levels. Former studies (Fiebig 1997) have shown that the collision of small, dense clumps onto the protostellar accretion disk around the deeply embedded YSO in L 1287 can account for the appearence of the associated position-velocity structure of identified 22 GHz {H_2O} masers. Recently another source was found which indicates a very similar structure (Torrelles et al. 1998). Since the origin of the 22 GHz {H_2O} maser emission is assigned to shock heated disk-clump collisions, a 2-D radiative transfer code was developed to calculate the population numbers of water molecules in the interaction region of a single collision event. Starting from the results obtained by a 2-D SPH-Code (Keller & Fiebig \ this conference) the spatial distribution of gas and dust temperature, density, and velocity were adopted as boundary conditions. Since dust was found to be the dominant cooling agent (Els & Fiebig \ this conference), the radiative transfer between water molecules could be decoupled from the heating/cooling processes incorporated into the SPH-Code. The 2-D radiative transfer code makes use of a modified short characteristics method, considers Rayleigh scattering, and is usual ly run for a number of 60 rotational levels covering corresponding levels temperatures up to 2600 K. The computational results clearly show the occurrence of compact regions of population number inversion to be identified with observable 22 GHz {H_2O} maser emission. Title: Differential Hanle effect and the spatial variation of turbulent magnetic fields on the Sun Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A. Bibcode: 1998A&A...329..319S Altcode: While diagnostic techniques based on the ordinary Zeeman effect (e.g. magnetograms) are almost ``blind'' to a turbulent magnetic field with mixed magnetic polarities within the spatial resolution element, the Hanle effect is sensitive to this domain of solar magnetism. We present observational evidence that the turbulent magnetic field that fills the 99\ts%\ of the volume between the kG flux tubes in quiet solar regions does not have a unique field-strength distribution, but the rms turbulent field strength can vary by an order of magnitude from one solar location to the next. The varying Hanle depolarization in combinations of spectral lines with different sensitivities to the Hanle effect is conspicuously evident from direct visual inspection of the spectra. To quantify these variations we have extracted the polarization amplitudes for a selection of spectral lines observed in 8 different solar regions with different turbulent field strengths, and then applied an inversion technique to find the field strengths and calibrate the selected lines. The inversion gives stable solutions for the turbulent field strengths, in the range 4--40\ts G, but the field-strength scale is presently very uncertain. The inversion exercise has helped to expose a number of problem areas which need to be attended to before the differential Hanle effect can become a standard, reliable diagnostic tool. One major problem is the extraction of the line polarization when the contributions from the line and continuum are of the same order of magnitude, which is the usual case. For exploratory purposes we have applied a heuristic, statistical approach to deal with this problem here. Title: The USAF Improved Solar Observing Optical Network (ISOON) and its Impact on Solar Synoptic Data Bases Authors: Neidig, D.; Wiborg, P.; Confer, M.; Haas, B.; Dunn, R.; Balasubramaniam, K. S.; Gullixson, C.; Craig, D.; Kaufman, M.; Hull, W.; McGraw, R.; Henry, T.; Rentschler, R.; Keller, C.; Jones, H.; Coulter, R.; Gregory, S.; Schimming, R.; Smaga, B. Bibcode: 1998ASPC..140..519N Altcode: 1998ssp..conf..519N No abstract at ADS Title: The Procyon campaign: Observations from Kitt Peak Authors: Pilachowski, C. A.; Barden, S.; Hill, F.; Harvey, J. W.; Keller, C. U.; Giampapa, M. S. Bibcode: 1998IAUS..185..319P Altcode: Time series spectra of the F5IV star Procyon (alpha CMi) were obtained at the Kitt Peak National Observatory during a 35-night observing run in January-February 1997. The observations were obtained as part of an international collaboration to detect and study acoustic oscillations in solar-type stars. Spectra covered the wavelength range from 4000 to 5300 AA , with a resolving power of approximately 3500 (1.3 AA resolution). The sampling rate was one observation per minute, and the typical S/N ratio per observation is in excess of 1000. At the time of writing, we have obtained over 10,000 spectra. The spectra will be analyzed to identify any periodic signals due to acoustic oscillations in Procyon. In addition to measuring the equivalent widths of the three Balmer lines (Kjeldsen et al. 1995) covered by our spectra (Hβ, Hγ, and Hdelta) we will also examine the spectra for variations in the average metal line strength. Preliminary power spectra will be presented. Title: Asteroseismology from equivalent widths: a test of the sun Authors: Keller, C. U.; Harvey, J. W.; Barden, S. C.; Giampapa, M. S.; Hill, F.; Pilachowski, C. A. Bibcode: 1998IAUS..185..375K Altcode: Kjeldsen et al. (1995) reported a probable detection of solar-like, low-amplitude, p-mode oscillations of eta Bootes using equivalent width measurements from low-resolution spectra of the hydrogen Balmer lines. We tested the usefulness of this technique using observations of the Sun in integrated light. Despite the very high signal-to-noise ratio of the data stretching over six continuous days, no solar oscillation signal was found so far in the equivalent width of Hβ. Spatially resolved observations of the Hβ equivalent width at solar disk center reveal that the oscillation signal is suppressed in the wings of Hβ as compared to the continuum. Extrapolation of the oscillation signal seen in the spatially resolved data suggests an amplitude of about 1ppm for integrated light measurements, which is about a factor of 5 lower than expected from simple theoretical arguments. We explore other methods to deduce an oscillation signal from all spectral lines simultaneously. cont has: Deng, L. et al.; auths fixed below Title: Center-to-limb variation of the second solar spectrum. Authors: Stenflo, J. O.; Bianda, M.; Keller, C. U.; Solanki, S. K. Bibcode: 1997A&A...322..985S Altcode: The linear polarization that is caused by scattering processes in the solar atmosphere has been refered to as the "second solar spectrum", since it is structurally as rich as the ordinary intensity spectrum but quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the SrI 4607Å line, which has served as a diagnostic tool for determinations of spatially unresolved, turbulent magnetic fields via the Hanle effect. Here we present the detailed center-to-limb variation of the scattering polarization in this line for a number of new data sets obtained both with an electrooptical modulation system (ZIMPOL) and a non-modulating beam splitter system (at IRSOL, Locarno), to provide improved observational constraints for theoretical modelling. The amplitude and width of the polarization profile, the amount of continuum polarization, as well as the depth and width of the intensity profile have been evaluated and carefully corrected for spectral broadening and stray light. While there is generally good agreement between the five data sets, some systematic differences are shown to be of solar rather than instrumental origin, most likely due to spatially varying Hanle depolarization across the solar disk. A number of other spectral lines have been observed with the ZIMPOL system at two different limb distances (μ=0.1 and 0.2) to allow us to compare the steepness of the center-to-limb variation of their polarization amplitudes. The steepest variation is exhibited by the continuum polarization, which declines by approximately a factor of 6 when going the 15 arcsec distance from μ=0.1 to μ=0.2. The spectral lines with the steepest center-to-limb variation are molecular lines, the CaII infrared triplet, and Hα. In contrast the SrI 4607 and BaII 4554Å lines have only moderately steeper center-to-limb variations than that of an ideal, purely dipole-scattering atmosphere, for which the polarization ratio between μ=0.1 and μ=0.2 is 1.38. These center-to-limb variations may be used to constrain temperature-density models of the upper photosphere and chromosphere. Title: ISOON: The Improved Solar Observing Optical Network Authors: Neidig, D.; Confer, M.; Wiborg, P.; Dunn, R.; Balasubramaniam, K. S.; Frederick, R.; Kutzman, R.; Soli, R.; Keller, C.; Gullixson, C.; Alios, Inter Bibcode: 1997SPD....28.0224N Altcode: 1997BAAS...29..897N Efforts are under way to replace the existing SOON system, which was designed in the 1970s, with a new system (ISOON) based on a fully tunable narrow-band filter and CCD detector. ISOON would feature autonomous, rapid-cadence solar imaging and remote operation at four sites, and would transmit solar images in near real time to central facilities at Falcon AFB and Boulder CO, for use in space weather forecasting. The ISOON technical approach is to retain the front end of the existing SOON telescope, but replace the optical bench, birefringent filter, and spectrograph with a dual Fabry-Perot filter system and secondary optics contained in a single pod. ISOON data products will include full-disk H-alpha, continuum, and line-of-sight magnetograms on 1-arcsecond pixels. High- resolution images (limited field, 0.3-arcsecond pixels) would be available via a future upgrade in the secondary optics. ISOON will also be capable of acquiring vector magnetic field images via a software upgrade to be added at a future time. Title: The second solar spectrum. A new window for diagnostics of the Sun. Authors: Stenflo, J. O.; Keller, C. U. Bibcode: 1997A&A...321..927S Altcode: The Sun's radiation becomes linearly polarized by coherent scattering processes in the solar atmosphere. With a novel polarimetry system that achieves a precision of 10^-5^ in the degree of polarization, the previously largely unexplored territory of scattering physics on the Sun is now fully accessible. The observations reveal a polarized spectrum that looks very different as compared with the ordinary, unpolarized solar spectrum but has an astounding wealth of spectral structures. It is therefore refered to as the "second solar spectrum". In the present paper we show how the second solar spectrum is governed by different physical processes, which provide new diagnostic opportunities and tools that are complementary to those of the ordinary intensity spectrum. We illustrate the effects of quantum interferences and hyperfine structure, isotope abundances, partial frequency redistribution, molecular contributions, and magnetic canopies. Also shown are polarization features, for which the underlying physics has not yet been identified. Title: Chromospheric Events in the Quiet Network Authors: Keller, C.; Bastian, T.; Benz, A.; Krucker, S. Bibcode: 1997SPD....28.1304K Altcode: 1997BAAS...29..917K Time sequences of a quiet network region close to disk center have been simultaneously recorded with the VLA, various instruments on SOHO, and the solar telescopes on Kitt Peak. The analysis of the Hα spectra obtained at the McMath-Pierce telescope revealed down-flows with apparent velocities of more than 2.5 km/s associated with magnetic field structures in the quiet network. During such events, the Hα spectra show a pronounced asymmetry. The photospheric magnetic field was determined from rapid scans in three iron lines with the Zurich Imaging Stokes Polarimeter. Up- and down-flow velocity excursions outside of magnetic field regions are compatible with chromospheric waves. We describe the properties of these events as seen in the observations of the visible part of the spectrum and their signatures at radio and UV wavelengths. The final goal of this study is the construction of a time-dependent 3-D picture of the quiet solar atmosphere and the understanding of the dynamical coupling of photospheric magnetic fields with the chromosphere and the corona. Title: Observations of the quiet Sun's magnetic field. Authors: Grossmann-Doerth, U.; Keller, C. U.; Schuessler, M. Bibcode: 1996A&A...315..610G Altcode: The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL, a new polarimeter, with very low noise level. The spatial resolution element of about 1Mm was essentially determined by seeing. In about 15% of the observed area, the signal-to-noise ratio was sufficiently large to determine the strength of the magnetic field and to derive various properties of the Stokes V profiles (asymmetries, zero-crossing shift and thermal line ratio). Our results show that, at least in this fraction of area, the magnetic field in the quiet Sun is dominated by strong flux concentrations which must be rather small. The data on the Stokes V properties provide observational constraints for theoretical models of flux concentrations. Title: Concept for a miniature solar magnetograph Authors: Keller, Christoph U.; Harvey, John W. Bibcode: 1996SPIE.2804...14K Altcode: We present a novel concept for a solar magnetograph that uses a photo-refractive crystal to reflect and focus the light from the wings of many spectral lines onto a camera. The crystal acts simultaneously as multiple, narrow-band filters and as an off-axis telescope. Polarization measurements are performed close to the final focus. Since this approach uses the light from many spectral lines simultaneously, the required telescope aperture is substantially reduced and exposure times can be so short that accurate tracking is not necessary. Such a concept is particularly attractive for NASA's Minimum Solar Mission where very compact, light-weight instruments are required. Title: Time series restoration from ground-based solar observations Authors: Seldin, John H.; Paxman, Richard G.; Keller, Christoph U. Bibcode: 1996SPIE.2804..166S Altcode: Many processes of interest in the solar atmosphere have spatial scales of much less than one second of arc. If the processes are related to magnetic fields, the relevant scales are even smaller. Noticeable evolutions of solar features occur on time-scales of less than a minute if a spatial resolution of better than one second of arc is reached. It is, therefore, of great interest to recover time-series imagery with near diffraction-limited spatial resolution and good temporal resolution on a consistent basis and over extended periods of time using ground-based techniques. Phase diversity is a post-collection technique for restoring fine-resolution detail when imaging in the presence of phase aberrations such as atmospheric turbulence. Incident energy is split into two channels: one is collected at the conventional focal plane, the other is intentionally defocussed a known amount and collected by a second detector array. Phase-diverse speckle is an extension of phase diversity whereby a time sequence of short-exposure image pairs is collected. The maximum-likelihood estimate of a common object and a set of phase aberrations is performed jointly using all images. A phase-diverse speckle set of images of a plage region was collected over a span of 13.5 minutes using the 76-cm Vacuum Tower Telescope at the National Solar Observatory on Sacramento Peak. A phase- diverse pair of broad-band images at 6563 angstroms was collected along a third, narrow-band image in the wing of H- (alpha) . A set of restorations was made into a movie depicting the highly dynamic photosphere at scales below 0.3 arcsec. We conclude that the combination of fine spatial and temporal resolution achieved with phase-diverse speckle opens a new window to the study of the dynamics of the solar atmosphere from ground-based observatories. Title: Flare Genesis Experiment Authors: Murphy, Graham A.; Rust, David M.; Strohbehn, Kim; Eaton, Harry A.; Keil, Stephen L.; Keller, Christoph U.; Wiborg, P. H. Bibcode: 1996SPIE.2804..141M Altcode: In January 1996, the Flare Genesis Experiment was carried for 19 days by a 29.4 M cu. ft helium-filled balloon in the stratosphere above Antarctica, during which over 14000 images of the Sun were recorded. Long-duration ballooning provides a relatively inexpensive means to observe the Sun under near-space conditions and to develop instrumentation and techniques that will be used on future solar space missions. The purpose of the flight was to improve understanding of the mechanisms involved in many different types of solar activity, particularly flares and solar filament eruptions. Achieving this goal demanded the development of a platform for an 80-cm F/1.5 optical telescope that would be stable to 10 arcseconds. In addition, we developed an image motion compensation system capable of holding the Sun's image to better than the system's 0.2 arcsecond diffraction limit. Other key elements on board included a lithium-niobate Fabry-Perot etalon filter to provide a tunable 0.016-nm bandpass over a wide wavelength range, a fast 1534 X 1024-pixel Kodak CCD camera, and 180 GBytes of on-board storage. There was also a system for sending commands and receiving telemetry and a high-speed downlink for sending images during periods when the payload was in line of sight of the ground station. On- board computers provided a command and control system capable of near-autonomous operation. During most of the flight, contact with the payload was sporadic, so operation was primarily under autonomous control. Title: Evaluation of Phase-Diversity Techniques for Solar-Image Restoration Authors: Paxman, Richard G.; Seldin, John H.; Loefdahl, Mats G.; Scharmer, Goeran B.; Keller, Christoph U. Bibcode: 1996ApJ...466.1087P Altcode: Phase-diversity techniques provide a novel observational method for overcoming the effects of turbulence and instrument-induced aberrations in ground-based astronomy. Two implementations of phase-diversity techniques that differ with regard to noise model, estimator, optimization algorithm, method of regularization, and treatment of edge effects are described. Reconstructions of solar granulation derived by applying these two implementations to common data sets are shown to yield nearly identical images. For both implementations, reconstructions from phase-diverse speckle data (involving multiple realizations of turbulence) are shown to be superior to those derived from conventional phase-diversity data (involving a single realization). Phase-diverse speckle reconstructions are shown to achieve near diffraction-limited resolution and are validated by internal and external consistency tests, including a comparison with a reconstruction using a well-accepted speckle-imaging method. Title: Imaging Coronal Emission Lines under High Sky-Background Conditions Authors: Keller, C. U.; Smartt, R. N. Bibcode: 1996SoPh..166..311K Altcode: We have obtained images in solar coronal emission lines under high sky-background conditions by making precise differential measurements between the coronal emission line and the near-by continuum, which is primarily due to scattered light from the solar disk. Chopping between the two wavelengths was performed at 100 kHz to avoid artifacts from fast-flying dust particles and other aerosols, and also from seeing effects. The differential signal was detected with a novel CCD camera that demodulates signals up to 100 kHz. These preliminary observations show coronal emission at the 0.2% level of the scattered-light background and pave the way to efficient and precise imaging of coronal emission features under less than ideal `coronal-sky' conditions. Title: The Flare Genesis Experiment Authors: Rust, D. M.; Murphy, G. A.; Strohbehn, K.; Keil, S. L.; Keller, C. U. Bibcode: 1996AAS...188.6705R Altcode: 1996BAAS...28Q.934R The goal of the Flare Genesis Experiment is to make solar observations at the highest practicable resolution in order to improve understanding of the mechanisms involved in many different types of solar activity, particularly flares and solar filament eruptions. Achieving this goal demanded the development of a balloon-borne platform for an 80-cm F/1.5 optical telescope that could maintain 10 arcsec pointing stability. The first flight of the Flare Genesis Experiment took place in January 1996. In the stratosphere, 37 km above Antarctica, for more than 19 days, the Flare Genesis telescope pointed at the Sun with the planned stability. While the primary science objective, to measure the vector magnetic fields using two liquid crystal polarization modulators, was not achieved on this flight, 18,000 continuum images were obtained. They demonstrate that the major engineering challenges for such a flight were overcome. In addition, we developed an image motion compensation system capable of limiting the motion of the Sun's image on the focal plane to less than the system's 0.2 arcsec diffraction limit. Other key elements on board included a lithium-niobate Fabry-Perot etalon filter to provide a tunable 0.016-nm bandpass over a wide wavelength range, a 1538 x 1024- pixel CCD camera and 100 GBytes of on-board storage. We will describe the payload design and how the instruments performed. We will discuss how the constraints of long duration Antarctic ballooning guided the final design and impacted the results. Two more flights are planned before the next solar maximum. Such long-duration balloon flights provide a relatively inexpensive means to observe the Sun at the highest resolution and to develop instrumentation and techniques for future space missions. Title: Asteroseismology via equivalent widths - tests on Procyon, Eta Bootis, and Alpha Trianguli. Authors: Harvey, J. W.; Pilachowski, C.; Barden, S.; Giampapa, M.; Keller, C. U.; Hill, F. Bibcode: 1996BAAS...28..917H Altcode: No abstract at ADS Title: The Second Solar Spectrum Authors: Keller, C. U.; Stenflo, J. O. Bibcode: 1996AAS...188.5704K Altcode: 1996BAAS...28..912K The Sun's radiation becomes linearly polarized by coherent scattering processes in the solar atmosphere. With a novel imaging polarimeter (Zurich Imaging Stokes Polarimeter I) that achieves a precision of 10(-5) in the degree of polarization, the previously largely unexplored territory of scattering physics on the Sun is now fully accessible. The observations reveal a polarized spectrum that looks very different from the ordinary, unpolarized solar spectrum but has an astounding wealth of spectral structures from atoms as well as molecules. This second solar spectrum will be used as a new source of information to learn more about atomic physics, abundances and isotope ratios, radiative transfer physics, solar magnetic fields, and the thermodynamics of the solar atmosphere. Title: Asteroseismology via Equivalent Widths -- Tests on Procyon, Eta Bootes, and Alpha Triangulum Authors: Harvey, J.; Pilachowski, C.; Barden, S.; Giampapa, M.; Keller, C.; Hill, F. Bibcode: 1996AAS...188.5903H Altcode: 1996BAAS...28S.917H Recently, Kjeldsen et al. reported a probable detection of solar-like low-amplitude p-mode oscillations of Eta Bootes using equivalent width measurements from low-resolution spectra of the H Balmer lines. This technique has the potential to provide stellar oscillation measurements good enough to allow the asteroseismic inference of stellar structure. Here we report on the preliminary analysis of data from three observing runs with the Kitt Peak Coude Feed and 2.1-m telescope in November 1995 (Alpha Triangulum), February 1996 (Procyon), and March 1996 (Eta Bootes). These runs are being used to develop observing and data reduction techniques, such as a synchronized timing system to maintain evenly spaced temporal samples, a continuous unshuttered CCD readout to increase the duty cycle of the observations, and a simulation of the probability of a detection as a function of observing run length. We observed the region around the H beta, gamma, and delta lines with a spectral dispersion of about 0.4 Angstroms per pixel, extracted equivalent widths, and performed time series analysis. The temporal spectrum of Alpha Triangulum contains a significant peak near the theoretical prediction, however, we do not yet know the origin of this peak. Title: SOLIS - A Modern Facility for Synoptic Solar Observations Authors: Harvey, J.; Keller, C.; November, L.; NSO Staff Bibcode: 1996AAS...188.6703H Altcode: 1996BAAS...28..934H SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a proposed suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory on behalf the solar and solar-terrestrial physics communities. The primary scientific goal is to provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. An operational goal is to produce real-time and near real-time data for forecasting space weather, and to augment the scientific yield from space mission such as SOHO and TRACE, and ground-based projects including RISE and GONG. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of data. A high degree of automation and remote control will provide faster user access to data and flexible interaction with the data-collection process. The instruments include a vector spectromagnetograph that will measure the magnetic field strength and direction over the full solar disk in 15 minutes, a full disk patrol delivering digital images in various spectral lines at a high cadence, a coronal emission line imager and photometer that will provide photometric and velocity images in at least five spectral lines, and a Sun-as-a-star precision spectrometer to measure changes in many spectral lines. The choice of sites for the instruments depends on potential partnerships with other observatories and the level of funding that can be obtained. The goal is to place the instruments at sites with large amounts of sunshine and coronal observing conditions as appropriate. The SOLIS proposal is currently under review by the National Science Foundation. Title: NIM-2 -- A Near Infrared Imaging Vector Magnetograph Authors: Rabin, D.; Keller, C.; Jaksha, D. Bibcode: 1996AAS...188.6706R Altcode: 1996BAAS...28R.934R NIM-1 is a spectrograph-based Stokes polarimeter for measuring the strength and orientation of magnetic fields in the solar photosphere using two Zeeman-sensitive Fe I lines (g = 3 and g_geff = 1.53) near 1565 nm. NIM-2, now under construction, also uses these spectral lines but is based on a high-resolution Fabry-Perot etalon. NIM-2 will eliminate the image scanning and consequent spatial distortions of NIM-1 and will be compact and light enough to serve as a prototype for balloon or space instruments. The Queensgate etalon will provide a spectral resolving power of 10(5) over a 1-nm free spectral range. The initial detector will be the 256(2) InSb array shared with NIM-1, but NIM-2 is designed to accommodate a 512(2) or 1024(2) ``Aladdin'' InSb array. The data system, also shared with NIM-1, is being upgraded to handle the faster switching speed ( ~ 8 ms) of improved liquid-crystal variable retarders. NOAO is operated for the NSF by the Association of Universities for Research in Astronomy. Near-infrared magnetometry at NSO is supported by the the NASA Space Physics Division through the SR&T program in solar physics. Title: Measurement of the full Stokes vector of He I 10830 Å Authors: Rüedi, I.; Keller, C. U.; Solanki, S. K. Bibcode: 1996SoPh..164..265R Altcode: First observations of the full Stokes vector in the upper chromosphere are presented. The He I 10830 Å line, which has been shown to give reliable measurements of the line-of-sight component of the magnetic field vector, has been used for this purpose. It is shown that the difference between the appearance of chromospheric and photospheric magnetic structures observed close to the solar limb is largely due to the difference in height to which they refer and projection effects. The observations do suggest, however, that the magnetic field above sunspot penumbrae is somewhat more vertical in the chromosphere than in the photosphere. Title: Recent Progress in Imaging Polarimetry Authors: Keller, C. U. Bibcode: 1996SoPh..164..243K Altcode: Recent instrumental developments in imaging polarimetry allow array detectors to reach a polarimetric sensitivity of 1 × 10−4 of the intensity. New instrumental effects appear at these levels of sensitivity and generate spurious polarization signals with amplitudes of up to 5 × 10−4. Here I discuss these effects and present methods to avoid them. Polarized spectra with an rms noise of 6 × 10−6 may then be obtained. Furthermore a method is brought to the reader's attention that allows polarization measurements at the 1 × 10−4 level with regular array detectors, e.g. in the near-infrared. Title: Balloon-Borne Polarimetry Authors: Rust, D. M.; Murphy, G.; Strohbehn, K.; Keller, C. U. Bibcode: 1996SoPh..164..403R Altcode: For about two weeks in 1995, the balloon-borne Flare Genesis Experiment will continuously observe the Sun well above the turbulent, image-blurring layers of the Earth's atmosphere. The polarization-free 80 cm telescope will supply images to a liquid-crystal based vector magnetograph, which will measure magnetic features at a resolution of 0.2 arcsec. An electrically tunable lithium-niobate Fabry-Perot provides a spectral resolution of about 0.015 nm. In a follow-up series of Antarctic balloon flights, the Flare Genesis Experiment (FGE) will provide unprecedented details about sunspots, flares, magnetic elements, filaments, and the quiet solar atmosphere. Title: Direct measurements of flux tube inclinations in solar plages. Authors: Bernasconi, P. N.; Keller, C. U.; Povel, H. P.; Stenflo, J. O. Bibcode: 1995A&A...302..533B Altcode: Observations of the full Stokes vector in three spectral lines indicate that flux tubes in solar plages have an average inclination in the photosphere of 14^o^ with respect to the local vertical. Most flux tubes are inclined in the eastwards direction, i.e., opposite to the solar rotation. We have recorded the Stokes vector of the FeI 5247.1A, FeI 5250.2A, and FeI 5250.7A lines in nine different plages with the polarization-free 20cm Zeiss coronagraph at the Arosa Astrophysical Observatory of ETH Zuerich. The telescope has been modified for solar disk observations. The chosen spectral lines are particularly sensitive to magnetic field strength and temperature. To determine the field strength and geometry of the flux tubes in the observed plages we use an inversion code that numerically solves the radiative transfer equations and derives the emergent Stokes profiles for one-dimensional model atmospheres consisting of a flux tube and its surrounding non-magnetic atmosphere. Our results confirm earlier indirect estimates of the inclination of the magnetic fields in plages. Title: Handling of huge multispectral image data volumes from a spectral hole burning device (SHBD) Authors: Graff, Werner; Rosselet, Armel C.; Wild, Urs P.; Gschwind, Rudolf; Keller, Christoph U. Bibcode: 1995SPIE.2480..445G Altcode: We use chlorin-doped polymer films at low temperatures as the primary imaging detector. Based on the principles of persistent spectral hole burning, this system is capable of storing spatial and spectral information simultaneously in one exposure with extremely high resolution. The sun as an extended light source has been imaged onto the film. The information recorded amounts to tens of GBytes. This data volume is read out by scanning the frequency of a tunable dye laser and reading the images with a digital CCD camera. For acquisition, archival, processing, and visualization, we use MUSIC (MUlti processor System with Intelligent Communication), a single instruction multiple data parallel processor system equipped with the necessary I/O facilities. The huge amount of data requires the developemnt of sophisticated algorithms to efficiently calibrate the data and to extract useful and new information for solar physics. Title: Speckle spectrography of extended objects. Authors: Keller, C. U.; Johannesson, A. Bibcode: 1995A&AS..110..565K Altcode: We present a method to obtain diffraction limited spectrograms of extended sources from a series of ground-based slit-spectrograms. The method is a combination of speckle deconvolution (Keller & von der Luehe 1992) and a rapid spectrograph scanning scheme (Johannesson et al. 1992). The slit of a spectrograph is scanned over the solar surface while simultaneous slit-jaw images and spectrograms are recorded. The exposure time is short with respect to seeing-induced variations. A Knox-Thompson speckle reconstruction scheme is applied to the slit jaw images. From the individual slit-jaw images and their speckle reconstruction the instantaneous point spread function can be determined for any location. The spectrograms are reconstructed by inverting a matrix that describes the action of the point-spread function on the spectrograms. The final product is a three-dimensional cube formed by the two spatial coordinates and the wavelength. In the spatial domain the data is nearly diffraction limited while the spectral resolution is not affected by the reconstruction. The method is applied to observations of the quiet solar granulation. Title: Weak Magnetic Fields in the Network Authors: Keller, C. U.; Harvey, J. W. Bibcode: 1995SPD....26..206K Altcode: 1995BAAS...27..952K No abstract at ADS Title: The spectral hole-burning device: a 3-dimensional photon detector. Authors: Keller, C. U.; Gschwind, R.; Renn, A.; Rosselet, A.; Wild, U. P. Bibcode: 1995A&AS..109..383K Altcode: We present the principles of a new device that combines a high-resolution spectrometer (0.001nm spectral resolution, 10nm bandwidth) with maximum efficiency, a large detector (pixel size 1μm by 1μm, overall size 2cm by 2cm), and a very high density data storage device (up to 400TB) in a single optical device. This device is based on persistent spectral hole-burning in a dye-doped polymer at temperatures of a few K. It is called a Spectral Hole-Burning Device (SHBD) in analogy to Charge Coupled Devices (CCD). A SHBD can be thought of as a color film with extremely high spectral resolution: it records the intensity in the visible or the near-infrared according to the two-dimensional position and the wavelength. Therefore it is a 3-dimensional photon detector. We show first results from laboratory measurements that prove the feasibility of a SHBD. Title: Visible and near-infrared polarimetry with LEST. Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Povel, H. P.; Steiner, P.; Stenflo, J. O. Bibcode: 1995LFTR...59.....K Altcode: This document describes the LEST vector polarimeters for the visible and the near-infrared part of the solar spectrum. After some general remarks on precise polarimetry with large telescopes, the authors present the specifications based on scientific reasons and some general design considerations. The proposed instrument design for the visible is based on the ZIMPOL II concept. They present two different concepts for vector polarimetry in the near infrared. One is based on a beam-splitter system combined with liquid crystal modulators, while the other is based on the same modulator package as used in the visible and optical demodulation in the final focus. Title: Infrared Capabilities of the Large Earth-Based Solar Telescope (LEST) Authors: Keller, C. U. Bibcode: 1995itsa.conf..215K Altcode: No abstract at ADS Title: Lest Detector and Data Acquisition System - Part One - Specifications Authors: Collados, M.; Keller, C. U.; Steiner, P. Bibcode: 1995lest.rept....1C Altcode: No abstract at ADS Title: Visible and Near Infrared Polarimetry with lest Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Powel, H. P.; Steiner, P.; Stenflo, J. O. Bibcode: 1995lest.rept....1K Altcode: No abstract at ADS Title: LEST detector and data acquisition systems. Authors: Collados, M.; Keller, C. U.; Steiner, P. Bibcode: 1995LFTR...61.....C Altcode: Contents: 1. Specifications (M. Collados, C. U. Keller, P. Steiner). 2. Design considerations for the data acquisition system (P. Steiner). Title: First light for an astronomical 3-D photon detector. Authors: Keller, C. U.; Graff, W.; Rosselet, A.; Gschwind, R.; Wild, U. P. Bibcode: 1994A&A...289L..41K Altcode: Spectroscopic studies of extended astronomical sources at optical wavelengths are hampered by the two-dimensional nature of current photon detectors. Most photons of interest are not utilized because a spectrometer must scan in the spatial or spectral domain. We present the first solar spectrum recorded with a three-dimensional, highly wavelength-sensitive photon detector based on a dye-doped polymer film at 1.6K. The achieved spectral resolution is comparable to the best solar spectra obtained with a Fourier transform spectrometer. Our experiment paves the way for a photon detector that improves the efficiency of spectroscopy of extended sources by orders of magnitudes. Title: Two-dimensional polarimeter with a charge-coupled-device image sensor and a piezoelastic modulator Authors: Povel, H. P.; Keller, C. U.; Yadigaroglu, I. -A. Bibcode: 1994ApOpt..33.4254P Altcode: No abstract at ADS Title: On the strength of solar intra-network fields. Authors: Keller, C. U.; Deubner, F. -L.; Egger, U.; Fleck, B.; Povel, H. P. Bibcode: 1994A&A...286..626K Altcode: The combination of the German Vacuum Tower Telescope and the prototype of ZIMPOL I (Zuerich Imaging Stokes Polarimeter I), a novel, very sensitive imaging polarimeter, has resulted in the first spectra of solar intra-network (IN) fields in circular polarization. The sensitivity in terms of flux density is 0.7 Mx/cm^2^. While magnetic fields in solar spots, pores, plages, and in the network have predominantly kG field strengths, the magnetic field strength of the IN flux is a controversial subject due to the absence of direct measurements. We first summarize the current ideas on IN fields and examine previous arguments for their field strength. Our measurements of the magnetic line ratio formed between the amplitudes of the Stokes V profiles of Fe I 5247.1 A and Fe I 5250.2 A are consistent with a field strength well below 1 kG. Since the sensitivity of the magnetic line ratio becomes low for small field strengths, we can only set an upper limit on the field strength of IN fields of 500 G at the level of line formation with a probability of 68% and an upper limit of 1 kG with a probability of 95%. We emphasize that these are the first observations of a magnetic line ratio of unity near disk center. Title: Application of multiframe iterative blind deconvolution for diverse astronomical imaging Authors: Christou, Julian C.; Hege, E. Keith; Jefferies, Stuart M.; Keller, Christoph U. Bibcode: 1994SPIE.2200..433C Altcode: 1994aisi.conf..433C We present applications of a recently developed iterative blind deconvolution algorithm to both simulated and real data. The applications demonstrate the algorithm's performance for a wide range of astronomical imaging. We demonstrate the effectiveness of using multiple observations of the same object convolved with different point spread functions. We also show the extension of the algorithm to phase retrieval when the object Fourier amplitude is available. Title: Speckle techniques for spectroscopic observations Authors: Keller, C. U. Bibcode: 1994ASIC..433...43K Altcode: No abstract at ADS Title: Direct Measurements of Fluxtube Inclinations in Plages Authors: Bernasconi, Pietro N.; Keller, Christoph U.; Stenflo, Jan Olof Bibcode: 1994ASPC...68..131B Altcode: 1994sare.conf..131B No abstract at ADS Title: High spatial resolution observations of solar magnetic fields Authors: Keller, C. U. Bibcode: 1994smf..conf..325K Altcode: No abstract at ADS Title: Some aspects of polarimetry with LEST Authors: Keller, C. U. Bibcode: 1994ASIC..433...37K Altcode: No abstract at ADS Title: Narrow-Band Speckle Imaging Authors: Keller, C.; von der Lühe, O. Bibcode: 1993rtpf.conf..129K Altcode: No abstract at ADS Title: Small-Scale Structure in Active Regions (Invited) Authors: Keller, C. Bibcode: 1993ASPC...46....3K Altcode: 1993IAUCo.141....3K; 1993mvfs.conf....3K No abstract at ADS Title: Resolution of magnetic flux tubes on the Sun Authors: Keller, C. U. Bibcode: 1992Natur.359..307K Altcode: MAGNETIC flux at the surface of the Sun is predominantly concentrated in discrete areas with kilogauss field strengths1. Except for sunspots, these areas are too small to have been resolved by conventional observations. These magnetic flux tubes are an essential part of the physics of the activity and heating of the outer atmosphere of the Sun and other late-type stars2, but although their average properties have been studied in considerable detail3,4, direct observations of them have been lacking because of turbulence in the Earth's atmosphere, which limits resolution to ~400 km. Using a newly developed technique of speckle inter-ferometry5, we have obtained simultaneous direct observations of the white-light and magnetic field signature of flux tubes. Individual flux tubes are seen, with resolved diameters of ~200 km and continuum brightness contrast of at least +30%. Magnetic features larger than 300 km in size tend, however, to be darker than their surroundings. Title: Solar speckle polarimetry Authors: Keller, C. U.; von der Luehe, O. Bibcode: 1992A&A...261..321K Altcode: The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. This reconstruction determines the instantaneous optical transfer function (OTF) for each individual broad-band exposure. Each simultaneously recorded image in the narrow-band channel is then corrected for the instantaneous OTE To recover all spatial frequencies in the narrow-band channel the so corrected single images are averaged. We have applied the method to polarimetric observations of a solar active region by tuning the narrow-band filter to the wing of a Zeeman sensitive spectral line. The most active part of the region shows no more normal granules but features with a diameter of 0.3 to 0.5 arcsec. The smallest magnetic fields are concentrated in regions with sizes at or below the diffraction limit of the telescope. Title: Application of Differential Speckle Imaging to Solar Polarimetry Authors: Keller, C. U.; von der Luhe, O. Bibcode: 1992ESOC...39..453K Altcode: 1992hrii.conf..453K No abstract at ADS Title: High Spatial Resolution Magnetograms of Solar Active Regions Authors: Keller, C. U.; Stenflo, J. O.; von der Luhe, O. Bibcode: 1992A&A...254..355K Altcode: Using the Universal Birefringent Filter at the Sacramento Peak Vacuum Tower Telescope we have obtained simultaneous observations of left and right circular polarization in various solar magnetic features with a resulting spatial resolution of 0".7 in the magnetograms. We describe the data reduction in some detail and discuss the various instrumental effects. In particular we show that seeing can create features in magnetograms. A penumbra near disk center shows small-scale features in the magnetogram which are associated with the bright filaments. Bright features in the umbra of a small spot exhibit considerable polarization signals. In a pore region opposite polarities are found within a few seconds of arc. Title: Zurich Imaging Stokes Polarimeter Zimpol-I - Design Review Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.; Steiner, P.; Stenflo, J. O. Bibcode: 1992lest.rept....1K Altcode: No abstract at ADS Title: GAP: yet another image processing system for solar observations. Authors: Keller, C. U. Bibcode: 1992lest.rept....3K Altcode: GAP is a versatile, interactive image processing system for analyzing solar observations, in particular extended time sequences, and for preparing publication quality figures. It consists of an interpreter that is based on a language with a control flow similar to PASCAL and C. The interpreter may be accessed from a command line editor and from user-supplied functions, procedures, and command scripts. GAP is easily expandable via external FORTRAN programs that are linked to the GAP interface routines. The current version of GAP runs on VAX, DECstation, Sun, and Apollo computers. Versions for MS-DOS and OS/2 are in preparation. Title: High resolution observation of solar magnetic fields Authors: Keller, Christoph Ulrich Bibcode: 1992PhDT.......203K Altcode: No abstract at ADS Title: Zürich Imaging Stokes Polarimeter - ZIMPOL I. Design review. Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.; Steiner, P.; Stenflo, J. O. Bibcode: 1992LFTR...53.....K Altcode: This document describes the design of ZIMPOL I, the first Zürich Imaging Stokes Polarimeter. This solar vector polarimeter will mainly be used for observations of the solar magnetic field at high spatial and/or spectral resolution. A brief overview of ZIMPOL I is given in the preface. The scientific requirements are then specified in detail. They lead to an instrument concept which consists of several parts: the optical system, the camera system, the real-time image processing system, and the graphical user interface. Data reduction and analysis of observations recorded with this polarimeter are also dealt with in detail. Prototypes of the modulator package and the CCD camera have been tested at various observatories. Results from these tests are presented and discussed. Title: Demodulation of all four Stokes parameters with a single CCD - ZIMPOL II. Conceptual design. Authors: Stenflo, J. O.; Keller, C. U.; Povel, H. P. Bibcode: 1992LFTR...54.....S Altcode: It is shown how it is possible to simultaneously record images of all four Stokes parameters with a single CCD detector chip when fast (50 kHz), piezoelastic modulation of the polarization state is used. As the four image planes use the identical pixels of the CCD, all gain-table or flat-field effects vanish when forming the fractional polarization images. For each group of four pixel rows, one row collects the photons, while the other three are used for fast buffer storage. There are no light losses caused by masking of the pixel rows used for buffer storage, sinced a microlens array collects all the photons and directs them to the unmasked pixel rows. The efficiency of the system for simultaneous recording of all four Stokes parameters is six times greater than that of ZIMPOL I, the first generation of the Zürich Imaging Stokes Polarimeter, since no beam splitter with three separate CCD cameras is needed and no significant light losses occur at the masked pixel rows. The theoretically possible efficiency limit is thereby practically reached. The system is planned to be developed as ZIMPOL II, the second generation of the Zürich Imaging Stokes Polarimeter. Title: Round table discussion. Authors: Darvann, T. A.; Keller, C. U. Bibcode: 1992lest.rept..131D Altcode: The discussion session was divided into two parts: 1) Image processing software requirements for the future LEST, and 2) strategies at the present time to increase the collaboration and communication on image processing software within the solar community, and for the successful achievement of point 1. Title: Multicolor Continuum Analysis of the Solar Granulation in Quiet and Active Regions Authors: Keller, C. U.; Koutchmy, S. Bibcode: 1991ApJ...379..751K Altcode: A set of narrow-band filtergrams of the solar photosphere recorded in three widely separated true continuum windows and in Mg I b1 with a spatial resolution of about 0.5 arcsec is analyzed. The influence of small-scale magnetic fields on the granulation is studied by comparing various statistical parameters in selected active and quiet regions, and the temperature stratification of faculae at the level of continuum formation is investigated. In active regions there exist more points with enhanced continuum intensity as compared with quiet regions, and there is more power at small spatial scales in active regions, whereas more power exists at large scales in quiet regions. Quiet regions near and far away from a large sunspot do not show any significant difference. It is concluded that changes in the granular pattern near sunspots are caused by the small-scale magnetic fields often found near sunspots. Faculae, identified by their brightness in the Mg I b1 wing, show an enhanced continuum intensity and a reduced ratio between the blue and the red continuum, which is consistent with current flux tube models. Title: Fine-Scale Magnetic Field in a Sunspot Penumbra and Adjacent Photosphere Authors: Frank, Z. A.; Scharmer, G. B.; Keller, C.; Lundstedt, H. Bibcode: 1991BAAS...23.1052F Altcode: No abstract at ADS Title: Two-dimensional polarimeter with CCD image sensors and piezo-elastic modulators. Authors: Povel, H. P.; Keller, C. U.; Stenflo, J. O. Bibcode: 1991sopo.work..100P Altcode: A new type of polarimeter for application in solar physics is described, which combines fast polarization modulation and high spatial resolution using piezoelastic modulators and charge coupled device (CCD) image sensors. The problem of incompatibility between the slow read-out of CCD sensors and fast modulation has been solved. First two-dimensional Stokes Q and V images, free from gain-table noise, and with an rms noise level of about 10-3 have been obtained. Title: High spatial resolution polarimetry using filtergrams. Authors: Keller, C. U. Bibcode: 1991sopo.work..124K Altcode: Using tunable filters at the Sacramento Peak Vacuum Tower Telescope Stokes V images of various solar magnetic features have been observed. Simultaneous recording of left and right circular polarization leads to high spatial resolution magnetograms. Various instrumental effects of these observations are discussed in detail. Title: Inversion of Stokes V profiles: The structure of solar magnetic fluxtubes and its dependence on the filling factor. Authors: Keller, C. U.; Solanki, S. K.; Stenflo, J. O.; Zayer, I. Bibcode: 1991sopo.work..387K Altcode: The authors present results from an inversion procedure that derive the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of magnetic fluxtubes from observed Stokes V spectra near disk center. In a first step the inversion has been applied to 10 Fe I and Fe II Stokes V profiles of a plage and a network region to obtain reliable models of the fluxtubes. In a second step the dependence of the fluxtube structure on the filling factor has been studied with spectra of 3 Fe I lines from 23 different regions based on the models derived in the first step. Title: Dependence of the properties of solar magnetic flux tubes on filling factor. II - Results of an inversion approach Authors: Zayer, I.; Stenflo, J. O.; Keller, C. U.; Solanki, S. K. Bibcode: 1990A&A...239..356Z Altcode: The dependence of the properties of solar magnetic elements on the magnetic filling factor is studied using Stokes V spectra of three lines observed near the center of the solar disk. The inversion technique developed by Keller et al. (1990) is applied to three neighboring spectral lines, and the average temperature difference, the magnetic field strength, and the nonstationary velocity in the relevant line-forming layers are quantitatively determined. Quantitative evidence is provided for the dependence of the temperature within flux tubes on the amount of magnetic flux. The flux tubes are found to become cooler and their field strengths, at a given optical depth, to become larger as the filling factor increases. The presence of kilogauss field strengths within flux tubes is reconfirmed. Title: Solar magnetic field strength determinations from high spatial resolution filtergrams Authors: Keller, C. U.; Stenflo, J. O.; Solanki, S. K.; Tarbell, T. D.; Title, A. M. Bibcode: 1990A&A...236..250K Altcode: Circularly polarized images with high spatial resolution (better than 1 arcsec) of a solar active region, obtained with a tunable filter in the wings of Fe I 5247.1 A and Fe I 5250.2 A, have been analyzed in terms of the magnetic line ratio technique introduced by Stenflo (1973). Whenever a measurable amount of polarization is present, the distribution of the observed magnetic-line ratio is compatible with a unique value, which is randomly blurred by noise due to the photon statistics, the CCD camera, and atmospheric distortions. There is no need for a distribution of field strengths to explain the observed distribution of the magnetic line ratio. Consequently, the observations are compatible with a unique magnetic field strength in solar small-scale magnetic elements of about 1000 G at the level of line formation. For a thin flux tube, this corresponds to a field strength of approximately 2000 G at the level of continuum formation, which is in excellent agreement with previous field strength determinations from low spatial resolution spectra (4-10 arcsec). Title: Structure of solar magnetic fluxtubes from the inversion of Stokes spectra at disk center Authors: Keller, C. U.; Steiner, O.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1990A&A...233..583K Altcode: The paper presents an inversion procedure that derives the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of small-scale magnetic fields from observed Stokes V spectra and the continuum intensity. The inversion is based on the determination of a small number of model flux parameters by a nonlinear least squares fitting algorithm. The minimization of the sum of the squared differences between observed and synthetic observables makes it possible to determine the temperature stratification and the magnetic field strength. Title: Empirical Photospheric Fluxtube Models from Inversion of Stokes V Data Authors: Keller, C. U. Bibcode: 1990IAUS..138..121K Altcode: No abstract at ADS Title: Restoration of distorted images as a variational problem: a dynamic programming approach Authors: Keller, C. U. Bibcode: 1989hsrs.conf..208K Altcode: No abstract at ADS Title: High resolution photographic Stokes polarimetry of small scale magnetic flux (poster) Authors: Keller, C. U.; Koutchmy, S. Bibcode: 1989hsrs.conf..443K Altcode: No abstract at ADS Title: Properties of solar magnetic fluxtubes from only two spectral lines Authors: Solanki, S. K.; Keller, C.; Stenflo, J. O. Bibcode: 1987A&A...188..183S Altcode: A method for the determination of the magnetic field strength, velocity, and temperatures inside solar flux tubes, in addition to their inclinations and filling factors, is presented which requires only the Stokes V and Q profiles of the Fe I 5250.2 A and Fe I 5247.1 A spectral lines. Application of the procedure to spectra of the two lines obtained at various distances from the solar limb shows that considerable velocity broadening is required at all positions on the disk in order to reproduce the polarimeter data. The center to limb variation of the 5250/5247 Stokes V and Q line ratios is found to contain little information on the height variation of the magnetic field in the context of one-dimensional models. Title: Comet Rudnicki (1966e) Authors: Nielsen, A. V.; Milet, B.; Pereyra, Z. M.; Keller; Rodriguez, J. J.; Mintz, B. Bibcode: 1967IAUC.1987....2N Altcode: Dr. Axel V. Nielsen, Ole Romer Observatory, points out that at its descending node Comet Rudnicki passes very near the orbit of the earth. Its heliocentric distance would then be 1.02 AU. He suggests the possibility of observing meteors associated with the comet on about 1967 June 7. Further precise positions have been reported as follows: 1966 UT R.A. (1950) Decl. Mag. Observer Nov. 16.81487 1 13 14.71 - 6 54 55.1 Milet 16.82318 1 13 12.28 - 6 55 08.3 " Dec. 1.03499 0 06 33.99 -11 24 24.3 9.0 Pereyra 1.04892 0 06 29.72 -11 24 35.6 9.0 " 4.06458 23 50 37.02 -12 12 58.6 Mintz 4.08542 23 50 30.51 -12 13 14.7 " 5.01250 23 45 31.56 -12 27 10.9 " Milet (Nice Observatory). Pereyra (Cordoba Observatory). Measurer: Keller. Computer: Rodriguez. Mintz (U.S. Naval Observatory, Washington). Measured on ADAMM.