Author name code: snik ADS astronomy entries on 2022-09-14 author:"Snik, Frans" ------------------------------------------------------------------------ Title: XPipeline: Starlight subtraction at scale for MagAO-X Authors: Long, Joseph D.; Males, Jared R.; Haffert, Sebastiaan Y.; Close, Laird M.; Morzinski, Katie M.; Van Gorkom, Kyle; Lumbres, Jennifer; Foster, Warren; Hedglen, Alexander; Kautz, Maggie; Rodack, Alex; Schatz, Lauren; Miller, Kelsey; Doelman, David; Bos, Steven; Kenworthy, Matthew A.; Snik, Frans; Otten, Gilles P. P. L. Bibcode: 2022arXiv220807354L Altcode: MagAO-X is an extreme adaptive optics (ExAO) instrument for the Magellan Clay 6.5-meter telescope at Las Campanas Observatory in Chile. Its high spatial and temporal resolution can produce data rates of 1 TB/hr or more, including all AO system telemetry and science images. We describe the tools and architecture we use for commanding, telemetry, and science data transmission and storage. The high data volumes require a distributed approach to data processing, and we have developed a pipeline that can scale from a single laptop to dozens of HPC nodes. The same codebase can then be used for both quick-look functionality at the telescope and for post-processing. We present the software and infrastructure we have developed for ExAO data post-processing, and illustrate their use with recently acquired direct-imaging data. 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: ABORAS: polarimetric, 10cm/s RV observations of the Sun as a star Authors: Farret Jentink, Casper; Mortier, Annelies; Snik, Frans; Dorval, Patrick; Thompson, Samantha J.; Navarro, Ramon; Naylor, Tim Bibcode: 2022arXiv220704804F Altcode: We present a description of A dual-Beam pOlarimetric Robotic Aperture for the Sun (ABORAS), to serve as a Solar input with a dedicated Stokes V polarimeter for the HARPS3 high-resolution spectrograph. ABORAS has three main science drivers: trying to understand the physics behind stellar variability, tracking the long-term stability of HARPS3, and serve as a benchmark for Earth-sized exoplanet detection with HARPS3 by injecting an Earth RV signal into the data. By design, ABORAS will (together with the HARPS3 instrument) be able to measure 10cm/s variations in RV of the integrated Solar disk and detect integrated magnetic field levels at sub 1 Gauss level through circularly polarized light. Title: L-band Integral Field Spectroscopy of the HR 8799 Planetary System Authors: Doelman, David S.; Stone, Jordan M.; Briesemeister, Zackery W.; Skemer, Andrew J. I.; Barman, Travis; Brock, Laci S.; Hinz, Philip M.; Bohn, Alexander; Kenworthy, Matthew; Haffert, Sebastiaan Y.; Snik, Frans; Ertel, Steve; Leisenring, Jarron M.; Woodward, Charles E.; Skrutskie, Michael F. Bibcode: 2022AJ....163..217D Altcode: 2022arXiv220308165D Understanding the physical processes sculpting the appearance of young gas-giant planets is complicated by degeneracies confounding effective temperature, surface gravity, cloudiness, and chemistry. To enable more detailed studies, spectroscopic observations covering a wide range of wavelengths are required. Here we present the first L-band spectroscopic observations of HR 8799 d and e and the first low-resolution wide-bandwidth L-band spectroscopic measurements of HR 8799 c. These measurements were facilitated by an upgraded LMIRCam/ALES instrument at the Large Binocular Telescope, together with a new apodizing phase plate coronagraph. Our data are generally consistent with previous photometric observations covering similar wavelengths, yet there exists some tension with narrowband photometry for HR 8799 c. With the addition of our spectra, each of the three innermost observed planets in the HR 8799 system has had its spectral energy distribution measured with integral field spectroscopy covering ~0.9-4.1 μm. We combine these spectra with measurements from the literature and fit synthetic model atmospheres. We demonstrate that the bolometric luminosity of the planets is not sensitive to the choice of model atmosphere used to interpolate between measurements and extrapolate beyond them. Combining luminosity with age and mass constraints, we show that the predictions of evolutionary models are narrowly peaked for effective temperature, surface gravity, and planetary radius. By holding these parameters at their predicted values, we show that more flexible cloud models can provide good fits to the data while being consistent with the expectations of evolutionary models. 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: Cryogenic characterization of the grating vector apodizing phase plate coronagraph for the enhanced resolution imager and spectrograph at the Very Large Telescope Authors: Boehle, Anna; Doelman, David; Konrad, Björn S.; Snik, Frans; Glauser, Adrian M.; Por, Emiel H.; Warriner, Nathaniel Z.; Shi, Shuojia; Escuti, Michael J.; Kenworthy, Matthew A.; Quanz, Sascha P. Bibcode: 2021JATIS...7d5001B Altcode: We present results on the laboratory characterization of the grating vector apodizing phase plate (gvAPP) coronagraph that will be included in the upcoming instrument enhanced resolution imager and spectrograph (ERIS) at the VLT. ERIS will include a 1 to 5 μm adaptive-optics-fed imager, NIX, that will greatly improve the capability of the VLT to perform high-contrast imaging of exoplanets especially in the 3 to 5 μm wavelength range. The gvAPP, one of the coronagraphs in the NIX suite, is a pupil plane coronagraph that uses a thin film of patterned liquid crystals to create two images of a star with a D-shaped dark hole on either side. The gvAPP is manufactured using an innovative direct-write system that produces precise patterns of liquid crystals. We utilized the upgraded infrared cryogenic test bench run by the Exoplanets and Habitability Group at ETH Zurich to measure the morphology of the gvAPP PSF and to test the accuracy of the liquid crystal manufacturing technique in the lab for the first time at contrast levels of ∼10 − 5. We find that the gvAPP can reach raw contrasts below ∼10 − 5 between ∼10 and 13 λ / D. This contrast upper limit translates to a writing accuracy of the orientation of the liquid crystal's fast axis of better than 0.3 deg for the spatial frequencies corresponding to those separations. This is a sufficient accuracy such that the gvAPP will not be the limiting factor in achieving the required contrasts to image exoplanets. Title: First on-sky demonstration of spatial Linear Dark Field Control with the vector-Apodizing Phase Plate at Subaru/SCExAO Authors: Bos, S. P.; Miller, K. L.; Lozi, J.; Guyon, O.; Doelman, D. S.; Vievard, S.; Sahoo, A.; Deo, V.; Jovanovic, N.; Martinache, F.; Currie, T.; Snik, F. Bibcode: 2021A&A...653A..42B Altcode: 2021arXiv210606286B Context. One of the key noise sources that currently limits high-contrast imaging observations for exoplanet detection is quasi-static speckles. Quasi-static speckles originate from slowly evolving non-common path aberrations (NCPA). These NCPA are related to the different optics encountered in the wavefront sensing path and the science path, and they also exhibit a chromatic component due to the difference in the wavelength between the science camera and the main wavefront sensor. These speckles degrade the contrast in the high-contrast region (or dark hole) generated by the coronagraph and make the calibration in post-processing more challenging.
Aims: The purpose of this work is to present a proof-of-concept on-sky demonstration of spatial Linear Dark Field Control (LDFC). The ultimate goal of LDFC is to stabilize the point spread function by addressing NCPA using the science image as additional wavefront sensor.
Methods: We combined spatial LDFC with the Asymmetric Pupil vector-Apodizing Phase Plate (APvAPP) on the Subaru Coronagraphic Extreme Adaptive Optics system at the Subaru Telescope. To allow for rapid prototyping and easy interfacing with the instrument, LDFC was implemented in Python. This limited the speed of the correction loop to approximately 20 Hz. With the APvAPP, we derive a high-contrast reference image to be utilized by LDFC. LDFC is then deployed on-sky to stabilize the science image and maintain the high-contrast achieved in the reference image.
Results: In this paper, we report the results of the first successful proof-of-principle LDFC on-sky tests. We present results from two types of cases: (1) correction of instrumental errors and atmospheric residuals plus artificially induced static aberrations introduced on the deformable mirror and (2) correction of only atmospheric residuals and instrumental aberrations. When introducing artificial static wavefront aberrations on the DM, we find that LDFC can improve the raw contrast by a factor of 3-7 over the dark hole. In these tests, the residual wavefront error decreased by ∼50 nm RMS, from ∼90 nm to ∼40 nm RMS. In the case with only residual atmospheric wavefront errors and instrumental aberrations, we show that LDFC is able to suppress evolving aberrations that have timescales of < 0.1-0.4 Hz. We find that the power at 10−2 Hz is reduced by a factor of ∼20, 7, and 4 for spatial frequency bins at 2.5, 5.5, and 8.5λ/D, respectively.
Conclusions: We have identified multiplied challenges that have to be overcome before LDFC can become an integral part of science observations. The results presented in this work show that LDFC is a promising technique for enabling the high-contrast imaging goals of the upcoming generation of extremely large telescopes.

Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Title: High-contrast observations of brown dwarf companion HR 2562 B with the vector Apodizing Phase Plate coronagraph Authors: Sutlieff, Ben J.; Bohn, Alexander J.; Birkby, Jayne L.; Kenworthy, Matthew A.; Morzinski, Katie M.; Doelman, David S.; Males, Jared R.; Snik, Frans; Close, Laird M.; Hinz, Philip M.; Charbonneau, David Bibcode: 2021MNRAS.506.3224S Altcode: 2021arXiv210614890S; 2021MNRAS.tmp.1644S The vector Apodizing Phase Plate (vAPP) is a class of pupil plane coronagraph that enables high-contrast imaging by modifying the Point Spread Function (PSF) to create a dark hole of deep flux suppression adjacent to the PSF core. Here, we recover the known brown dwarf HR 2562 B using a vAPP coronagraph, in conjunction with the Magellan Adaptive Optics (MagAO) system, at a signal-to-noise of S/N = 3.04 in the lesser studied L-band regime. The data contained a mix of field and pupil-stabilized observations, hence we explored three different processing techniques to extract the companion, including Flipped Differential Imaging (FDI), a newly devised Principal Component Analysis (PCA)-based method for vAPP data. Despite the partial field-stabilization, the companion is recovered sufficiently to measure a 3.94 $\mu\mathrm{ m}$ narrow-band contrast of (3.05 ± 1.00) × 10-4 ($\Delta \, {\rm m}_{3.94 \mu {\rm m}}$ = 8.79 ± 0.36 mag). Combined with archival GPI and SPHERE observations, our atmospheric modelling indicates a spectral type at the L/T transition with mass M = 29 ± 15 MJup, consistent with literature results. However, effective temperature and surface gravity vary significantly depending on the wavebands considered (1200 ≤ Teff(K) ≤ 1700 and 4.0 ≤ log(g)(dex) ≤ 5.0), reflecting the challenges of modelling objects at the L/T transition. Observations between 2.4 and 3.2 $\mu\mathrm{ m}$ will be more effective in distinguishing cooler brown dwarfs due to the onset of absorption bands in this region. We explain that instrumental scattered light and wind-driven halo can be detrimental to FDI+PCA and thus must be sufficiently mitigated to use this processing technique. We thus demonstrate the potential of vAPP coronagraphs in the characterization of high-contrast substellar companions, even in sub-optimal conditions, and provide new complementary photometry of HR 2562 B. Title: The Young Suns Exoplanet Survey: imaging infant planets around young, solar analogs Authors: Kenworthy, Matthew; Bohn, Alexander; Ginski, Christian; Reggiani, Maddalena; Meshkat, Tiffany; Mamajek, Eric; Pecaut, Mark; Snik, Frans Bibcode: 2021EPSC...15...35K Altcode: Within the Young Suns Exoplanet Survey (YSES) we are observing a homogeneous sample of 70 solar-mass members of the approximately 16 Myr-old Lower Centaurus-Crux subgroup of the Scorpius-Centaurus association to search for sub-stellar companions.High-contrast imaging observations with VLT/SPHERE/IRDIS revealed (i) a shadowed transition disk around Wray 15-788 that shows significant signs of ongoing planet formation and (ii) one of the lowest-mass companions imaged to date: YSES-2 b has a mass of 6.5 Jupiter masses and is orbiting its solar-mass primary at a separation of 110 au. Most intriguing, though, was (iii) the discovery of the first directly imaged multi-planet system around a Sun-like star. The detection of two gas-giant companions of 14±3 and 6±1 Jupiter masses that are orbiting YSES-1 (TYC 8998-760-1) at separations of 160 au and 320 au, respectively, provides important implications for the outer architecture of planetary systems and the underlying formation mechanisms.In addition to the SPHERE observations, we identified further companions to our `Young Suns' outside the instrument's field of view in the third early data release of the Gaia mission. Based on parallaxes and proper motions provided in this catalogue, we detected eight additional sub-stellar companions at separations larger than 500 au amongst our sample.By combining Gaia astrometry with the high-contrast imaging capabilities of SPHERE, our survey will provide a complete census of wide-orbit sub-stellar companions for a statistically highly significant sample of young, solar analogues. From the current results we derived a preliminary probability of 14.3±3.1% for our solar-type stars to host wide-orbit, sub-stellar companions. As follow-up observations of 45 YSES targets are still pending, this ratio can be interpreted as a lower limit, which is tentatively indicating a higher companion yield than previous surveys. 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: The Santa Cruz Extreme AO Lab (SEAL): design and first light Authors: Jensen-Clem, Rebecca; Dillon, Daren; Gerard, Benjamin; van Kooten, M. A. M.; Fowler, J.; Kupke, Renate; Cetre, Sylvain; Sanchez, Dominic; Hinz, Philip; Laguna, Cesar; Doelman, David; Snik, Frans Bibcode: 2021SPIE11823E..1DJ Altcode: 2021arXiv210903318J The Santa Cruz Extreme AO Lab (SEAL) is a new visible-wavelength testbed designed to advance the state of the art in wavefront control for high contrast imaging on large, segmented, ground-based telescopes. SEAL provides multiple options for simulating atmospheric turbulence, including a custom spatial light modulator. A 37-segment deformable mirror simulates the W. M. Keck Observatory segmented primary mirror. The adaptive optics system consists of a woofer/tweeter DM system, and four wavefront sensor arms: 1) a high-speed Shack-Hartmann WFS, 2) a reflective pyramid WFS, 3) vector-Zernike mask, and 4) a Fast Atmospheric SCC Technique demonstration arm. Finally, a science arm preliminarily includes a classical Lyot-style coronagraph. SEAL's real time control system is based on the CACAO package, and is designed to support the efficient transfer of software between SEAL and the Keck II AO system. In this paper, we present an overview of the design and first light performance of SEAL. 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: Full characterization of the instrumental polarization effects of the spectropolarimetric mode of SCExAO-CHARIS Authors: Joost `t Hart, G. J.; van Holstein, Rob G.; Bos, Steven P.; Ruigrok, Jasper; Snik, Frans; Lozi, Julien; Guyon, Olivier; Kudo, Tomoyuki; Zhang, Jin; Jovanovic, Nemanja; Norris, Barnaby; Martinod, Marc-Antoine; Groff, Tyler D.; Chilcote, Jeffrey; Currie, Thayne; Tamura, Motohide; Vievard, Sébastien; Sahoo, Ananya; Deo, Vincent; Ahn, Kyohoon; Martinache, Frantz; Kasdin, Jeremy Bibcode: 2021arXiv210804833J Altcode: 2021arXiv210804833H SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integral-field spectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by a Wollaston prism and is unique among high-contrast imagers. We present a detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument. From measurements with the internal light source, we find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ~95% of the incident linear polarization to circularly polarized light that cannot be measured. Observations of an unpolarized star show that the magnitude of the instrumental polarization of the telescope varies with wavelength between 0.5% and 1%, and that its angle is exactly equal to the altitude angle of the telescope. Using physical models of the fold mirror of the telescope, the half-wave plate, and the derotator, we simultaneously fit the instrumental polarization effects in the 22 wavelength bins. Over the full wavelength range, our model currently reaches a total polarimetric accuracy between 0.08% and 0.24% in the degree of linear polarization. We propose additional calibration measurements to improve the polarimetric accuracy to <0.1% and plan to integrate the complete Mueller matrix model into the existing CHARIS post-processing pipeline. Our calibrations of CHARIS' spectropolarimetric mode will enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions. Title: Full characterization of the instrumental polarization effects of the spectropolarimetric mode of SCExAO/CHARIS Authors: 't Hart, Joost G. J.; van Holstein, Rob G.; Bos, Steven P.; Ruigrok, Jasper; Snik, Frans; Lozi, Julien; Guyon, Olivier; Kudo, Tomoyuki; Zhang, Jin; Jovanovic, Nemanja; Norris, Barnaby; Martinod, Marc-Antoine; Groff, Tyler D.; Chilcote, Jeffrey; Currie, Thayne; Tamura, Motohide; Vievard, Sébastien; Sahoo, Ananya; Deo, Vincent; Ahn, Kyohoon; Martinache, Frantz; Kasdin, Jeremy Bibcode: 2021SPIE11833E..0OT Altcode: 2021SPIE11833E..0O' SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integralfield spectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by a Wollaston prism and is unique among high-contrast imagers. We present a detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument. From measurements with the internal light source, we find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ∼95% of the incident linear polarization to circularly polarized light that cannot be measured. Observations of an unpolarized star show that the magnitude of the instrumental polarization of the telescope varies with wavelength between 0.5% and 1%, and that its angle is exactly equal to the altitude angle of the telescope. Using physical models of the fold mirror of the telescope, the half-wave plate, and the derotator, we simultaneously fit the instrumental polarization effects in the 22 wavelength bins. Over the full wavelength range, our model currently reaches a total polarimetric accuracy between 0.08% and 0.24% in the degree of linear polarization. We propose additional calibration measurements to improve the polarimetric accuracy to <0.1% and plan to integrate the complete Mueller matrix model into the existing CHARIS post-processing pipeline. Our calibrations of CHARIS' spectropolarimetric mode will enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions. Title: Pupil-Plane Phase Apodization Authors: Kenworthy, Matthew A.; Codona, Johanan L.; Snik, Frans Bibcode: 2021hai3.book..377K Altcode: 2018arXiv180711242K Phase apodization coronagraphs are implemented in a pupil plane to create a dark hole in the science camera focal plane. They are successfully created as "Apodizing Phase Plates" (APPs) using classical optical manufacturing, and as "vector-APPs" using liquid-crystal patterning with essentially achromatic performance. This type of coronagraph currently delivers excellent broadband contrast ($\sim$10$^{-5}$) at small angular separations (few $\lambda/D$) at ground-based telescopes, owing to their insensitivity to tip/tilt errors. 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: The 13CO-rich atmosphere of a young accreting super-Jupiter Authors: Zhang, Yapeng; Snellen, Ignas A. G.; Bohn, Alexander J.; Mollière, Paul; Ginski, Christian; Hoeijmakers, H. Jens; Kenworthy, Matthew A.; Mamajek, Eric E.; Meshkat, Tiffany; Reggiani, Maddalena; Snik, Frans Bibcode: 2021Natur.595..370Z Altcode: 2021arXiv210706297Z Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis1,2. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System1,3, but do vary on galactic scales with a 12C/13C isotopologue ratio of around 68 in the current local interstellar medium4-6. In molecular clouds and protoplanetary disks, 12CO/13CO ratios can be altered by ice and gas partitioning7, low-temperature isotopic ion-exchange reactions8 and isotope-selective photodissociation9. Here we report observations of 13CO in the atmosphere of the young, accreting super-Jupiter TYC 8998-760-1 b, at a statistical significance of more than six sigma. Marginalizing over the planet's atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a 12CO/13CO ratio of 31−10+17?(90% confidence), a substantial enrichment in 13C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC 8998-760-1 b at greater than or equal to 160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in 13C through fractionation. 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: 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: Discovery of a directly imaged planet to the young solar analog YSES 2 Authors: Bohn, Alexander J.; Ginski, Christian; Kenworthy, Matthew A.; Mamajek, Eric E.; Pecaut, Mark J.; Mugrauer, Markus; Vogt, Nikolaus; Adam, Christian; Meshkat, Tiffany; Reggiani, Maddalena; Snik, Frans Bibcode: 2021A&A...648A..73B Altcode: 2021arXiv210408285B Context. To understand the origin and formation pathway of wide-orbit gas giant planets, it is necessary to expand the limited sample of these objects. The mass of exoplanets derived with spectrophotometry, however, varies strongly as a function of the age of the system and the mass of the primary star.
Aims: By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association.
Methods: Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The subtraction of the stellar point spread function (PSF) is based on reference star differential imaging using the other targets (with similar colors and magnitudes) in the survey in combination with principal component analysis. Two astrometric epochs that are separated by more than one year are used to confirm co-moving companions by proper motion analysis.
Results: We report the discovery of YSES 2b, a co-moving, planetary-mass companion to the K1 star YSES 2 (TYC 8984-2245-1, 2MASS J11275535-6626046). The primary has a Gaia EDR3 distance of 110 pc, and we derive a revised mass of 1.1 M⊙ and an age of approximately 14 Myr. We detect the companion in two observing epochs southwest of the star at a position angle of 205° and with a separation of ~1."05, which translates to a minimum physical separation of 115 au at the distance of the system. Photometric measurements in the H and Ks bands are indicative of a late L spectral type, similar to the innermost planets around HR 8799. We derive a photometric planet mass of 6.3−0.9+1.6 MJup using AMES-COND and AMES-dusty evolutionary models; this mass corresponds to a mass ratio of q = (0.5 ± 0.1)% with the primary. This is the lowest mass ratio of a direct imaging planet around a solar-type star to date. We discuss potential formation mechanisms and find that the current position of the planet is compatible with formation by disk gravitational instability, but its mass is lower than expected from numerical simulations. Formation via core accretion must have occurred closer to the star, yet we do not find evidence that supports the required outward migration, such as via scattering off another undiscovered companion in the system. We can exclude additional companions with masses greater than 13 MJup in the full field of view of the detector (0."15<ρ<5."50), at 0."5 we can rule out further objects that are more massive than 6 MJup, and for projected separations ρ >2" we are sensitive to planets with masses as low as 2 MJup.
Conclusions: YSES 2b is an ideal target for follow-up observations to further the understanding of the physical and chemical formation mechanisms of wide-orbit Jovian planets. The YSES strategy of short snapshot observations (≤5 min) and PSF subtraction based on a large reference library proves to be extremely efficient and should be considered for future direct imaging surveys.

Data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/648/A73

Based 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), 0101.C-0341(A), and 106.20X2.001. 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: VizieR Online Data Catalog: Discovery of the directly imaged planet YSES 2b (Bohn+, 2021) Authors: Bohn, A. J.; Ginski, C.; Kenworthy, M. A.; Mamajek, E. E.; Pecaut, M. J.; Mugrauer, M.; Vogt, N.; Adam, C.; Meshkat, T.; Reggiani, M.; Snik, F. Bibcode: 2021yCat..36480073B 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. Furthermore, the achieved contrast performance that is presented in the paper is included in this catalogue.

(6 data files). 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: Spatial linear dark field control on Subaru/SCExAO. Maintaining high contrast with a vAPP coronagraph Authors: Miller, K. L.; Bos, S. P.; Lozi, J.; Guyon, O.; Doelman, D. S.; Vievard, S.; Sahoo, A.; Deo, V.; Jovanovic, N.; Martinache, F.; Snik, F.; Currie, T. Bibcode: 2021A&A...646A.145M Altcode: Context. One of the key challenges facing direct exoplanet imaging is the continuous maintenance of the region of high contrast within which light from the exoplanet can be detected above the stellar noise. In high-contrast imaging systems, the dominant source of aberrations is the residual wavefront error that arises due to non-common path aberrations (NCPA) to which the primary adaptive optics (AO) system is inherently blind. Slow variations in the NCPA generate quasi-static speckles in the post-AO corrected coronagraphic image resulting in the degradation of the high-contrast dark hole created by the coronagraph.
Aims: In this paper, we demonstrate spatial linear dark field control (LDFC) with an asymmetric pupil vector apodizing phase plate (APvAPP) coronagraph as a method to sense time-varying NCPA using the science image as a secondary wavefront sensor (WFS) running behind the primary AO system. By using the science image as a WFS, the NCPA to which the primary AO system is blind can be measured with high sensitivity and corrected, thereby suppressing the quasi-static speckles which corrupt the high contrast within the dark hole.
Methods: On the Subaru Coronagraphic Extreme Adaptive Optics instrument (SCExAO), one of the coronagraphic modes is an APvAPP which generates two PSFs, each with a 180° D-shaped dark hole with approximately 10-4 contrast at λ = 1550 nm. The APvAPP was utilized to first remove the instrumental NCPA in the system and increase the high contrast within the dark holes. Spatial LDFC was then operated in closed-loop to maintain this high contrast in the presence of a temporally-correlated, evolving phase aberration with a root-mean-square wavefront error of 80 nm. In the tests shown here, an internal laser source was used, and the deformable mirror was used both to introduce random phase aberrations into the system and to then correct them with LDFC in closed-loop operation.
Results: The results presented here demonstrate the ability of the APvAPP combined with spatial LDFC to sense aberrations in the high amplitude regime (∼80 nm). With LDFC operating in closed-loop, the dark hole is returned to its initial contrast and then maintained in the presence of a temporally-evolving phase aberration. We calculated the contrast in 1 λ/D spatial frequency bins in both open-loop and closed-loop operation, and compared the measured contrast in these two cases. This comparison shows that with LDFC operating in closed-loop, there is a factor of ∼3x improvement (approximately a half magnitude) in contrast across the full dark hole extent from 2-10 λ/D. This improvement is maintained over the full duration (10 000 iterations) of the injected temporally-correlated, evolving phase aberration.
Conclusions: This work marks the first deployment of spatial LDFC on an active high-contrast imaging instrument. Our SCExAO testbed results show that the combination of the APvAPP with LDFC provides a powerful new focal plane wavefront sensing technique by which high-contrast imaging systems can maintain high contrast during long observations. This conclusion is further supported by a noise analysis of LDFC's performance with the APvAPP in simulation.

Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. 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: Status of the SCExAO instrument: recent technology upgrades and path to a system-level demonstrator for PSI Authors: Lozi, Julien; Guyon, Olivier; Vievard, Sébastien; Sahoo, Ananya; Deo, Vincent; Jovanovic, Nemanja; Norris, Barnaby; Martinod, Marc-Antoine; Mazin, Ben; Walter, Alex; Fruitwala, Neelay; Steiger, Sarah; Davis, Kristina; Tuthill, Peter; Kudo, Tomoyuki; Kawahara, Hajime; Kotani, Takayuki; Ireland, Michael; Anagnos, Theodoros; Schwab, Chrstian; Cvetojevic, Nick; Huby, Elsa; Lacour, Sylvestre; Barjot, Kevin; Groff, Tyler D.; Chilcote, Jeffrey; Kasdin, Jeremy; Martinache, Frantz; Laugier, Romain; N'Diaye, Mamadou; Knight, Justin; Males, Jared; Bos, Steven; Snik, Frans; Doelman, David; Miller, Kelsey; Bendek, Eduardo; Belikov, Ruslan; Pluzhnik, Eugene; Currie, Thayne; Kuzuhara, Masayuki; Uyama, Taichi; Nishikawa, Jun; Murakami, Naoshi; Hashimoto, Jun; Minowa, Yosuke; Clergeon, Christophe; Ono, Yoshito; Takato, Naruhisa; Tamura, Motohide; Takami, Hideki; Hayashi, Masa Bibcode: 2020SPIE11448E..0NL Altcode: The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a high-contrast imaging system installed at the 8-m Subaru Telescope on Maunakea, Hawaii. Due to its unique evolving design, SCExAO is both an instrument open for use by the international scientific community, and a testbed validating new technologies, which are critical to future high-contrast imagers on Giant Segmented Mirror Telescopes (GSMTs). Through multiple international collaborations over the years, SCExAO was able to test the most advanced technologies in wavefront sensors, real-time control with GPUs, low-noise high frame rate detectors in the visible and infrared, starlight suppression techniques or photonics technologies. Tools and interfaces were put in place to encourage collaborators to implement their own hardware and algorithms, and test them on-site or remotely, in laboratory conditions or on-sky. We are now commissioning broadband coronagraphs, the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera (MEC) for high-speed speckle control, as well as a C-RED ONE camera for both polarization differential imaging and IR wavefront sensing. New wavefront control algorithms are also being tested, such as predictive control, multi-camera machine learning sensor fusion, and focal plane wavefront control. We present the status of the SCExAO instrument, with an emphasis on current collaborations and recent technology demonstrations. We also describe upgrades planned for the next few years, which will evolve SCExAO —and the whole suite of instruments on the IR Nasmyth platform of the Subaru Telescope— to become a system-level demonstrator of the Planetary Systems Imager (PSI), the high-contrast instrument for the Thirty Meter Telescope (TMT). Title: New NIR spectro-polarimetric modes for the SCExAO instrument Authors: Lozi, Julien; Guyon, Olivier; Kudo, Tomoyuki; Zhang, Jin; Jovanovic, Nemanja; Norris, Barnaby; Martinod, Marc-Antoine; Groff, Tyler D.; Chilcote, Jeffrey; Tamura, Motohide; Bos, Steven; Snik, Frans; Vievard, Sébastien; Sahoo, Ananya; Deo, Vincent; Martinache, Frantz; Kasdin, Jeremy Bibcode: 2020SPIE11448E..7CL Altcode: Polarization Differential Imaging (PDI) is one of the most productive modes of current high-contrast imagers. Dozens of new protoplanetary, transition and debris disks were imaged recently for the first time, helping us understand the processes of planet formation, and giving clues on the mass of potential planets inside these disks, even if they cannot be imaged directly. The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is equipped with a fast visible dual-camera polarimetric module, VAMPIRES, already producing valuable scientific observations of protoplanetary disks and dust shells. In addition, we recently commissioned two new polarimetric modules in the infrared. The first one is a spectro-polarimetric mode using the CHARIS Integral Field Spectrograph (IFS). A Wollaston prism was added in front of the IFS, reducing the field-of-view to 2x1 arcsec to accommodate for the imaging of both polarizations on the same detector without sacrificing the spectral resolution of the instrument, in any of its spectral modes. The second module, similar to VAMPIRES, uses a low-noise high frame rate C-RED ONE camera combined with a Ferroelectric Liquid Crystal (FLC) device to modulate and record the polarization at high-speed, freezing effectively the atmospheric speckles for higher precision. We present on-sky results of the new polarimetric capabilities taken during the commissioning phase. In addition, we show future capabilities that are already scheduled to increase the performance of these modules, especially the addition of non-redundant masks, as well as a polarimetric vector Apodizing Phase Plate (vAPP) coronagraph. 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: A Search for Polarized Thermal Emission from Directly Imaged Exoplanets and Brown Dwarf Companions to Nearby Stars Authors: Jensen-Clem, Rebecca; Millar-Blanchaer, Maxwell A.; van Holstein, Rob G.; Mawet, Dimitri; Graham, James; Sengupta, Sujan; Marley, Mark S.; Snik, Frans; Vigan, Arthur; Hinkley, Sasha; de Boer, Jos; Girard, Julien H.; De Rosa, Robert J.; Bowler, Brendan P.; Wiktorowicz, Sloane J.; Perrin, Marshall D.; Crepp, Justin R.; Macintosh, Bruce Bibcode: 2020AJ....160..286J Altcode: Aerosols in the atmospheres of cloudy gas giant exoplanets and brown dwarfs scatter and polarize these objects' thermal emission. If such an object has an oblate shape or nonuniform cloud distribution, the net degree of linear polarization can show an increase ranging from several tenths of a percent to a few percent. Modern high-contrast imaging polarimeters are now poised to detect such low-polarization signals, opening up a new window into the rotational velocities and cloud properties of substellar companions to nearby stars. In this paper, we present the results of a near-IR survey searching for linearly polarized thermal emission from a sample of two planetary-mass companions and five brown dwarf companions using GPI and SPHERE-IRDIS. We probe the subpercent linear polarization regime that typifies polarized free-floating brown dwarfs and place limits on each object's degree of linear polarization. We relate our upper limits on each target's degree of linear polarization to its rotation rate, and place our results in the context of rotation rates measured using high-resolution spectroscopy. 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: New concepts in vector-apodizing phase plate coronagraphy Authors: Bos, Steven P.; Doelman, David S.; Miller, Kelsey L.; Snik, Frans Bibcode: 2020SPIE11448E..3WB Altcode: 2020arXiv201200462B The vector-Apodizing Phase Plate (vAPP) is a pupil-plane coronagraph that manipulates phase to create darkholes in the stellar PSF. The phase is induced on the circular polarization states through the inherently achromatic geometric phase by spatially varying the fast axis orientation of a half-wave liquid-crystal layer. The two polarized PSFs can be separated, either by a quarter-wave plate (QWP) followed by a polarizing beamsplitter (PBS) for broadband operation, or a polarization sensitive grating (PSG) for narrowband or IFS operation. Here we present new vAPP concepts that lift the restrictions of previous designs and report on their performance. We demonstrated that the QWP+PBS combination puts tight tolerances on the components to prevent leakage of non-coronagraphic light into the dark-hole. We present a new broadband design using an innovative two-stage patterned liquid-crystal element system based on multi-color holography, alleviating the leakage problem and relaxing manufacturing tolerances. Furthermore, we have shown that focal-plane wavefront sensing (FPWFS) can be integrated into the vAPP by an asymmetric pupil. However, such vAPPs suffer from a reduced throughput and have only been demonstrated with a PSG in narrowband operation. We present advanced designs that maintain throughput and enable phase and amplitude wavefront sensing. We also present broadband vAPP FPWFS designs and outline a broadband FPWFS algorithm. Finally, previous dual-beam vAPP designs for sensitive polarimetry with one-sided dark holes were very complex. We show new dual-beam designs that significantly reduce the complexity. Title: Calibration of the instrumental polarization effects of SCExAO-CHARIS' spectropolarimetric mode Authors: van Holstein, Rob G.; Bos, Steven P.; Ruigrok, Jasper; Lozi, Julien; Guyon, Olivier; Norris, Barnaby; Snik, Frans; Chilcote, Jeffrey; Currie, Thayne; Groff, Tyler D.; 't Hart, Joost; Jovanovic, Nemanja; Kasdin, Jeremy; Kudo, Tomoyuki; Martinache, Frantz; Mazin, Ben; Sahoo, Ananya; Tamura, Motohide; Vievard, Sébastien; Walter, Alex; Zhang, Jin Bibcode: 2020SPIE11447E..5BV Altcode: 2020arXiv201200475V SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integral field spectrograph CHARIS. Recently, a Wollaston prism was added to CHARIS' optical path, giving CHARIS a spectropolarimetric capability that is unique among high-contrast imaging instruments. We present a comprehensive and detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument, using measurements with the internal source and observations of standard stars. The 22 wavelength bins of CHARIS provide a unique opportunity to investigate in detail the wavelength dependence of the instrumental polarization effects. We find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ~95% of the incident linear polarization to circularly polarized light that cannot be measured. We fit the crosstalk of the half-wave plate (HWP) for all wavelengths with a simple two-parameter model of an achromatic HWP consisting of a layer of quartz and a layer of MgF2. While the magnitude of the telescope-induced polarization varies with wavelength, its angle varies solely with the altitude angle of the telescope. We show initial steps toward correcting on-sky data for the instrumental polarization effects, with which we aim to achieve a polarimetric accuracy <0.1% in the degree of linear polarization. Our calibrations of CHARIS' spectropolarimetric mode enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions. Title: Validating advanced wavefront control techniques on the SCExAO testbed/instrument Authors: Guyon, Olivier; Lozi, Julien; Vievard, Sebastien; Belikov, Ruslan; Bendek, Eduardo; Bos, Steven; Currie, Thayne; Deo, Vincent; Fitzgerald, Michael; Gratadour, Damien; Groff, Tyler; Jovanovic, Nemanja; Kawahara, Hajime; Kotani, Takayuki; Kudo, Tomoyuki; Lopez, Coline; Ltaief, Hatem; Males, Jared; Martinache, Frantz; Martinod, Marc-Antoine; Mazin, Benjamin A.; Miller, Kelsey; Norris, Barnaby; Ndiaye, Mamadou; Pluzhnyk, Eugene; Sahoo, Ananya; Sevin, Arnaud; Skaf, Nour; Snik, Frans; Tamura, Motohide; Wong, Alison Bibcode: 2020SPIE11448E..1ZG Altcode: The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) serves both a science instrument in operation, and a prototyping platform for integrating and validating advanced wavefront control techniques. It provides a modular hardware and software environment optimized for flexible prototyping, reducing the time from concept formulation to on-sky operation and validation. This approach also enables external research group to deploy and test new hardware and algorithms. The hardware architecture allows for multiple subsystems to run concurrently, sharing starlight by means of dichroics. The multiplexing lends itself to running parallel experiments simultaneously, and developing sensor fusion approaches for increased wavefront sensing sensitivity and reliability. Thanks to a modular realtime control software architecture designed around the CACAO package, users can deploy WFS/C routines with full low-latency access to all cameras data streams. Algorithms can easily be shared with other cacao-based AO systems at Magellan (MagAO-X) and Keck. We highlight recent achievements and ongoing activities that are particularly relevant to the development of high contrast imaging instruments for future large ground-based telescopes (ELT, TMT, GMT) and space telescopes (HabEx, LUVOIR). These include predictive control and sensor fusion, PSF reconstruction from AO telemetry, integrated coronagraph/WFS development, focal plane speckle control with photon counting MKIDS camera, and fiber interferometry. We also describe upcoming upgrades to the WFS/C architecture: a new 64x64 actuator first stage DM, deployment of a beam switcher for concurrent operation of SCExAO with other science instruments, and the ULTIMATE upgrade including deployment of multiple LGS WFSs and an adaptive secondary mirror. 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: METIS high-contrast imaging: design and expected performance (Erratum) Authors: Carlomagno, Brunella; Delacroix, Christian; Absil, Olivier; Cantalloube, Faustine; de Xivry, Gilles Orban; Pathak, Prashant; Agocs, Tibor; Bertram, Thomas; Brandl, Bernhard; Burtscher, Leonard; Doelman, David; Feldt, Markus; Glauser, Adrian; Hippler, Stefan; Kenworthy, Matthew; Por, Emiel; Snik, Frans; Stuik, Remko; van Boekel, Roy Bibcode: 2020JATIS...6d9801C Altcode: This erratum corrects the omission of authors and references from the paper as it was originally published. Title: LOUPE: Spectropolarimetry in the Search for (Extra)Terrestrial Life Authors: Klindžić, Dora; Stam, Daphne; Snik, Frans; Pallichadath, Vidhya; van Dijk, Chris; Esposito, Marco Bibcode: 2020EPSC...14..887K Altcode: Since the discovery of the first planets orbiting other stars in the 1990"s, the field of exoplanetary study has come to understand that Earth-like planets, i.e. rocky planets in the habitable zone, are more common than previously thought. Astronomical spectropolarimetry will hold the key in characterizing these exoplanets, including deriving their atmospheric and surface properties from linearly polarized light, such as the presence of oxygen, liquid water, continents, oceans, clouds, vegetation, ice, deserts; and even their circularly polarized biomarkers - signs of homochiral extraterrestrial life. But, in order to recognize life in the Universe, we ought to first use life on Earth as an observational benchmark.We present a novel spectropolarimetric instrument design: LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth (Klindžić+, submitted), 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: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.Fig 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: Detection of polarization neutral points in observations of the combined corona and sky during the 21 August 2017 total solar eclipse Authors: Snik, Frans; Bos, Steven P.; Brackenhoff, Stefanie A.; Doelman, David S.; Por, Emiel H.; Bettonvil, Felix; Rodenhuis, Michiel; Vorobiev, Dmitry; Eshelman, Laura M.; Shaw, Joseph A. Bibcode: 2020ApOpt..59F..71S Altcode: 2020arXiv200712482S We report the results of polarimetric observations of the total solar eclipse of 21 August 2017 from Rexburg, Idaho (USA). We use three synchronized DSLR cameras with polarization filters oriented at 0°, 60°, and 120° to provide high-dynamic-range RGB polarization images of the corona and surrounding sky. We measure tangential coronal polarization and vertical sky polarization, both as expected. These observations provide detailed detections of polarization neutral points above and below the eclipsed Sun where the coronal polarization is canceled by the sky polarization. We name these special polarization neutral points after Minnaert and Van de Hulst. 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: VSTpol: the first large survey telescope for optical polarimetry Authors: Covino, S.; Smette, A.; Snik, F. Bibcode: 2020vstb.conf...20C Altcode: No abstract at ADS Title: First Images of the Protoplanetary Disk around PDS 201 Authors: Wagner, Kevin; Stone, Jordan; Dong, Ruobing; Ertel, Steve; Apai, Daniel; Doelman, David; Bohn, Alexander; Najita, Joan; Brittain, Sean; Kenworthy, Matthew; Keppler, Miriam; Webster, Ryan; Mailhot, Emily; Snik, Frans Bibcode: 2020AJ....159..252W Altcode: 2020arXiv200406157W Scattered light imaging has revealed nearly a dozen circumstellar disks around young Herbig Ae/Be stars—enabling studies of structures in the upper disk layers as potential signs of ongoing planet formation. We present the first images of the disk around the variable Herbig Ae star PDS 201 (V* V351 Ori) and an analysis of the images and spectral energy distribution through 3D Monte Carlo radiative transfer simulations and forward modeling. The disk is detected in three data sets with the Large Binocular Telescope Interferometer/Large Binocular Telescope mid-infrared camera at the Large Binocular Telescope, including direct observations in the Ks and L' filters, and an L' observation with the 360° vector apodizing phase plate coronagraph. The scattered light disk extends to a very large radius of ∼250 au, which places it among the largest of such disks. Exterior to the disk, we establish detection limits on substellar companions down to ∼5 MJup at ≳1"5 (≳500 au), assuming the Baraffe et al. models. The images show a radial gap extending to ∼0"4 (∼140 au at a distance of 340 pc) that is also evident in the spectral energy distribution. The large gap is a possible signpost of multiple high-mass giant planets at orbital distances (∼60-100 au) that are unusually massive and widely separated compared to those of planet populations previously inferred from protoplanetary disk substructures. Title: Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary Authors: Millar-Blanchaer, Maxwell A.; Girard, Julien H.; Karalidi, Theodora; Marley, Mark S.; van Holstein, Rob G.; Sengupta, Sujan; Mawet, Dimitri; Kataria, Tiffany; Snik, Frans; de Boer, Jos; Jensen-Clem, Rebecca; Vigan, Arthur; Hinkley, Sasha Bibcode: 2020ApJ...894...42M Altcode: Brown dwarfs exhibit patchy or spatially varying banded cloud structures that are inferred through photometric and spectroscopic variability modeling techniques. However, these methods are insensitive to rotationally invariant structures, such as the bands seen in Jupiter. Here, we present H-band Very Large Telescope/NaCo linear polarization measurements of the nearby Luhman 16 L/T transition binary, which suggest that Luhman 16A exhibits constant longitudinal cloud bands. The instrument was operated in pupil tracking mode, allowing us to unambiguously distinguish between a small astrophysical polarization and the ∼2% instrumental linear polarization. We measure the degree and angle of linear polarization of Luhman 16A and B to be pA = 0.031% ± 0.004% and ψA = -32° ± 4°, and pB = 0.010% ± 0.004% and ${\psi }_{B}={73}_{-11}^{+13\circ} $ , respectively. Using known physical parameters of the system, we demonstrate that an oblate homogeneous atmosphere cannot account for the polarization measured in Luhman 16A, but could be responsible for that of the B component. Through a nonexhaustive search of banded cloud morphologies, we demonstrate a two-banded scenario that can achieve a degree of linear polarization of p = 0.03% and conclude that the measured polarization of the A component must be predominantly due to cloud banding. For Luhman 16B, either oblateness or cloud banding could be the dominant source of the measured polarization. The misaligned polarization angles of the two binary components tentatively suggest spin-orbit misalignment. These measurements provide new evidence for the prevalence of cloud banding in brown dwarfs while at the same time demonstrating a new method—complementary to photometric and spectroscopic variability methods—for characterizing the cloud morphologies of substellar objects without signs of variability. 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: Minimizing the Polarization Leakage of Geometric-phase Coronagraphs with Multiple Grating Pattern Combinations Authors: Doelman, David S.; Por, Emiel H.; Ruane, Garreth; Escuti, Michael J.; Snik, Frans Bibcode: 2020PASP..132d5002D Altcode: 2020arXiv200205622D The design of liquid-crystal diffractive phase plate coronagraphs for ground-based and space-based high-contrast imaging systems is limited by the trade-off between spectral bandwidth and polarization leakage. We demonstrate that by combining phase patterns with a polarization grating (PG) pattern directly followed by one or several separate PGs, we can suppress the polarization leakage terms by additional orders of magnitude by diffracting them out of the beam. Using two PGs composed of a single-layer liquid crystal structure in the lab, we demonstrate a leakage suppression of more than an order of magnitude over a bandwidth of 133 nm centered around 532 nm. At this center wavelength we measure a leakage suppression of three orders of magnitude. Furthermore, simulations indicate that a combination of two multi-layered liquid-crystal PGs can suppress leakage to <10-5 for 1-2.5 μm and <10-10 for 650-800 nm. We introduce multi-grating solutions with three or more gratings that can be designed to have no separation of the two circular polarization states, and offer even deeper suppression of polarization leakage. We present simulations of a triple-grating solution that has <10-10 leakage on the first Airy ring from 450 to 800 nm. We apply the double-grating concept to the Vector-vortex coronagraph of charge 4, and demonstrate in the lab that polarization leakage no longer limits the on-axis suppression for ground-based contrast levels. Lastly, we report on the successful installation and first-light results of a double-grating vector Apodizing Phase Plate pupil-plane coronagraph installed at the Large Binocular Telescope. We discuss the implications of these new coronagraph architectures for high-contrast imaging systems on the ground and in space. 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: 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: New NIR spectro-polarimetric modes for the SCExAO instrument Authors: Lozi, J.; Guyon, O.; Jovanovic, N.; Norris, B.; Groff, T.; Chilcote, J.; Kasdin, N.; Kudo, T.; Tamura, M.; Zhang, J.; Bos, S.; Snik, F.; Doelman, D.; Vievard, S.; Sahoo, A.; Currie, T.; Martinache, F. Bibcode: 2020AAS...23516107L Altcode: The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a high-contrast imaging system installed at Subaru. It is equipped with a fast visible dual-camera polarimetric module, VAMPIRES, already producing valuable observations of protoplanetary disks and dust shells. We present two new polarimetric modules that were recently implemented,. using the NIR light from y- to K-band and Wollaston prisms. The fast polarization module, similarly to VAMPIRES, uses a fast IR camera that can run at kilohertz frame rates, and a Ferroelectric Liquid Crystal (FLC) device modulating the polarization in a synchronized way with the acquisition. This allows to freeze atmospheric speckles and to calibrate more precisely the degree of polarization of the target, as already demonstrated by VAMPIRES. For the second module, we perform spectro-polarimetric measurements at a slower rate, using the CHARIS Integral Field Spectrograph (IFS). The field-of-view is reduced by a factor 2 in one direction to 2x1 arcsec, to accommodate for the imaging of both polarizations on the same detector without sacrificing the spectral resolution of the instrument. This is the first demonstration of a high-contrast spectro-polarimeter using an IFS. This mode is now available for open-use. We present on-sky results of the new polarimetric capabilities taken during the commissioning phase, on strongly polarized targets. 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: Circular Spectropolarimetric Sensing of Vegetation in the Field: Possibilities for the Remote Detection of Extraterrestrial Life Authors: Patty, C. H. Lucas; Loes ten Kate, Inge; Buma, Wybren Jan; van Spanning, Rob J. M.; Steinbach, Gábor; Ariese, Freek; Snik, Frans Bibcode: 2019AsBio..19.1221P Altcode: 2019arXiv190205859P Homochirality is a generic and unique property of all biochemical life, and the fractional circular polarization of light it induces therefore constitutes a potentially unambiguous biosignature. However, while high-quality circular polarimetric spectra can be easily and quickly obtained in the laboratory, accurate measurements in the field are much more challenging due to large changes in illumination and target movement. In this study, we measured various targets in the field, up to distances of a few kilometers, using the dedicated circular spectropolarimeter TreePol. We show how photosynthetic life can readily be distinguished from abiotic matter. We underline the potential of circular polarization signals as a remotely accessible means to characterize and monitor terrestrial vegetation, for example, for agriculture and forestry. In addition, we discuss the potential of circular polarization for the remote detection of extraterrestrial life. 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: 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: Standardized spectral and radiometric calibration of consumer cameras Authors: Burggraaff, Olivier; Schmidt, Norbert; Zamorano, Jaime; Pauly, Klaas; Pascual, Sergio; Tapia, Carlos; Spyrakos, Evangelos; Snik, Frans Bibcode: 2019OExpr..2719075B Altcode: 2019arXiv190604155B Consumer cameras, particularly onboard smartphones and UAVs, are now commonly used as scientific instruments. However, their data processing pipelines are not optimized for quantitative radiometry and their calibration is more complex than that of scientific cameras. The lack of a standardized calibration methodology limits the interoperability between devices and, in the ever-changing market, ultimately the lifespan of projects using them. We present a standardized methodology and database (SPECTACLE) for spectral and radiometric calibrations of consumer cameras, including linearity, bias variations, read-out noise, dark current, ISO speed and gain, flat-field, and RGB spectral response. This includes golden standard ground-truth methods and do-it-yourself methods suitable for non-experts. Applying this methodology to seven popular cameras, we found high linearity in RAW but not JPEG data, inter-pixel gain variations >400% correlated with large-scale bias and read-out noise patterns, non-trivial ISO speed normalization functions, flat-field correction factors varying by up to 2.79 over the field of view, and both similarities and differences in spectral response. Moreover, these results differed wildly between camera models, highlighting the importance of standardization and a centralized database. 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: Simultaneous phase and amplitude aberration sensing with a liquid-crystal vector-Zernike phase mask Authors: Doelman, David S.; Fagginger Auer, Fedde; Escuti, Michael J.; Snik, Frans Bibcode: 2019OptL...44...17D Altcode: 2018arXiv181108805D We present an enhanced version of the Zernike wavefront sensor, that simultaneously measures phase and amplitude aberrations. The 'vector-Zernike' wavefront sensor consists of a patterned liquid-crystal mask, which imposes $\pm \pi/2$ phase on the point spread function core through the achromatic geometric phase acting with opposite sign on opposite circular polarizations. After splitting circular polarization, the ensuing pupil intensity images are used to reconstruct the phase and the amplitude of the incoming wavefront. We demonstrate reconstruction of the complex wavefront with monochromatic lab measurements and show in simulation the high accuracy and sensitivity over a bandwidth up to $100\%$ Title: The SCExAO High Contrast Imaging Platform: Current and Upcoming Capabilities Authors: Guyon, Olivier; Lozi, Julien; Vievard, Sébastien; Sahoo, Ananya; Jovanovic, Nemanja; Currie, Thayne; Pathak, Prashant; Martinache, Frantz; Kudo, Tomoyuki; Tamura, Motohide; Singh, Garima; Clergeon, Christophe; Minowa, Yosuke; Ono, Yoshito; Mieda, Etsuko; Mazin, Benjamin; Walter, Alexander; Cvetojevic, Nick; Lacour, Sylvestre; Huby, Elsa; Norris, Barnaby; Wong, Alison; Ireland, Michael; Schwab, Christian; Feger, Tobias; Tuthill, Peter; Lagadec, Tiphaine; Groff, Tyler; Chilcote, Jeff; Brandt, Timothy; Hall, Don; Goebel, Sean; Snik, Frans; Doelman, David; Bos, Steven; Kawahara, Hajime; Kotani, Takayuki; Mawet, Dimitri; Belikov, Ruslan; Bendek, Eduardo; Sevin, Arnaud; Gratadour, Damien; Ltaief, Hatem; Males, Jared; Jin, Zhang; Murakami, Naoshi; Knight, Justin; Kasdin, Jeremy Bibcode: 2019AAS...23310403G Altcode: The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, currently in science operation on the Subaru Telescope, combines high performance wavefront control and starlight suppression to image exoplanets and disks. In addition to its flagship science cameras, CHARIS (near-IR spectro-imaging) and VAMPIRES (visible polarimetric imaging), new capabilities are being deployed, including advanced small inner working angle coronagraphy, high frame rate low noise imaging, near-IR polarimetric imaging, interferometric imaging/nulling and high resolution diffraction-limited spectroscopy. A significant part of SCExAO's development and performance improvements is realized through wavefront control algorithms (focal plane wavefront control, sensor fusion, predictive control, multi-star WFC) implemented through the cacao software package and enabled by advances in detector technologies (photon counting with MKIDs camera) and modern computer hardware. Further improvements are also expected from the use of real-time WFS telemetry to enhance removal of residual starlight in post-processing. Much of SCExAO's development activities are aimed at direct imaging of giant planets in reflected light with current large telescopes, and imaging/spectroscopy of habitable planets with future giant segmented mirror telescopes. Title: HiCIBaS: A precursor mission for high contrast imaging balloon systems Authors: Marchis, F.; Thibault, S.; Côté, O.; Brousseau, D.; Allain, G.; Lord, M. P.; Ouellet, M.; Patel, D.; Vallée, C.; Belikov, R.; Bendek, E.; Blain, C.; Bradley, C.; Doelman, D.; Daigle, O.; Doyon, R.; Grandmont, F. J.; Helmbrecht, M.; Kenworthy, M.; Lafrenière, D.; Marois, C.; Montminy, S.; Snik, F.; de Jonge, C.; Vasisht, G.; Veran, J. P.; Vincent, P. Bibcode: 2018AGUFM.P41C3747M Altcode: Even though space-based astronomical observations provides valuable data in optical regimes that are not possible on ground, they are still scarce and difficult to plan because of the high cost that is involved with space-based instrumentation. Flying scientific instruments aboard sub-orbital balloons can be achieve at a fraction of the cost. HiCIBaS, the High-Contrast Imaging Balloon System, provides a platform to develop instrumentation and imaging techniques for future balloon-borne high-contrast coronagraph to detect and characterize extrasolar planets.

HiCIBaS consists of an off-the-shelf 14-inch Schmidt-Cassegrain telescope, a custom Nasmyth mount and a high-contrast instrument. The optical payload will be flown at an altitude of 36 km aboard a pointed multi-instrument gondola (CARMEN) provided by CNES using CSA's Stratos program from Timmins, Ontario (Canada). High-contrast imaging becomes interesting at this altitude because there is only 1% of the atmosphere left to induce wavefront error. The gondola can be stabilized in azimuth to ±1 deg, so the front-end optics include two cameras used for a coarse pointing system. The telescope mount can rotate to ±20 degrees in azimuth and from -20 degrees to +60 degrees in elevation. The optical payload consists of two instruments, a pyramidal-style low-order wavefront sensor (LOWFS) and a coronagraphic wavefront sensor (CWS). Both the telescope mount and the optical bench were designed by the HiCIBaS team in collaboration with OMP Inc, science EMCCD's were provided by Nüvü Caméras and the segmented deformable mirror was provided by Iris AO Inc.

HiCIBaS launch is scheduled between August 13th and August 25th, just after 23h00 local time. The mission will be in two parts, a night and an early-day phase. During the night phase Cappella will be used to record data on atmospheric turbulence and test the new technology used on this first flight. During the day, the objective will mainly be to obtain data on the luminosity of the background compared to Polaris, the second target star. 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: 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: A unique infrared spectropolarimetric unit for CRIRES+ Authors: Piskunov, Nikolai; Stempels, Eric; Lavail, Alexis; Escuti, Michael; Snik, Frans; Dolgopolov, Andrey; Rozel, Milan; Durandet, Candice; Hatzes, Artie; Bristow, Paul; Brucalassi, Anna; Cumani, Claudio; Dorn, Reinhold J.; Haimerl, Andreas; Heiter, Ulrike; Hinterschuster, Renate; Follert, Roman; Ives, Derek; Jung, Yves; Kerber, Florian; Klein, Barbara; Lizon, Jean-Louis; Marquart, Thomas; Molina-Conde, Ignacio; Pasquini, Luca; Paufique, Jérôme; Oliva, Ernesto; Reiners, Ansgar; Seemann, Ulf; Stegmeier, Jörg; Tordo, Sebastien; Valenti, Elena Bibcode: 2018SPIE10702E..34P Altcode: High-resolution infrared spectropolarimetry has many science applications in astrophysics. One of them is measuring weak magnetic fields using the Zeeman effect. Infrared domain is particularly advantageous as Zeeman splitting of spectral lines is proportional to the square of the wavelength while the intrinsic width of the line cores increases only linearly. Important science cases include detection and monitoring of global magnetic fields on solar-type stars, study of the magnetic field evolution from stellar formation to the final stages of the stellar life with massive stellar winds, and the dynamo mechanism operation across the boundary between fully- and partially-convective stars. CRIRES+ (the CRIRES upgrade project) includes a novel spectropolarimetric unit (SPU) based on polar- ization gratings. The novel design allows to perform beam-splitting very early in the optical path, directly after the tertiary mirror of the telescope (the ESO Very Large Telescope, VLT), minimizing instrumental polariza- tion. The new SPU performs polarization beam-splitting in the near-infrared while keeping the telescope beam mostly unchanged in the optical domain, making it compatible with the adaptive optics system of the CRIRES+ instrument. The SPU consists of four beam-splitters optimized for measuring circular and linear polarization of spectral lines in YJ and HK bands. The SPU can perform beam switching allowing to correct for throughput in each beam and for variations in detector pixel sensitivity. Other new features of CRIRES+, such as substantially increased wavelength coverage, stability and advanced data reduction pipeline will further enhance the sensitivity of the polarimetric mode. The combination of the SPU, CRIRES+ and the VLT is a unique facility for making major progress in understanding stellar activity. In this article we present the design of the SPU, laboratory measurements of individual components and of the whole unit as well as the performance prediction for the operation at the VLT. 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: A review of high contrast imaging modes for METIS Authors: Kenworthy, Matthew A.; Absil, Olivier; Carlomagno, Brunella; Agócs, Tibor; Por, Emiel H.; Bos, Steven; Brandl, Bernhard; Snik, Frans Bibcode: 2018SPIE10702E..A3K Altcode: The Mid-infrared E-ELT Imager and Spectrograph (METIS) for the European Extremely Large Telescope (E- ELT) consists of diffraction-limited imagers that cover 3 to 14 microns with medium resolution (R 5000) long slit spectroscopy, and an integral field spectrograph for high spectral resolution spectroscopy (R 100,000) over the L and M bands. We present our approach for high contrast imaging with METIS, covering diffraction suppression with coronagraphs, the removal of residual aberrations using QACITS1, 2 and Phase Sorting Interferometry (PSI),3 and simulations demonstrating the expected contrast. 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: Analysis of the polarimetric performance of the HARPS3 Cassegrain adaptor unit Authors: Dorval, Patrick; Snik, Frans; Piskunov, Nikolai; Navarro, Ramon; Kragt, Jan; ter Horst, Rik; Kunst, Peter; Snellen, Ignas; Naylor, Tim; Thompson, Samantha Bibcode: 2018SPIE10702E..6BD Altcode: The third version of the High Accuracy Radial velocity Planet Searcher (HARPS3) instrument is built for a ten-year programme aimed at achieving 10 cm/sec radial velocity precision on nearby stars to search for Earth-like planets. HARPS3 will be commissioned on the to-be-roboticized 2.54-m Isaac Newton Telescope at La Palma in 2021. One of the main changes compared to its predecessors is the novel dual-beam Cassegrain focus, featuring a stabilised beam feed into the HARPS3 spectrograph and an insertable polarimetric sub-unit. This polarimetric sub-unit enables HARPS3 to directly measure stellar activity signatures, which can be useful for correcting activity-induced radial velocity jitter in the search for Earth-like planets. The sub-unit consists of superachromatic polymer quarter- and half-wave retarders for circular and linear polarizations respectively, designed to suppress polarized fringing, and a novel polarimetric beam splitter based on a wire-grid design, separating the two polarimetric beams by 30 mm and feeding two separate science fibers. The dual-beam exchange implementation in combination with the extreme stability of the HARPS3 spectrograph enables a polarimetric sensitivity of 10-5 on bright stars. One of the main challenges of such a system is in the characterization of instrumental polarization effects which limit the polarimetric accuracy of the polarimetric observing mode. By design and characterization of this subsystem and by pre-emptively mitigating possible noise sources, we can minimize the noise characteristics of the polarization sub-unit to allow for precise observations. In this paper we report on the design, realization, assembly, alignment, and testing of the polarimetric unit to be installed in the Cassegrain Adaptor Unit of the HARPS3 spectrograph Title: Fully broadband vAPP coronagraphs enabling polarimetric high contrast imaging Authors: Bos, Steven P.; Doelman, David S.; de Boer, Jos; Por, Emiel H.; Norris, Barnaby; Escuti, Michael J.; Snik, Frans Bibcode: 2018SPIE10706E..5MB Altcode: 2018arXiv181109775B We present designs for fully achromatic vector Apodizing Phase Plate (vAPP) coronagraphs, that implement low polarization leakage solutions and achromatic beam-splitting, enabling observations in broadband filters. The vAPP is a pupil plane optic, inducing the phase through the inherently achromatic geometric phase. We discuss various implementations of the broadband vAPP and set requirements on all the components of the broadband vAPP coronagraph to ensure that the leakage terms do not limit a raw contrast of 10-5. Furthermore, we discuss superachromatic QWPs based of liquid crystals or quartz/MgF2 combinations, and several polarizer choices. As the implementation of the (broadband) vAPP coronagraph is fully based on polarization techniques, it can easily be extended to furnish polarimetry by adding another QWP before the coronagraph optic, which further enhances the contrast between the star and a polarized companion in reflected light. We outline several polarimetric vAPP system designs that could be easily implemented in existing instruments, e.g. SPHERE and SCExAO. 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: The hunt for Sirius Ab: comparison of algorithmic sky and PSF estimation performance in deep coronagraphic thermal-IR high contrast imaging Authors: Long, Joseph D.; Males, Jared R.; Morzinski, Katie M.; Close, Laird M.; Snik, Frans; Kenworthy, Matthew A.; Otten, Gilles P. P. L.; Monnier, John; Tolls, Volker; Weinberger, Alycia Bibcode: 2018SPIE10703E..2TL Altcode: 2018arXiv180704361L Despite promising astrometric signals, to date there has been no success in direct imaging of a hypothesized third member of the Sirius system. Using the Clio instrument and MagAO adaptive optics system on the Magellan Clay 6.5 m telescope, we have obtained extensive imagery of Sirius through a vector apodizing phase plate (vAPP) coronagraph in a narrowband filter at 3.9 microns. The vAPP coronagraph and MagAO allow us to be sensitive to planets much less massive than the limits set by previous non-detections. However, analysis of these data presents challenges due to the target's brightness and unique characteristics of the instrument. We present a comparison of dimensionality reduction techniques to construct background illumination maps for the whole detector using the areas of the detector that are not dominated by starlight. Additionally, we describe a procedure for sub-pixel alignment of vAPP data using a physical-optics-based model of the coronagraphic PSF. Title: Focal plane wavefront sensing and control strategies for high-contrast imaging on the MagAO-X instrument Authors: Miller, Kelsey; Males, Jared R.; Guyon, Olivier; Close, Laird M.; Doelman, David; Snik, Frans; Por, Emiel; Wilby, Michael J.; Bohlman, Chris; Lumbres, Jennifer; Van Gorkom, Kyle; Kautz, Maggie; Rodack, Alexander; Knight, Justin; Jovanovic, Nemanja; Morzinski, Katie; Schatz, Lauren Bibcode: 2018SPIE10703E..1TM Altcode: 2018arXiv180704381M The Magellan extreme adaptive optics (MagAO-X) instrument is a new extreme adaptive optics (ExAO) system designed for operation in the visible to near-IR which will deliver high contrast-imaging capabilities. The main AO system will be driven by a pyramid wavefront sensor (PyWFS); however, to mitigate the impact of quasi-static and non-common path (NCP) aberrations, focal plane wavefront sensing (FPWFS) in the form of low-order wavefront sensing (LOWFS) and spatial linear dark field control (LDFC) will be employed behind a vector apodizing phase plate (vAPP) coronagraph using rejected starlight at an intermediate focal plane. These techniques will allow for continuous high-contrast imaging performance at the raw contrast level delivered by the vAPP coronagraph ( 6 x 10-5). We present simulation results for LOWFS and spatial LDFC with a vAPP coronagraph, as well as laboratory results for both algorithms implemented with a vAPP coronagraph at the University of Arizona Extreme Wavefront Control Lab. 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: Modeling coronagraphic extreme wavefront control systems for high contrast imaging in ground and space telescope missions Authors: Lumbres, Jennifer; Males, Jared; Douglas, Ewan; Close, Laird; Guyon, Olivier; Cahoy, Kerri; Carlton, Ashley; Clark, Jim; Doelman, David; Feinberg, Lee; Knight, Justin; Marlow, Weston; Miller, Kelsey; Morzinski, Katie; Por, Emiel; Rodack, Alexander; Schatz, Lauren; Snik, Frans; Van Gorkom, Kyle; Wilby, Michael Bibcode: 2018SPIE10703E..4ZL Altcode: 2018arXiv180704729L The challenges of high contrast imaging (HCI) for detecting exoplanets for both ground and space applications can be met with extreme adaptive optics (ExAO), a high-order adaptive optics system that performs wavefront sensing (WFS) and correction at high speed. We describe 2 ExAO optical system designs, one each for ground- based telescopes and space-based missions, and examine them using the angular spectrum Fresnel propagation module within the Physical Optics Propagation in Python (POPPY) package. We present an end-to-end (E2E) simulation of the MagAO-X instrument, an ExAO system capable of delivering 6x10-5 visible-light raw contrast for static, noncommon path aberrations without atmosphere. We present an E2E simulation of a laser guidestar (LGS) companion spacecraft testbed demonstration, which uses a remote beacon to increase the signal available for WFS and control of the primary aperture segments of a future large space telescope, providing of order 10 factor improvement for relaxing observatory stability requirements. Title: Cryogenic characterization of the grating vector APP coronagraph for the upcoming ERIS instrument at the VLT Authors: Boehle, Anna; Glauser, Adrian M.; Kenworthy, Matthew A.; Snik, Frans; Doelman, David; Quanz, Sascha P.; Meyer, Michael R. Bibcode: 2018SPIE10702E..3YB Altcode: We present results from a cryogenic characterization of the grating vector Apodizing Phase Plate (gvAPP) coro- nagraph that will be used in the upcoming instrument ERIS (Enhanced Resolution Imager and Spectrograph) at the VLT. ERIS consists of a 1-5 μm imager (NIX) and a 1 2.5 μm integral field spectrograph (SPIFFIER), both fed by the Adaptive Optics Facility of UT4 to yield diffraction-limited spatial resolution. A gvAPP coronagraph will be included in the NIX imager to enable high-contrast imaging observations, which will be particularly powerful for the direct imaging of exoplanets at L and M bands ( 3-5 μm) and will compliment the current capabilities of VLT/SPHERE and surpass the capabilities of VLT/NACO. We utilize the near-infrared test bench of the Star and Planet Formation group at ETH Zurich to measure key properties of the gvAPP coronagraph at its operating wavelengths and under the vacuum/cryogenic ( 70 K) conditions of the future ERIS instrument. Title: SCExAO, an instrument with a dual purpose: perform cutting-edge science and develop new technologies Authors: Lozi, Julien; Guyon, Olivier; Jovanovic, Nemanja; Goebel, Sean; Pathak, Prashant; Skaf, Nour; Sahoo, Ananya; Norris, Barnaby; Martinache, Frantz; N'Diaye, Mamadou; Mazin, Ben; Walter, Alex B.; Tuthill, Peter; Kudo, Tomoyuki; Kawahara, Hajime; Kotani, Takayuki; Ireland, Michael; Cvetojevic, Nick; Huby, Elsa; Lacour, Sylvestre; Vievard, Sébastien; Groff, Tyler D.; Chilcote, Jeffrey K.; Kasdin, Jeremy; Knight, Justin; Snik, Frans; Doelman, David; Minowa, Yosuke; Clergeon, Christophe; Takato, Naruhisa; Tamura, Motohide; Currie, Thayne; Takami, Hideki; Hayashi, Masa Bibcode: 2018SPIE10703E..59L Altcode: 2018arXiv180908301L The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is an extremely modular high- contrast instrument installed on the Subaru telescope in Hawaii. SCExAO has a dual purpose. Its position in the northern hemisphere on a 8-meter telescope makes it a prime instrument for the detection and characterization of exoplanets and stellar environments over a large portion of the sky. In addition, SCExAO's unique design makes it the ideal instrument to test innovative technologies and algorithms quickly in a laboratory setup and subsequently deploy them on-sky. SCExAO benefits from a first stage of wavefront correction with the facility adaptive optics AO188, and splits the 600-2400 nm spectrum towards a variety of modules, in visible and near infrared, optimized for a large range of science cases. The integral field spectrograph CHARIS, with its J, H or K-band high-resolution mode or its broadband low-resolution mode, makes SCExAO a prime instrument for exoplanet detection and characterization. Here we report on the recent developments and scientific results of the SCExAO instrument. Recent upgrades were performed on a number of modules, like the visible polarimetric module VAMPIRES, the high-performance infrared coronagraphs, various wavefront control algorithms, as well as the real-time controller of AO188. The newest addition is the 20k-pixel Microwave Kinetic Inductance Detector (MKIDS) Exoplanet Camera (MEC) that will allow for previously unexplored science and technology developments. MEC, coupled with novel photon-counting speckle control, brings SCExAO closer to the final design of future high-contrast instruments optimized for Giant Segmented Mirror Telescopes (GSMTs). Title: A precursor mission to high contrast imaging balloon system Authors: Côté, Olivier; Allain, Guillaume; Brousseau, Denis; Lord, Marie-Pier; Ouahbi, Samy; Ouellet, Mireille; Patel, Deven; Thibault, Simon; Vallée, Cédric; Belikov, Ruslan; Bendek, Eduardo; Blain, Célia; Bradley, Collin; Daigle, Olivier; de Jong, Chris; Doelman, David; Doyon, René; Grandmont, Frédéric; Helmbrecht, Michael; Kenworthy, Matthew; Lafrenière, David; Marchis, Frank; Marois, Christian; Montminy, Steeve; Snik, Frans; Vasisht, Gautam; Véran, Jean-Pierre; Vincent, Philippe Bibcode: 2018SPIE10702E..48C Altcode: The HiCIBaS (High-Contrast Imaging Balloon System) project aims at launching a balloon borne telescope up to 36km to test high contrast imaging equipment and algorithms. The payload consists of a off the shelf 14-inch telescope with a custom-built Alt-Az mount. This telescope provides lights to two sensors, a pyramidal low order wave front sensor, and a coronagraphic wavefront sensor. Since the payload will reach its cruise altitude at about midnight mission, two target stars have been designated for observations, Capella as the night target, and Polaris as the early morning target. Data will be collected mainly on the magnitude of atmospheric and gondola's turbulences, the luminosity of the background. The whole system is already built and ready to ship to Timmins for the launch in mid-August 2018. 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: VST: The First Large Survey Telescope for Optical Polarimetry Authors: Smette, Alain; Bagnulo, Stefano; Snik, Frans; Cox, Nick; Hainaut, Olivier; Hutsemekers, Damien; Magalhaes, Antonio Mario Bibcode: 2018vels.confE..44S Altcode: Replacing the unused ADC by a polarizing filter would transform the VST into the first large polarimetric optical survey telescope, without affecting the current capabilities of the VST+OmegaCAM system.

Scientific cases include: mapping the Milky-Way and Magellanic clouds magnetic fields, surveys of quasar polarization, identification and variability of polarized brown dwarfs, polarimetric characterization and mapping of solar-system objects (incl. the moon, comets, and asteroids), detection of polarized signal around the L4 and L5 Lagrange points, a census of magnetic white dwarfs and Herbig Ae/Be stars, etc.

The design of the VST limits the technical implementation for the polarimetric unit to a single-beam system: therefore, we will develop data-driven calibration methods to achieve a sub-percent polarimetric accuracy.

The proposed implementation is of low cost and requires no major development, as polarizing filters of the needed size are readily available. In particular, it does not require major modification neither of VST, nor of OmegaCAM. In fact, the currently offered capabilities of the VST+OmegaCAM are preserved as the polarizing filter can be removed from the optical beam. Would this project be accepted, efforts will be needed on the design of the polarimeter unit to optimize accuracy and execution times, on the design of the calibration plan, and on the extension of the data reduction pipeline to extract the polarization signals. 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: Spectropolarimetry for earth observations: a novel method for characterization of aerosols and clouds Authors: Van der Togt, Oana; Verlaan, Ad; Moddemeijer, Kees; Smit, Martijn; Rietjens, Jeroen; Hasekamp, Otto P.; Stam, Daphne; Snik, Frans; van Harten, Gerard Bibcode: 2017SPIE10564E..1SV Altcode: Aerosols affect Earth's energy level by scattering and absorbing radiation and by changing the properties of clouds. Such effects influence the precipitation patterns and lead to modifications of the global circulation systems that constitute Earth's climate. The aerosol effects on our climate cannot be at full scale estimated due to the insufficient knowledge of their properties at a global scale. Achieving global measurement coverage requires an instrument with a large instantaneous field of view that can perform polarization measurements with high accuracy, typically better than 0.1%. Developing such an instrument can be considered as the most important challenge in polarimetric aerosol remote sensing. Using a novel technique to measure polarization, we have designed an instrument for a low-Earth orbit, e.g. International Space Station, that can simultaneously characterize 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° viewing angle. In this article we present the instrument's optical design concept. 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: Polarimetric Imaging Of Protoplanetary Disks From The Optical To Sub-Mm Authors: De Boer, Jos; Ménard, F.; Pinte, C.; van der Plas, G.; Snik, F. Bibcode: 2017ques.workE...5D Altcode: To learn how planets form from the smallest building blocks within protoplanetary disks, we first need to know how dust grains grow from micron to mm sizes. Polarimetry across the spectrum has proven to be sensitive to grain properties like dust size distribution and composition and thus can be used to characterize the scattering grains. However, polarization measured with radio interferometric arrays is rarely studied in concert with optical polarimetry. Our team has successfully calibrated the NIR polarimetric imaging mode of VLT/SPHERE, hence upgrading the instrument from a high-contrast imager to a robust tool for quantitative characterization. In this presentation, we will discuss which lessons can be learned by comparing polarimetry in the optical and sub-mm and explore for which science cases both techniques can complement each other. When we combine the polarimetric capabilities of the most advanced optical high-contrast imagers (e.g., Gemini GPI or VLT SPHERE) with that of ALMA we will be able to study the spatial distribution of an extensive range of different grains, which allows us to take an essential step towards a deeper understanding of planet formation. 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: Patterned liquid-crystal optics for broadband coronagraphy and wavefront sensing Authors: Doelman, David S.; Snik, Frans; Warriner, Nathaniel Z.; Escuti, Michael J. Bibcode: 2017SPIE10400E..0UD Altcode: 2017arXiv170909897D The direct-write technology for liquid-crystal patterns allows for manufacturing of extreme geometric phase patterned coronagraphs that are inherently broadband, e.g. the vector Apodizing Phase Plate (vAPP). We present on-sky data of a double-grating vAPP operating from 2-5 μm with a 360-degree dark hole and a decreased leakage term of ∼ 10-4 . We report a new liquid-crystal design used in a grating-vAPP for SCExAO that operates from 1-2.5μm. Furthermore, we present wavelength-selective vAPPs that work at specific wavelength ranges and transmit light unapodized at other wavelengths. Lastly, we present geometric phase patterns for advanced implementations of WFS (e.g. Zernike-type) that are enabled only by this liquid-crystal technology. 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: Investigating circular patterns in linear polarization observations of Venus Authors: Mahapatra, Gourav; Stam, Daphne; Rossi, Loic; Rodenhuis, Michiel; Snik, Frans Bibcode: 2017EGUGA..1915926M Altcode: ESA's Venus Express mission has revealed our neighbouring planet to be a highly dynamic world, with ever-changing cloud properties and structures, wind speeds that increase in time, and variable concentrations of atmospheric trace gases such as SO2. The SPICAV-IR instrument on Venus Express has provided us with close-up linear polarization data of sunlight reflected by Venus's clouds and hazes, that allows a characterisation of their composition and particle sizes. Here, we analyse linear polarization data of the planet at a distance, obtained with the Extreme Polarimeter (ExPo) on the William Herschel Telescope on La Palma. These spatially resolved, high-accuracy polarization observations of Venus show faint circular patterns centered on the sub-solar point that are absent in the flux observations. So far, careful analyses have ruled out instrumental effects which leaves us to wonder about atmospheric properties as the cause of the circular patterns. Using numerical simulations of the flux and polarization of sunlight that is reflected by Venus, we have investigated the relation between the observed patterns and several atmospheric properties, such as variations in particle sizes, composition, density and altitude. We discuss the plausibility of the possible causes in the view of the current knowledge of the composition and dynamical processes in Venus's atmosphere. Title: Circular spectropolarimetric sensing of chiral photosystems in decaying leaves Authors: Patty, C. H. Lucas; Visser, Luuk J. J.; Ariese, Freek; Buma, Wybren Jan; Sparks, William B.; van Spanning, Rob J. M.; Röling, Wilfred F. M.; Snik, Frans Bibcode: 2017JQSRT.189..303P Altcode: 2017arXiv170101297P Circular polarization spectroscopy has proven to be an indispensable tool in photosynthesis research and (bio)molecular research in general. Oxygenic photosystems typically display an asymmetric Cotton effect around the chlorophyll absorbance maximum with a signal ≤ 1 % . In vegetation, these signals are the direct result of the chirality of the supramolecular aggregates. The circular polarization is thus directly influenced by the composition and architecture of the photosynthetic macrodomains, and is thereby linked to photosynthetic functioning. Although ordinarily measured only on a molecular level, we have developed a new spectropolarimetric instrument, TreePol, that allows for both laboratory and in-the-field measurements. Through spectral multiplexing, TreePol is capable of fast measurements with a sensitivity of ∼ 1 *10-4 and is therefore suitable of non-destructively probing the molecular architecture of whole plant leaves. We have measured the chiroptical evolution of Hedera helix leaves for a period of 22 days. Spectrally resolved circular polarization measurements (450-900 nm) on whole leaves in transmission exhibit a strong decrease in the polarization signal over time after plucking, which we accredit to the deterioration of chiral macro-aggregates. Chlorophyll a levels measured over the same period by means of UV-vis absorption and fluorescence spectroscopy showed a much smaller decrease. With these results we are able to distinguish healthy from deteriorating leaves. Hereby we indicate the potency of circular polarization spectroscopy on whole and intact leaves as a nondestructive tool for structural and plant stress assessment. Additionally, we underline the establishment of circular polarization signals as remotely accessible means of detecting the presence of extraterrestrial life. 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: 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: 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: HARPS3 for a roboticized Isaac Newton Telescope Authors: Thompson, Samantha J.; Queloz, Didier; Baraffe, Isabelle; Brake, Martyn; Dolgopolov, Andrey; Fisher, Martin; Fleury, Michel; Geelhoed, Joost; Hall, Richard; González Hernández, Jonay I.; ter Horst, Rik; Kragt, Jan; Navarro, Ramón; Naylor, Tim; Pepe, Francesco; Piskunov, Nikolai; Rebolo, Rafael; Sander, Louis; Ségransan, Damien; Seneta, Eugene; Sing, David; Snellen, Ignas; Snik, Frans; Spronck, Julien; Stempels, Eric; Sun, Xiaowei; Santana Tschudi, Samuel; Young, John Bibcode: 2016SPIE.9908E..6FT Altcode: 2016arXiv160804611T We present a description of a new instrument development, HARPS3, planned to be installed on an upgraded and roboticized Isaac Newton Telescope by end-2018. HARPS3 will be a high resolution (R≃115,000) echelle spectrograph with a wavelength range from 380-690 nm. It is being built as part of the Terra Hunting Experiment - a future 10- year radial velocity measurement programme to discover Earth-like exoplanets. The instrument design is based on the successful HARPS spectrograph on the 3.6m ESO telescope and HARPS-N on the TNG telescope. The main changes to the design in HARPS3 will be: a customised fibre adapter at the Cassegrain focus providing a stabilised beam feed and on-sky fibre diameter ≍1:4 arcsec, the implementation of a new continuous ow cryostat to keep the CCD temperature very stable, detailed characterisation of the HARPS3 CCD to map the effective pixel positions and thus provide an improved accuracy wavelength solution, an optimised integrated polarimeter and the instrument integrated into a robotic operation. The robotic operation will optimise our programme which requires our target stars to be measured on a nightly basis. We present an overview of the entire project, including a description of our anticipated robotic operation. Title: OCTOCAM: a fast multi-channel imager and spectrograph proposed for the Gemini Observatory Authors: de Ugarte Postigo, A.; Roming, P.; Thöne, C. C.; van der Horst, A. J.; Pope, S.; García Vargas, M. L.; Sánchez-Blanco, E.; Maldonado Medina, M.; Content, R.; Snik, F.; Killough, R.; Winters, G.; Persson, K.; Jeffers, S.; Riva, A.; Bianco, A.; Zanutta, A. Bibcode: 2016SPIE.9908E..40D Altcode: OCTOCAM has been proposed to the Gemini Observatory as a workhorse imager and spectrograph that will fulfill the needs of a large number of research areas in the 2020s. It is based on the use of high-efficiency dichroics to divide the incoming light in eight different channels, four optical and four infrared, each optimized for its wavelength range. In its imaging mode, it will observe a field of 3'x3' simultaneously in g, r, i, z, Y, J, H, and KS bands. It will obtain long-slit spectroscopy covering the range from 3700 to 23500 Å with a resolution of 4000 and a slit length of 3 arcminutes. To avoid slit losses, the instrument it will be equipped with an atmospheric dispersion corrector for the complete spectral range. Thanks to the use of state of the art detectors, OCTOCAM will allow high time-resolution observations and will have negligible overheads in classical observing modes. It will be equipped with a unique integral field unit that will observe in the complete spectral range with an on-sky coverage of 9.7"x6.8", composed of 17 slitlets, 0.4" wide each. Finally, a state-of-the-art polarimetric unit will allow us to obtain simultaneous full Stokes spectropolarimetry of the range between 3700 and 22000 Å. Title: High-contrast imaging with METIS Authors: Kenworthy, Matthew A.; Absil, Olivier; Agócs, Tibor; Pantin, Eric; Quanz, Sascha; Stuik, Remko; Snik, Frans; Brandl, Bernhard Bibcode: 2016SPIE.9908E..A6K Altcode: The Mid-infrared E-ELT Imager and Spectrograph (METIS) for the European Extremely Large Telescope (E-ELT) consists of diffraction-limited imagers that cover 3 to 14 microns with medium resolution (R 5000) long slit spectroscopy, and an integral field spectrograph for high spectral resolution spectroscopy (R 100,000) over the L and M bands. One of the science cases that METIS addresses is the characterization of faint circumstellar material and exoplanet companions through imaging and spectroscopy. We present our approach for high contrast imaging with METIS, covering diffraction suppression with coronagraphs, the removal of slowly changing optical aberrations with focal plane wavefront sensing, interferometric imaging with sparse aperture masks, and observing strategies for both the imagers and IFU image slicers. Title: The path to visible extreme adaptive optics with MagAO-2K and MagAO-X Authors: Males, Jared R.; Close, Laird M.; Guyon, Olivier; Morzinski, Katie M.; Hinz, Philip; Esposito, Simone; Pinna, Enrico; Xompero, Marco; Briguglio, Runa; Riccardi, Armando; Puglisi, Alfio; Mazin, Ben; Ireland, Michael J.; Weinberger, Alycia; Conrad, Al; Kenworthy, Matthew; Snik, Frans; Otten, Gilles; Jovanovic, Nemanja; Lozi, Julien Bibcode: 2016SPIE.9909E..52M Altcode: The next generation of extremely large telescopes (ELTs) have the potential to image habitable rocky planets, if suitably optimized. This will require the development of fast high order "extreme" adaptive optics systems for the ELTs. Located near the excellent site of the future GMT, the Magellan AO system (MagAO) is an ideal on-sky testbed for high contrast imaging development. Here we discuss planned upgrades to MagAO. These include improvements in WFS sampling (enabling correction of more modes) and an increase in speed to 2000 Hz, as well as an H2RG detector upgrade for the Clio infrared camera. This NSF funded project, MagAO-2K, is planned to be on-sky in November 2016 and will significantly improve the performance of MagAO at short wavelengths. Finally, we describe MagAO-X, a visible-wavelength extreme-AO "afterburner" system under development. MagAO-X will deliver Strehl ratios of over 80% in the optical and is optimized for visible light coronagraphy. 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: Modelling the circular polarisation of Earth-like exoplanets: constraints on detecting homochirality Authors: Hogenboom, Michael; Stam, Daphne; Rossi, Loic; Snik, Frans Bibcode: 2016EGUGA..18.4721H Altcode: The circular polarisation of light is a property of electromagnetic radiation from which extensive information can be extracted. It is oft-neglected due to its small signal relative to linear polarisation and the need for advanced instrumentation in measuring it. Additionally, numerical modelling is complex as the full Stokes vector must always be computed. Circular polarisation is commonly induced through the multiple scattering of light by aerosols te{hansen} and multiple reflections of light by rough surfaces te{circplanets}. Most interestingly, distinctive spectral circular polarimetric behaviour is exhibited by light reflected by organisms due to the homochiral molecular structure of all known organisms te{chiralbailey}. Especially fascinating is the unique circular polarimetric behaviour of light reflected by photosynthesising organisms at the absorption wavelength of the chlorophyll pigment te{circpolchar}. This presents the previously unexplored possibility of circular polarimetry as a method for identifying and characterising the presence of organisms, a method which could be applied in the hunt for extraterrestrial life. To date, few telescopes exist that measure circular polarisation and none that have been deployed in space. Observations of the circular polarisation reflected by other planets in the solar system have been made with ground-based telescopes, with significant results te{circplanets}. However, none of these observations have been made at the phase angles at which exoplanets will be observed. Also, none have been made of the Earth, which is the logical starting point for the study of biologically induced circular polarisation signals. This introduces the need for numerical modelling to determine the extent to which circular polarisation is present in light reflected by exoplanets or the Earth. In this study, we model the multiple scattering and reflection of light using the doubling-adding method te{dehaan}. We will present circular polarisation signals for both spatially resolved and spatially unresolved planets, using various atmospheric and surface properties and across a range of phase angles. As a test, the calculated degree of circular polarisation resulting from the multiple scattering of light in an atmosphere with varying properties was compared with results presented by Kawata te{circatmos} and was found to be in agreement. Initial modelling of the atmospheric scattering of light by a planetary disk has shown a presence of degree of circular polarisation in the order of 10-4. This represents a static case with one cloudy hemisphere, one cloudless hemisphere and a Lambertian surface. Results containing varied patchy cloud patterns shall also be presented in a bid to reflect the random nature of planetary cloud cover. We will also present the calculated degree of circular polarisation of planets with various cloud coverage and a circularly polarising surface in order to discover the influence of organisms on the numerical results. {1} {hansen} J. E. {Hansen} and L. D. {Travis}. {Light scattering in planetary atmospheres}. {Space Science Reviews}, 16:527-610, October 1974. {circplanets} J. C. {Kemp} and R. D. {Wolstencroft}. {Circular Polarization: Jupiter and Other Planets}. {Nature}, 232:165-168, July 1971. {chiralbailey} J. {Bailey}. {Circular Polarization and the Origin of Biomolecular Homochirality}. In G. {Lemarchand} and K. {Meech}, editors, {Bioastronomy 99}, volume 213 of {Astronomical Society of the Pacific Conference Series}, 2000. {circpolchar} L. {Nagdimunov}, L. {Kolokolova}, and D. {Mackowski}. {Characterization and remote sensing of biological particles using circular polarization}. {Journal of Quantitative Spectroscopy and Radiative Transfer}, 131:59-65, December 2013. dehaan} J. F. {de Haan}, P. B. {Bosma}, and J. W. {Hovenier}. {The adding method for multiple scattering calculations of polarized light}. {Astronomy and Astrophysics}, 183:371-391, September 1987. {circatmos} Y. {Kawata}. {Circular polarization of sunlight reflected by planetary atmospheres}. {Icarus}, 33:217-232, January 1978. Title: Preliminary design of the full-Stokes UV and visible spectropolarimeter for UVMag/Arago Authors: Pertenais, Martin; Neiner, Coralie; Parès, Laurent; Petit, Pascal; Snik, Frans; van Harten, Gerard Bibcode: 2015IAUS..305..168P Altcode: 2015arXiv150200856P The UVMag consortium proposed the space mission project Arago to ESA at its M4 call. Arago is dedicated to the study of the dynamic 3D environment of stars and planets. This space mission will be equipped with a high-resolution spectropolarimeter working from 119 to 888 nm. A preliminary optical design of the whole instrument has been prepared and is presented here. The design consists of the telescope, the instrument itself, and the focusing optics. Considering not only the scientific requirements, but also the cost and size constraints to fit an M-size mission, the telescope has a 1.3 m diameter primary mirror and is a classical Cassegrain-type telescope that allows a polarization-free focus. The polarimeter is placed at this Cassegrain focus. This is the key element of the mission and the most challenging one to be designed. The main challenge lies in the huge spectral range offered by the instrument; the polarimeter has to deliver the full Stokes vector with a high precision from the FUV (119 nm) to the NIR (888 nm). The polarimeter module is then followed by a high-resolution echelle-spectrometer achieving a resolution of 35000 in the visible range and 25000 in the UV. The two channels are separated after the echelle grating, allowing specific cross-dispersion and focusing optics for the UV and the visible ranges. Considering the large field of view and the high numerical aperture, the focusing optics for both the UV and the visible channels is a Three-Mirror-Anastigmatic (TMA) telescope, needed to focus the various wavelengths and many orders onto the detectors. Title: Instrumemtation Authors: Keller, Christoph U.; Snik, Frans; Harrington, David M.; Packham, Chris Bibcode: 2015psps.book...35K Altcode: No abstract at ADS 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: Performance characterization of a broadband vector Apodizing Phase Plate coronagraph Authors: Otten, Gilles P. P. L.; Snik, Frans; Kenworthy, Matthew A.; Miskiewicz, Matthew N.; Escuti, Michael J. Bibcode: 2014OExpr..2230287O Altcode: 2014arXiv1412.0660O One of the main challenges for the direct imaging of planets around nearby stars is the suppression of the diffracted halo from the primary star. Coronagraphs are angular filters that suppress this diffracted halo. The Apodizing Phase Plate coronagraph modifies the pupil-plane phase with an anti-symmetric pattern to suppress diffraction over a 180 degree region from 2 to 7 {\lambda}/D and achieves a mean raw contrast of 10^-4 in this area, independent of the tip-tilt stability of the system. Current APP coronagraphs implemented using classical phase techniques are limited in bandwidth and suppression region geometry (i.e. only on 1 side of the star). In this paper, we show the vector-APP (vAPP) whose phase pattern is implemented by the orientation of patterned liquid crystals. Beam-splitting according to circular polarization states produces two, complementary PSFs with dark holes on either side. We have developed a prototype vAPP that consists of a stack of 3 twisting liquid crystal layers with a bandwidth of 500-900 nm. We characterize the properties of this device using reconstructions of the pupil-plane pattern, and of the ensuing PSF structures. By imaging the pupil between crossed and parallel polarizers we reconstruct the fast axis pattern, transmission, and retardance of the vAPP, and use this as input for a PSF model. This model includes aberrations of the laboratory set-up, and matches the measured PSF, which shows a raw contrast of 10^-3.8 between 2 and 7 {\lambda}/D in a 135 degree wedge. The vAPP coronagraph is relatively easy to manufacture and can be implemented together with a broadband quarter-wave plate and Wollaston prism in a pupil wheel in high-contrast imaging instruments. The manufacturing techniques permit the application of phase patterns with deeper contrasts inside the dark holes and enables unprecedented spectral bandwidths for phase-manipulation coronagraphy. 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: Combining vector-phase coronagraphy with dual-beam polarimetry Authors: Snik, Frans; Otten, Gilles; Kenworthy, Matthew; Mawet, Dimitri; Escuti, Michael Bibcode: 2014SPIE.9147E..7US Altcode: Utilizing the so-called vector phase of polarized light, both focal-plane coronagraphs (e.g. the Vector Vortex Coronagraph) and pupil-plane coronagraphs (e.g. the vector Apodizing Phase Plate) are powerful components for high-contrast imaging. These coronagraphs can be built and optimized with polarization techniques and liquid crystal technology, that enable patterning at the micron level and furnish broad-band performance. The contrast between the residual starlight and the (polarized) reflected light off exoplanets can be further bridged by incorporating sensitive, dual-beam imaging polarimetry. As vector-phase coronagraphs use polarizers to enhance their performance, we introduce optimally integrated solutions that combine advanced coronagraphy and polarimetry. For both the VVC and the vAPP we present polarization beam-splitting concepts, with polarization analyzers either behind or in front of the coronagraphic optics. We discuss design solutions for the implementation of polarization optics, and set the stage for a trade-off between the improvement of coronagraphic and polarimetric performance and the ensuing degradation on the high-contrast imaging performance due to wavefront errors. Title: UVMag: Space UV and visible spectropolarimetry Authors: Pertenais, Martin; Neiner, Coralie; Parès, Laurent P.; Petit, Pascal; Snik, Frans; van Harten, Gerard Bibcode: 2014SPIE.9144E..3BP Altcode: 2014arXiv1406.4378P UVMag is a project of a space mission equipped with a high-resolution spectropolarimeter working in the UV and visible range. This M-size mission will be proposed to ESA at its M4 call. The main goal of UVMag is to measure the magnetic fields, winds and environment of all types of stars to reach a better understanding of stellar formation and evolution and of the impact of stellar environment on the surrounding planets. The groundbreaking combination of UV and visible spectropolarimetric observations will allow the scientists to study the stellar surface and its environment simultaneously. The instrumental challenge for this mission is to design a high-resolution space spectropolarimeter measuring the full- Stokes vector of the observed star in a huge spectral domain from 117 nm to 870 nm. This spectral range is the main difficulty because of the dispersion of the optical elements and of birefringence issues in the FUV. As the instrument will be launched into space, the polarimetric module has to be robust and therefore use if possible only static elements. This article presents the different design possibilities for the polarimeter at this point of the project. 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: Successes and challenges of the APP Coronagraph Authors: Kenworthy, Matthew A.; Quanz, Sascha; Otten, Gilles; Meshkat, Tiffany; Codona, Johanan; Snik, Frans; Meyer, Michael E.; Kasper, Markus; Girard, Julien Bibcode: 2014IAUS..299...40K Altcode: The Apodizing Phase Plate (APP) coronagraph has been used to image the exoplanet β Pictoris b and the protoplanet candidate around HD 100546, and is currently in use in surveys with NaCo at the VLT. Its success is due to its tolerance to tip-tilt pointing errors in current AO systems, which degrade the performance of nearly all other coronagraphs. Currently the sensitivity of the APP is limited by non-common path errors in the science camera systems and by its chromatic behaviour. We present the achromatized Vector APP coronagraph and address how we will measure and minimise non-common path errors with Focal Plane Wavefront Sensing algorithms. 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: 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: 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: The polarimeters for HARPS and X-shooter Authors: Snik, F.; Harpspol Team; X-Shooter-Pol Team Bibcode: 2013ASPC..470..401S Altcode: Spectropolarimetry enables observations of stellar magnetic fields and circumstellar asymmetries, e.g. in disks and supernova explosions. To furnish better diagnostics of such stellar physics, we designed and commissioned a polarimetric unit at the successful HARPS spectrograph at ESO's 3.6-m telescope at La Silla. We present the design and performance of HARPSpol, and show some first science results. The most striking achievement of HARPSpol is its capability to measure stellar magnetic fields as small as 0.1 G. Finally, we give a sneak preview of the polarimeter we are currently designing for X-shooter at the VLT. It contains a novel type of polarimetric modulator that is able to efficiently measure all the Stokes parameters over the huge wavelength range of 300-2500 nm. 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: 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: Innovative technology for optical and infrared astronomy Authors: Cunningham, Colin R.; Evans, Christopher J.; Molster, Frank; Kendrew, Sarah; Kenworthy, Matthew A.; Snik, Frans Bibcode: 2012SPIE.8450E..31C Altcode: 2012arXiv1207.4892C Advances in astronomy are often enabled by adoption of new technology. In some instances this is where the technology has been invented specifically for astronomy, but more usually it is adopted from another scientific or industrial area of application. The adoption of new technology typically occurs via one of two processes. The more usual is incremental progress by a series of small improvements, but occasionally this process is disruptive, where a new technology completely replaces an older one. One of the activities of the OPTICON Key Technology Network over the past few years has been a technology forecasting exercise. Here we report on a recent event which focused on the more radical, potentially disruptive technologies for ground-based, optical and infrared astronomy. Title: Design of a full-Stokes polarimeter for VLT/X-shooter Authors: Snik, Frans; van Harten, Gerard; Navarro, Ramon; Groot, Paul; Kaper, Lex; de Wijn, Alfred Bibcode: 2012SPIE.8446E..25S Altcode: 2012arXiv1207.2965S X-shooter is one of the most popular instruments at the VLT, offering instantaneous spectroscopy from 300 to 2500 nm. We present the design of a single polarimetric unit at the polarization-free Cassegrain focus that serves all three spectrograph arms of X-shooter. It consists of a calcite Savart plate as a polarizing beam-splitter and a rotatable crystal retarder stack as a "polychromatic modulator". Since even "superachromatic" wave plates have a wavelength range that is too limited for X-shooter, this novel modulator is designed to offer close-to-optimal polarimetric efficiencies for all Stokes parameters at all wavelengths. We analyze the modulator design in terms of its polarimetric performance, its temperature sensitivity, and its polarized fringes. Furthermore, we present the optical design of the polarimetric unit. The X-shooter polarimeter will furnish a myriad of science cases: from measuring stellar magnetic fields (e.g., Ap stars, white dwarfs, massive stars) to determining asymmetric structures around young stars and in supernova explosions. 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: 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: The vector-APP: a broadband apodizing phase plate that yields complementary PSFs Authors: Snik, Frans; Otten, Gilles; Kenworthy, Matthew; Miskiewicz, Matthew; Escuti, Michael; Packham, Christopher; Codona, Johanan Bibcode: 2012SPIE.8450E..0MS Altcode: 2012arXiv1207.2970S The apodizing phase plate (APP) is a solid-state pupil optic that clears out a D-shaped area next to the core of the ensuing PSF. To make the APP more efficient for high-contrast imaging, its bandwidth should be as large as possible, and the location of the D-shaped area should be easily swapped to the other side of the PSF. We present the design of a broadband APP that yields two PSFs that have the opposite sides cleared out. Both properties are enabled by a half-wave liquid crystal layer, for which the local fast axis orientation over the pupil is forced to follow the required phase structure. For each of the two circular polarization states, the required phase apodization is thus obtained, and, moreover, the PSFs after a quarter-wave plate and a polarizing beam-splitter are complementary due to the antisymmetric nature of the phase apodization. The device can be achromatized in the same way as half-wave plates of the Pancharatnam type or by layering self-aligning twisted liquid crystals to form a monolithic film called a multi-twist retarder. As the VAPP introduces a known phase diversity between the two PSFs, they may be used directly for wavefront sensing. By applying an additional quarter-wave plate in front, the device also acts as a regular polarizing beam-splitter, which therefore furnishes high-contrast polarimetric imaging. If the PSF core is not saturated, the polarimetric dual-beam correction can also be applied to polarized circumstellar structure. The prototype results show the viability of the vector-APP concept. Title: Compact and robust method for full Stokes spectropolarimetry Authors: Sparks, William; Germer, Thomas A.; MacKenty, John W.; Snik, Frans Bibcode: 2012ApOpt..51.5495S Altcode: 2012arXiv1206.7106S We present an approach to spectropolarimetry which requires neither moving parts nor time dependent modulation, and which offers the prospect of achieving high sensitivity. The technique applies equally well, in principle, in the optical, UV or IR. The concept, which is one of those generically known as channeled polarimetry, is to encode the polarization information at each wavelength along the spatial dimension of a 2D data array using static, robust optical components. A single two-dimensional data frame contains the full polarization information and can be configured to measure either two or all of the Stokes polarization parameters. By acquiring full polarimetric information in a single observation, we simplify polarimetry of transient sources and in situations where the instrument and target are in relative motion. The robustness and simplicity of the approach, coupled to its potential for high sensitivity, and applicability over a wide wavelength range, is likely to prove useful for applications in challenging environments such as space. 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: 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: 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: Polarimetry at Current and Future Ground-based Telescopes for Detection and Characterization of Exoplanets Authors: Snik, F. Bibcode: 2011AGUFM.P14C..07S Altcode: Reflected light off exoplanets is generally linearly polarized. This offers a unique possibility to distinguish this light from the halo of unpolarized starlight, which is many orders of magnitude brighter. Polarimetry is therefore a powerful method that enables direct imaging of exoplanets. Moreover, the spectral dependence of the degree of polarization of the reflected starlight contains unambiguous information on the exoplanetary atmosphere and surface. Polarimetry is therefore also a very promising future technique for exoplanet characterization. This contribution reviews polarimetric instrumentation at current 4-10 m ground-based telescopes that is particularly suited for exoplanet research. Furthermore, it provides an outlook for exoplanet polarimetry with 30-40 m extremely large telescopes. Title: SPEX: a multi-angle Spectropolarimeter for Planetary EXploration Authors: Smit, J. M.; Hasekamp, O. P.; Rietjens, J.; Stam, D.; Snik, F.; Van Harten, G.; Verlaan, A.; Voors, R.; Moon, S.; Wielinga, K. Bibcode: 2011AGUFM.P11F1635S Altcode: We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass multi-viewing angle spectropolarimeter designed to operate from an orbiting satellite platform. Its purpose is to simultaneously measure, with high accuracy, the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere or reflected by a planetary surface. 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 results in a modulation of the radiance spectrum in both amplitude and phase by the degree and angle of the linear polarization spectrum, respectively. The polarization optics consists of an achromatic quarter-wave retarder, an a-thermal multiple-order retarder, and a polarizing beam splitter. Such a configuration is implemented for a range of viewin directions, which allows sampling the full scattering phase function of each ground pixel under investigation, while orbiting the planetary body. The present design of SPEX is tuned to a Mars mission, as a payload on a satellite in a low orbit. However, the concept is perfectly applicable for Earth remote sensing from an orbiting platform like ISS or a dedicated mission, for which we are developing a breadboard. A similar concepts is under study for a mission to the Jovian system including the Galilean Moons. We will show first test results obtained with recently developed prototype of the SPEX instrument, demonstrating excellent performance and overall behavior as compared with design parameters and SPEX instrument simulator. In addition, we present results of multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with one of AERONET's sun photometers. Title: Astronomical Applications for ``Radial Polarimetry'' Authors: Snik, F. Bibcode: 2011ASPC..449...21S Altcode: Many objects on the sky exhibit a centrosymmetric polarization pattern, particularly in cases involving single scattering around a central source. Utilizing a novel liquid crystal device (the “theta cell'') that transforms the coordinate system of linear polarization in an image plane from Cartesian to polar, the observation of centrosymmetric polarization patterns can be improved: instead of measuring Stokes Q and U on the sky, one only needs to measure Stokes Q' in the new instrument coordinate system. This reduces the effective exposure time by a factor of two and simplifies the polarization modulator design. According to the manufacturer's specifications and to measurements in the lab, the liquid crystal device can be applied in the visible and NIR wavelength range. Astronomical science cases for a“radial polarimeter'' include exoplanet detection, imaging of circumstellar disks, reflection nebulae and light echos, characterization of planetary atmospheres and diagnostics of the solar K-corona. The first astronomical instrument that utilizes a theta cell for radial polarimetry is the S5T (Small Synoptic Second Solar Spectrum Telescope), which accurately measures scattering polarization signals near the limb of the sun. These observations are crucial for understanding the nature and origin of weak, turbulent magnetic fields in the solar photosphere and elsewhere in the universe. A “radial polarimeter'' observing a slightly defocused point source performs one-shot full linear polarimetry. With a theta cell in a pupil plane, a beam's linear polarization properties (e.g. for calibration purposes) can be fully controlled through pupil masking. 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: 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: 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: 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: 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: 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: 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 Irkutsk Barium filter for narrow-band wide-field high-resolution solar images at the Dutch Open Telescope Authors: Hammerschlag, Robert H.; Skomorovsky, Valery I.; Bettonvil, Felix C. M.; Kushtal, Galina I.; Olshevsky, Vyacheslav L.; Rutten, Robert J.; Jägers, Aswin P. L.; Sliepen, Guus; Snik, Frans Bibcode: 2010SPIE.7735E..85H Altcode: 2010SPIE.7735E.265H A wide-field birefringent filter for the barium II line at 455.4nm is developed in Irkutsk. The Barium line is excellent for Doppler-shift measurements because of low thermal line-broadening and steep flanks of the line profile. The filter width is 0.008nm and the filter is tunable over 0.4nm through the whole line and far enough in the neighboring regions. A fast tuning system with servomotor is developed at the Dutch Open Telescope (DOT). Observations are done in speckle mode with 10 images per second and Keller-VonDerLühe reconstruction using synchronous images of a nearby bluecontinuum channel at 450.5nm. Simultaneous observation of several line positions, typically 3 or 5, are made with this combination of fast tuning and speckle. All polarizers are birefringent prisms which largely reduced the light loss compared to polarizing sheets. The advantage of this filter over Fabry-Perot filters is its wide field due to a large permitted entrance angle and no need of polishing extremely precise surfaces. The BaII observations at the DOT occur simultaneously with those of a fast-tunable birefringent H-alpha filter. This gives the unique possibility of simultaneous speckle-reconstructed observations of velocities in photosphere (BaII) and chromosphere (H-alpha). 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: 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: 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: Astronomical Polarimetry : new concepts, new instruments, new measurements & observations Authors: Snik, F. Bibcode: 2009PhDT.......489S Altcode: All astronomical sources are polarized to some degree. Polarimetry is therefore a powerful astronomical technique. It furnishes unique diagnostics of e.g. magnetic fields and scattering media. This thesis presents new polarimetric concepts, instruments, and measurements targeting astronomical science questions. The first part of the thesis describes three novel polarimetric concepts. -A dedicated passive liquid crystal device known as a theta cell is introduced to enable one-shot observations of astronomical targets exhibiting a centrosymmetric polarization pattern. -A new passive measurement concept for broad-band linear polarization is introduced. It is based on a sinusoidal modulation of the spectrum, and is particularly suitable for instruments for which classical spatial and/or temporal polarization modulation is unfavorable. -Calibration of polarimetric instruments is usually limited by non-ideal effects of the calibration optics themselves. A mathematical frame-work based on Fourier analysis is introduced to tackle various non-ideal effects in polarimetric calibration. The second part of the thesis presents the designs and first results of three very different astronomical polarimeters. -The ultra-stable high-resolution HARPS spectrograph is successfully upgraded with a dual-beam polarimetric module. It furnishes direct observations of magnetic fields on stars. -The Small Synoptic Second Solar Spectrum Telescope (S5T) is designed to accurately monitor the variation of weak, turbulent magnetic fields on the Sun during a solar cycle. Such measurements are crucial for the understanding of local dynamo action in the solar photosphere. The prototype shows the feasibility of the instrument concept. -The Spectropolarimeter for Planetary EXpolaration (SPEX) is designed to study a planet's or moon's atmosphere from orbit. The additional information from the polarization measurement of scattered sunlight allows for determination of microphysical properties of atmospheric aerosols. The third part of the thesis presents new polarimetric measurements and observations. -A precise ellipsometer was constructed to measure the polarization properties of various optical components. The instrument was employed to measure the complete Mueller matrix of an aluminum mirror at various durations after evaporation. It is concluded that the aluminum oxide layer has a significant effect on the polarization properties, which is constant in time after a day from evaporation. -Next, an aluminized mirror was contaminated with dust. This also has a significant effect on the measured Mueller matrix, for which a linear model is presented as a function of dust absorption. -Observations of solar scattering polarization at high spatial resolution were obtained with the Hinode space telescope. A significant difference of ~10% in scattering polarization was detected between granules and intergranules, which constrains models of quiet Sun magnetoconvection. 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: 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: 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: Astronomical Applications for "Radial Polarimetry" Authors: Snik, Frans Bibcode: 2009arXiv0903.2734S Altcode: Many objects on the sky exhibit a centrosymmetric polarization pattern, particularly in cases involving single scattering around a central source. Utilizing a novel liquid crystal device (the ``theta cell'') that transforms the coordinate system of linear polarization in an image plane from Cartesian to polar, the observation of centrosymmetric polarization patterns can be improved: instead of measuring Stokes Q and U on the sky, one only needs to measure Stokes Q' in the new instrument coordinate system. This reduces the effective exposure time by a factor of two and simplifies the polarization modulator design. According to the manufacturer's specifications and to measurements in the lab, the liquid crystal device can be applied in the visible and NIR wavelength range. Astronomical science cases for a``radial polarimeter'' include exoplanet detection, imaging of circumstellar disks, reflection nebulae and light echos, characterization of planetary atmospheres and diagnostics of the solar K-corona. The first astronomical instrument that utilizes a theta cell for radial polarimetry is the S5T (Small Synoptic Second Solar Spectrum Telescope), which accurately measures scattering polarization signals near the limb of the sun. These observations are crucial for understanding the nature and origin of weak, turbulent magnetic fields in the solar photosphere and elsewhere in the universe. A ``radial polarimeter'' observing a slightly defocused point source performs one-shot full linear polarimetry. With a theta cell in a pupil plane, a beam's linear polarization properties (e.g. for calibration purposes) can be fully controlled through pupil masking. 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: Astronomical polarimetry: New concepts; new instruments; new measurements & observations Authors: Snik, Frans Bibcode: 2009PhDT.......584S 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: 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: 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: 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: Calibration strategies for instrumental polarization at the 10 -5 level Authors: Snik, Frans Bibcode: 2006SPIE.6269E..5PS Altcode: 2006SPIE.6269E.182S Many current instrumental developments for both solar and nighttime telescopes are directed at measuring the polarization state of the incoming light in addition to determining its spatial, temporal and/or spectral properties. Such polarimeters need to be sensitive down to a polarization degree of the order of 10 -5 e.g. to employ the full range of diagnostics to accurately measure solar magnetic fields or to enable direct imaging of extrasolar planetary systems. At the low polarization degree of these observations, it is crucial to accurately know the polarization properties of the instrument itself. Since instrumental polarization is inevitable for any telescopic configuration with oblique reflections or refractions, it is always necessary to cope with it by means of calibration in combination with (limited) forward modeling. I present general strategies based on discrete Fourier analysis for the calibration of instrumental polarization to enable astronomical (spectro-)polarimetry at the 10 -5 level. The technique only assumes the presence of a freely rotatable polarizer and (quarter) wave plate to create known input polarization states. The Fourier components of the observed output polarization contain information about the full instrumental polarization, as well as about non-ideal effects in the calibration elements, polarized input to the calibration unit and non-linear response of the detector. Title: GISOT: a giant solar telescope Authors: Hammerschlag, Robert H.; von der Lühe, Oskar F.; Bettonvil, Felix C.; Jägers, Aswin P.; Snik, Frans Bibcode: 2004SPIE.5489..491H Altcode: A concept is presented for an extremely large high-resolution solar telescope with an aperture of 11 m and diffraction limited for visual wavelengths. The structure of GISOT will be transparent to wind and placed on a transparent stiff tower. For efficient wind flushing, all optics, including the primary mirror, will be located above the elevation axis. The aperture will be of the order of 11 m, not rotatively symmetrical, but of an elongated shape with dimensions 11 x 4 m. It consists of a central on-axis 4 m mirror with on both sides 3 pieces of 2 m mirrors. The optical layout will be kept simple to guarantee quality and minimize stray light. A Coudé room for instruments is planned below the telescope. The telescope will not be housed in a dome-like construction, which interferes with the open principle. Instead the telescope will be protected by a foldable tent construction with a diameter of the order of 30 m, which doesn"t form any obstruction during observations, but can withstand the severe weather circumstances on mountain sites. Because of the nature of the solar scene, extremely high resolution in only one dimension is sufficient to solve many exciting problems in solar physics and in this respect the concept of GISOT is very promising. Title: DOT++: the Dutch Open Telescope with 1.4-m aperture Authors: Bettonvil, Felix C.; Hammerschlag, Robert H.; Sütterlin, Peter; Rutten, Robert J.; Jägers, Aswin P.; Snik, Frans Bibcode: 2004SPIE.5489..362B Altcode: The Dutch Open Telescope (DOT; http://dot.astro.uu.nl) on La Palma is a revolutionary open solar telescope, on an excellent site, on top of a transparent steel tower, and uses natural air flow to minimize local seeing. The aim is long-duration high-resolution imaging with a multi-wavelength camera system. In order to achieve this, the DOT is equipped with a diffraction limited imaging system and uses the speckle reconstruction technique for removing the remaining atmospheric turbulence. The DOT optical system is simple and consists currently of a 0.45m/F4.44 parabolic mirror and a 10x enlargement lens system. We present our plans to increase the aperture of the DOT from 0.45m to 1.4m. The mirror support and telescope top shall be redesigned, but telescope, tower, multi-wavelength camera system and speckle system remain intact. The new optical design permits user selectable choice between angular resolution and field size, as well as transversal pupil shift introducing the possibility to use obstruction free apertures up to 65cm. The design will include a low order AO system, which improves the speckle S/N substantially during moderate seeing conditions. Title: The Dutch Open Telescope on La Palma Authors: Rutten, R. J.; Bettonvil, F. C. M.; Hammerschlag, R. H.; Jägers, A. P. L.; Leenaarts, J.; Snik, F.; Sütterlin, P.; Tziotziou, K.; de Wijn, A. G. Bibcode: 2004IAUS..223..597R Altcode: 2005IAUS..223..597R The Dutch Open Telescope (DOT) on La Palma is an innovative solar telescope combining open telescope structure and an open support tower with a multi-wavelength imaging assembly and with synchronous speckle cameras to generate high-resolution movies which sample different layers of the solar atmosphere simultaneously and co-spatially at high resolution over long durations. The DOT test and development phase is nearly concluded. The installation of an advanced speckle processor enables full science utilization including "Open-DOT" time allocation to the international community. Co-pointing with spectropolarimeters at other Canary Island telescopes and with TRACE furnishes valuable Solar-B precursor capabilities.