explanation      blue bibcodes open ADS page with paths to full text
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.
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.
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
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.
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
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.
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<SUP> − 5</SUP>. We find that the gvAPP
  can reach raw contrasts below ∼10<SUP> − 5</SUP> 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.
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. <BR
  /> 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. <BR /> 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. <BR /> 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 &lt; 0.1-0.4 Hz. We find that the power at 10<SUP>−2</SUP>
  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. <BR /> 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. <P />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
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<SUP>-4</SUP> ($\Delta \, {\rm m}_{3.94 \mu {\rm m}}$ = 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 M<SUB>Jup</SUB>, consistent with
  literature results. However, effective temperature and surface
  gravity vary significantly depending on the wavebands considered
  (1200 ≤ T<SUB>eff</SUB>(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
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
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
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.
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<SUP>®</SUP>,
  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
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
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
  &lt;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
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 &lt;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
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
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.
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 ($&gt;96\%$) 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 <SUP>13</SUP>CO-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
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
  nucleosynthesis<SUP>1,2</SUP>. In contrast to deuterium/hydrogen ratios,
  carbon isotope ratios are found to be roughly constant (around 89)
  in the Solar System<SUP>1,3</SUP>, but do vary on galactic scales
  with a <SUP>12</SUP>C/<SUP>13</SUP>C isotopologue ratio of around 68
  in the current local interstellar medium<SUP>4-6</SUP>. In molecular
  clouds and protoplanetary disks, <SUP>12</SUP>CO/<SUP>13</SUP>CO
  ratios can be altered by ice and gas partitioning<SUP>7</SUP>,
  low-temperature isotopic ion-exchange reactions<SUP>8</SUP> and
  isotope-selective photodissociation<SUP>9</SUP>. Here we report
  observations of <SUP>13</SUP>CO 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 <SUP>12</SUP>CO/<SUP>13</SUP>CO ratio of
  31<SUB>−10</SUB><SUP>+17</SUP>?(90% confidence), a substantial
  enrichment in <SUP>13</SUP>C 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 <SUP>13</SUP>C 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
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. <BR
  /> 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. <BR /> Methods: In this study, we present sensitive
  circular spectropolarimetric measurements of various landscape elements
  taken from a fast-moving helicopter. <BR /> Results: We demonstrate
  that during flight, within mere seconds of measurements, we can
  differentiate (S∕N &gt; 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. <BR /> 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
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%. <BR /> 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. <BR /> 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. <BR /> 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 (λ<SUB>0</SUB>
  = 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. <BR /> 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
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. <BR /> 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. <BR /> 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. <BR /> 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<SUB>⊙</SUB> 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 K<SUB>s</SUB>
  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<SUB>−0.9</SUB><SUP>+1.6</SUP> M<SUB>Jup</SUB> 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 M<SUB>Jup</SUB> in the full
  field of view of the detector (0."15&lt;ρ&lt;5."50), at 0."5 we can
  rule out further objects that are more massive than 6 M<SUB>Jup</SUB>,
  and for projected separations ρ &gt;2" we are sensitive to planets with
  masses as low as 2 M<SUB>Jup</SUB>. <BR /> 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. <P />Data are only available at the CDS via anonymous ftp to
  <A href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/648/A73">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/648/A73</A>
  <P />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.
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.
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. <P />(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.
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. <BR /> Aims: We aim to measure the near-infrared linear
  polarization of 20 known directly imaged exoplanets and brown dwarf
  companions. <BR /> 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 &lt;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. <BR /> 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. <BR /> 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. <P />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.
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. <BR />
  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. <BR /> 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<SUP>-4</SUP>
  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. <BR /> 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. <BR /> 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. <P />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.
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. <P />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
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
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
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.
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
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
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 (&lt; 70%), high resolution (19 mas),
  and high contrast (&lt; 1 × 10<SUP>-4</SUP>) 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
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
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 &lt;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
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
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<SUP>-4</SUP>. 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
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
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.
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
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 M<SUB>Jup</SUB>, which is equivalent to a mass ratio of q = 0.57
  ± 0.10% with respect to the primary. With the previously detected 14
  ± 3 M<SUB>Jup</SUB> 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
  &gt; 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. <SUP>*</SUP>
  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.
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. <BR /> Aims: In this article, we aim to show
  that the focal-plane wavefront sensing (FPWFS) algorithm, Fast and
  Furious (F&amp;F), can be used to measure and correct the IE/LWE. The
  F&amp;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. <BR
  /> 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&amp;F can
  correct a wide variety of LWE phase screens. Subsequently, F&amp;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). <BR />
  Results: The internal source results show that F&amp;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 &gt; 90% and an VAR ⪅ 0.05. Furthermore, the
  on-sky results show that F&amp;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&amp;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. <BR /> Conclusions:
  The F&amp;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.
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
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 M<SUB>Jup</SUB> 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
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
  p<SUB>A</SUB> = 0.031% ± 0.004% and ψ<SUB>A</SUB> = -32° ± 4°, and
  p<SUB>B</SUB> = 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
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.
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
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 &lt;10<SUP>-5</SUP> for 1-2.5 μm and &lt;10<SUP>-10</SUP>
  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 &lt;10<SUP>-10</SUP> 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.
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<SUP>-4</SUP>, and the 180 SCAR reaches a null of 1 ×
  10<SUP>-4</SUP>. 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.
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.
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.
2020A&A...634A..69H    Altcode: 2019arXiv191112759H
  <BR /> 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. <BR /> 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. <BR /> 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. <P />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.
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. <P />(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.
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.
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. <BR /> 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. <BR />
  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 (U<SUB>ϕ</SUB>),
  and a correction method based on a polarimetric model that we have
  developed, as presented in Paper II of this study. <BR /> 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. <BR
  /> 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. <P />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.
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. <BR /> Aims: We aim to characterize and correct
  the instrumental polarization effects of the complete optical system,
  that is, the telescope and SPHERE/IRDIS. <BR /> Methods: We created
  a detailed Mueller matrix model in the broadband filters Y, J, H,
  and K<SUB>s</SUB> 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. <BR /> 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 (&gt; 90% loss in the H- and K<SUB>s</SUB>-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. <BR /> 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. <P
  />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). <P />The data-reduction pipeline IRDAP is available at
  <A href="https://irdap.readthedocs.io">https://irdap.readthedocs.io</A>

---------------------------------------------------------
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
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.
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. <BR />
  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. <BR />
  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. <BR /> Results: In idealized simulations we show that for
  10<SUP>7</SUP> 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. <BR /> 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
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<SUP>
  - 6</SUP>. 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
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
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
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
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 &gt;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.
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. <BR /> Aims:
  We detect and characterize circumstellar disks and potential
  companions around solar-type, pre-main sequence stars in the
  Scorpius-Centaurus association (Sco-Cen). <BR /> 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<SUB>-7</SUB><SUP>+16</SUP> 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. <BR /> 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° &lt;
  ϕ &lt; 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). <BR /> 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 M<SUB>jup</SUB> 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. <P
  />Reduced images are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A87">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A87</A>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), 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.
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. <P />(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
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
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.
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. <P />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. <P />HiCIBaS launch is scheduled between August 13<SUP>th</SUP>
  and August 25<SUP>th</SUP>, 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.
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. <BR /> 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. <BR /> 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 ≈ 6<SUP>m</SUP>. 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 (&lt; 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. <BR /> Results: SPHERE/ZIMPOL reaches
  routinely an angular resolution (FWHM) of 22-28 mas, and a normalized
  peak surface brightness of SB<SUB>0</SUB> - m<SUB>star</SUB> ≈
  -6.5<SUP>m</SUP> arcsec<SUP>-2</SUP> for the V-, R- and I-band. The
  AO performance is worse for mediocre ≳1.0″ seeing conditions,
  faint stars m<SUB>R</SUB> ≳ 9<SUP>m</SUP>, or in the presence of the
  "low wind" effect (telescope seeing). The coronagraphs are effective in
  attenuating the PSF peak by factors of &gt; 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 &lt; 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 SB<SUB>pol</SUB>( ρ)-m<SUB>star</SUB> ≈ 8<SUP>m</SUP>
  arcsec<SUP>-2</SUP> and for the point source contrast m<SUB>pol</SUB>(
  ρ)-m<SUB>star</SUB> ≈ 15<SUP>m</SUP> and much lower limits are
  achievable with deeper observations. <BR /> 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. <P
  />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.
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.
2018A&A...616A..79G    Altcode: 2018arXiv180502261G
  <BR /> 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. <BR />
  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. <BR /> 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 M<SUB>Jup</SUB>. 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 M<SUB>Jup</SUB>) brown dwarf or high-mass planet with an
  unresolved disk and dust envelope. <P />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
  <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/616/A79">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/616/A79</A>

---------------------------------------------------------
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
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.
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
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
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
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<SUP>-5</SUP>
  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
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<SUP>-5</SUP>. 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
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 (&gt; 70%), high
  resolution (19 mas), and high contrast (&lt; 1 × 10<SUP>-4 </SUP>)
  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.
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
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
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<SUP>-5</SUP>). 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
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<SUP>-10</SUP>. 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
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<SUP>-5</SUP> 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.
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
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
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.
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
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. <P />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. <P />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. <P />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.
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.
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.
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 P<SUB>L</SUB> is very stringent: the absolute
  error in P<SUB>L</SUB> must be smaller then 0.001+0.005.P<SUB>L</SUB>
  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
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
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.
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
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.
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
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 &lt;=
  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.
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<SUP>-4</SUP> . 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.
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
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
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<SUP>-4</SUP> 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<SUP>★</SUP>
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.
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.
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 (m<SUB>L</SUB> = 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}<SUP>-3.3</SUP>) at 2 λ /D and 12.2
  (={10}<SUP>-4.8</SUP>) 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.
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<SUP>-6</SUP>
  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
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. <BR /> 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). <BR /> 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). <BR /> 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. <BR /> 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. <P />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 <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114</A>

---------------------------------------------------------
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.
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
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
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.
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
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
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.
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<SUP>-4</SUP>, 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
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<SUP>-4</SUP>. 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
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
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.
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. <BR /> 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. <BR />
  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. <BR /> Results: Based on high-precision measurements of the
  mean magnetic field modulus, we refined the rotational period of HD
  75049 to P<SUB>rot</SUB> = 4.048267 ± 0.000036 d. We also derived
  basic stellar parameters, T<SUB>eff</SUB> = 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. <BR /> 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. <P />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.
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.
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
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
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
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 --&gt;
  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.
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.
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. <P />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.
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
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.
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. <BR /> 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. <BR /> 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 10<SUP>5</SUP>. The multiline technique of least-squares
  deconvolution (LSD) was applied to combine information from the
  spectral lines of Fe-peak and rare earth elements. <BR /> 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,
  ⟨B<SUB>z</SUB>⟩, with an accuracy of 5-10 G. We also determined the
  net linear polarization from the LSD Stokes Q and U profiles. Combining
  previous ⟨B<SUB>z</SUB>⟩ measurements with our data allowed us to
  determine an improved rotational period of the star, P<SUB>rot</SUB>
  = 12.45812 ± 0.00019 d. We also measured the longitudinal
  magnetic field from the cores of Hα and Hβ lines. The analysis of
  ⟨B<SUB>z</SUB>⟩ measurements showed no evidence for a significant
  radial magnetic field gradient in the atmosphere of HD 24712. We used
  our ⟨B<SUB>z</SUB>⟩ 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. <P />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 <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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.
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. <BR /> 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. <BR /> 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. <BR
  /> 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. <BR /> 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. <P />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.
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.
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.
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
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.
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.
2012A&A...547A..34F    Altcode: 2012arXiv1209.0983F
  <BR /> 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. <BR /> 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. <BR /> 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<SUP>-1</SUP> to 14 km s<SUP>-1</SUP> 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.
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 (&lt;25 pc) with masses ranging from a few Jupiter
  masses to Super Earths (∼2 Earth radii, ∼10 M<SUB>⊕</SUB>)
  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
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
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.
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&amp;m's code
Authors: de Juan Ovelar, M.; Diamantopoulou, S.; Roelfsema, R.;
   van Werkhoven, T.; Snik, F.; Pragt, Johan; Keller, C.
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&amp;m's code, an error budget
  and performance simulator for polarimetric systems. In this study
  we present the realistic Mueller matrices calculated with M&amp;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
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
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.
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.
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.
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
  &amp; 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 &amp;
  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
  &amp; 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.
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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Based on
  observations collected at the European Southern Observatory, Chile
  (ESO programme 084.D-0338). Figures 9-12 are available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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.
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.
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
  S<SUP>5</SUP>T (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
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&amp;m's: an error budget and performance simulator code
    for polarimetric systems
Authors: de Juan Ovelar, Maria; Snik, Frans; Keller, Christoph U.
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 &amp; Snik, in 2009, we have developed the M&amp;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&amp;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
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<SUP>-4</SUP>. 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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />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.
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.
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. <BR /> 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. <BR />
  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. <BR />
  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. <P />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.
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<SUP>-4</SUP>
  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<SUP>-5</SUP>,
  indicating the absence of a net longitudinal magnetic field stronger
  than 0.2 G. <P />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.
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.
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<SUP>-5</SUP> (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.
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.
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. <BR />
  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. <BR /> 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. <BR /> 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. <BR /> 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. <P />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 <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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. <BR /> 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. <BR /> 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. <BR /> 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 &lt; μ ≤ 0.3, although systematic effects cannot
  be completely excluded. <BR /> 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
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
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
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.
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.
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 &amp; observations
Authors: Snik, F.
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.
2009epsc.conf..536S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Polarimetry from the Ground Up
Authors: Keller, C. U.; Snik, F.
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.
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<SUP>-5</SUP>
  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.
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.
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
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
2009astro2010S.296T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astronomical polarimetry: New concepts; new instruments;
    new measurements &amp; observations
Authors: Snik, Frans
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.
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
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
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
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.
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 <SUP>-5</SUP> level
Authors: Snik, Frans
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 <SUP>-5</SUP> 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 <SUP>-5</SUP>
  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
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
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.
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.