42 research outputs found
The Vector-APP: a Broadband Apodizing Phase Plate that yields Complementary PSFs
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.Comment: Proc. SPIE 8450-2
Fully broadband vAPP coronagraphs enabling polarimetric high contrast imaging
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 1E-5. Furthermore, we
discuss superachromatic QWPs based of liquid crystals or quartz/MgF2
combinations, and several polarizer choices. As the implementation of the
(broadband) vAPP coronagraph is fully based on polarization techniques, it can
easily be extended to furnish polarimetry by adding another QWP before the
coronagraph optic, which further enhances the contrast between the star and a
polarized companion in reflected light. We outline several polarimetric vAPP
system designs that could be easily implemented in existing instruments, e.g.
SPHERE and SCExAO.Comment: 11 pages, 5 figures, presented at SPIE Astronomical Telescopes and
Instrumentation 201
Combining vector-phase coronagraphy with dual-beam polarimetry
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
Minimizing the polarization leakage of geometric-phase coronagraphs with multiple grating pattern combinations
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. \textcolor{black}{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 for 1-2.5 m and 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 leakage on the first Airy ring
from 450 nm 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.Comment: 23 pages, 15 figures, accepted for publication in PAS
An overview of polarimetric sensing techniques and technology with applications to different research fields
We report the main conclusions from an interactive, multidisciplinary workshop on “Polarimetric Techniques and Technology”, held on March 24-28 2014 at the Lorentz Center in Leiden, the Netherlands. The work- shop brought together polarimetrists from different research fields. Participants had backgrounds ranging from academia to industrial RD. Here we provide an overview of polarimetric instrumentation in the optical regime geared towards a wide range of applications: atmospheric remote sensing, target detection, astronomy, biomedical applications, etc. We identify common approaches and challenges. We list novel polarimetric techniques and polarization technologies that enable promising new solutions. We conclude with recommendations to the polarimetric community at large on joint efforts for exchanging expertise
WIRC+Pol: A Low-resolution Near-infrared Spectropolarimeter
WIRC+Pol is a newly commissioned low-resolution (R ~ 100), near-infrared (J and H bands) spectropolarimetry mode of the Wide-field InfraRed Camera (WIRC) on the 200 inch Hale Telescope at Palomar Observatory. The instrument utilizes a novel polarimeter design based on a quarter-wave plate and a polarization grating (PG), which provides full linear polarization measurements (Stokes I, Q, and U) in one exposure. The PG also has high transmission across the J and H bands. The instrument is situated at the prime focus of an equatorially mounted telescope. As a result, the system only has one reflection in the light path providing minimal telescope induced polarization. A data reduction pipeline has been developed for WIRC+Pol to produce linear polarization measurements from observations. WIRC+Pol has been on-sky since 2017 February. Results from the first year commissioning data show that the instrument has a high dispersion efficiency as expected from the polarization grating. We demonstrate the polarimetric stability of the instrument with rms variation at 0.2% level over 30 minutes for a bright standard star (J = 8.7). While the spectral extraction is photon noise limited, polarization calibration between sources remain limited by systematics, likely related to gravity dependent pointing effects. We discuss instrumental systematics we have uncovered in the data, their potential causes, along with calibrations that are necessary to eliminate them. We describe a modulator upgrade that will eliminate the slowly varying systematics and provide polarimetric accuracy better than 0.1%
WIRC+Pol: A Low-resolution Near-infrared Spectropolarimeter
WIRC+Pol is a newly commissioned low-resolution (R ~ 100), near-infrared (J and H bands) spectropolarimetry mode of the Wide-field InfraRed Camera (WIRC) on the 200 inch Hale Telescope at Palomar Observatory. The instrument utilizes a novel polarimeter design based on a quarter-wave plate and a polarization grating (PG), which provides full linear polarization measurements (Stokes I, Q, and U) in one exposure. The PG also has high transmission across the J and H bands. The instrument is situated at the prime focus of an equatorially mounted telescope. As a result, the system only has one reflection in the light path providing minimal telescope induced polarization. A data reduction pipeline has been developed for WIRC+Pol to produce linear polarization measurements from observations. WIRC+Pol has been on-sky since 2017 February. Results from the first year commissioning data show that the instrument has a high dispersion efficiency as expected from the polarization grating. We demonstrate the polarimetric stability of the instrument with rms variation at 0.2% level over 30 minutes for a bright standard star (J = 8.7). While the spectral extraction is photon noise limited, polarization calibration between sources remain limited by systematics, likely related to gravity dependent pointing effects. We discuss instrumental systematics we have uncovered in the data, their potential causes, along with calibrations that are necessary to eliminate them. We describe a modulator upgrade that will eliminate the slowly varying systematics and provide polarimetric accuracy better than 0.1%