The Wide Area Linear Optical Polarimeter North is an optical polarimeter designed for the needs of the Polar-Areas Stellar Imaging in Polarimetry High-Accuracy Experiment survey. It will be installed on the 1.3-m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the 30×30 arcmin2 polarization of millions of stars at high galactic latitude, aiming to measure hundreds of stars per square degree. The astronomical filter used in the instrument is a modified, polarimetrically neutral broadband Sloan Digital Sky Survey-r. This instrument will be a pioneering one due to its large field of view (FoV) of and high-accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1% and 0.05% levels, respectively. Four separate 4k×4k charge-coupled devices will be used as the instrument detectors, each imaging one of the 0-, 45-, 90-, and 135-deg polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing the aspects of the instrument that are different from Wide Area Linear Optical Polarimeter South. We also present a customized design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.
High-contrast imaging in the next decade aims to image exoplanets at smaller angular separations and deeper contrasts than ever before. A problem that has recently garnered attention for telescopes equipped with high-contrast coronagraphs is polarization aberration arising from the optics. These aberrations manifest as low-order aberrations of different magnitudes for orthogonal polarization states and spread light into the dark hole of the coronagraph that cannot be fully corrected. The origin of polarization aberrations has been modeled at the telescope level. However, we don't fully understand how polarization aberrations arise at the instrument level. To directly measure this effect, we construct a dual-rotating-retarder polarimeter around the SCoOB high-contrast imaging testbed to measure its Mueller matrix. With this matrix, we directly characterize the diattenuation, retardance, and depolarization of the instrument as a function of position in the exit pupil. We measure the polarization aberrations in the Lyot plane, both with and without the Vector Vortex Coronagraph, to understand how polarization couples into high-contrast imaging residuals.
Future space observatories will likely have segmented primaries, causing diffraction effects that reduce coronagraph performance. Reflective binary pupil apodizer masks can mitigate these, with the metamaterial black silicon (BSi) showing promise as a strong absorber. To bring contrast ratios to the 10−10 level as needed to observe Earth-like exoplanets, feature sizes on these BSi masks will need to be less than 5 microns when paired with MEMS (micro-electromechanical systems) deformable mirrors. As scalar diffraction cannot reliably model this feature size, we developed a Finite-Difference Time-Domain (FDTD) model of BSi masks using Meep software. We characterize the FDTD-derived polarization-dependent bidirectional reflectance distribution function of BSi and discuss the model’s shortcomings.
Polarization aberrations originating from the telescope and high-contrast imaging instrument optics introduce polarization-dependent speckles and associated errors in the image plane, affecting the measured exoplanet signal. Understanding this effect is critical for future space-based high-contrast imaging instruments that aim to image the Earth analogs with 10−10 raw contrast and characterize their atmospheres. We present end-to-end modeling of the polarization aberrations for a high-contrast imaging testbed, SCoOB. We use a vector vortex coronagraph (VVC) as the focal plane mask, incorporate polarization filtering, and estimate the peak contrast in the dark hole region 3-10 λ/D. The dominant polarization aberrations in the system are retardance defocus and tilt due to the OAPs and fold mirrors. Although the mean contrast in the dark hole region remains unaffected by the polarization aberrations, we see brighter speckles limiting the contrast to 1×10−9 at 1-2 λ/D. We extend the simulations using the measured retardance maps for the VVC and find that the mean contrast in SCoOB is more sensitive to retardance errors of the VVC and the QWP than the polarization aberrations.
Polarimetric differential imaging observations provide the highest contrast images of circumstellar disks in addition to providing information on dust grain scattering properties. The upcoming Nancy Grace Roman Space Telescope Coronagraph is expected to measure the linear polarization fraction of disks greater than 0.3 with an uncertainty of 0.03. One of the critical problems with polarimetric observations is the polarization aberrations generated by the telescope and polarimetric optics, which introduce errors when measuring lower SNR polarized signals. A modeling pipeline was previously developed to simulate the polarization observations of higher SNR debris disks similar without accounting for polarization aberrations. Here, we present the simulated polarimetric disk images of fainter debris disks (∼0.1mJy/arcsec2) through the Roman telescope and the HLC and SPC coronagraphs, incorporating polarization aberrations, jitter, detector, and speckle noise. The Point Response Functions are generated using PROPER for each orthogonal polarization state to account for the polarization aberrations. Finally, we compare the recovered polarization fraction of the debris disk with the input to demonstrate the polarimetric capability of the Roman Coronagraph.
The Space Coronagraph Optical Bench (SCoOB) is a high-contrast imaging testbed built to demonstrate starlight suppression techniques at visible wavelengths in a space-like vacuum environment. The testbed is designed to achieve <10−8 contrast from 3 − 10λ/D in a one-sided dark hole using a liquid crystal vector vortex waveplate and a 952-actuator Kilo-C deformable mirror (DM) from Boston Micromachines (BMC). We have recently expanded the testbed to include a field stop for mitigation of stray/scattered light, a precision-fabricated pinhole in the source simulator, a Minus K passive vibration isolation table for jitter reduction, and a low-noise vacuum-compatible CMOS sensor. We report the latest contrast performance achieved using implicit electric field conjugation (iEFC) at a vacuum of ∼10−6 Torr and over a range of bandpasses with central wavelengths from 500 to 650nm and bandwidths (BW) from ≪ 1% to 15%. Our jitter in vacuum is < 3 × 10−3λ/D, and the best contrast performance to-date in a half-sided D-shaped dark hole is 2.2 × 10−9 in a ≪ 1% BW, 4 × 10−9 in a 2% BW, and 2.5 × 10−8 in a 15% BW.
The Coronagraphic Instrument onboard the Nancy Grace Roman Space Telescope is an important stepping stone towards the characterization of habitable, rocky exoplanets. In a technology demonstration phase conducted during the first 18 months of the mission (expected to launch in late 2026), novel starlight suppression technology may enable direct imaging of a Jupiter analog in reflected light. Here we summarize the current activities of the Observation Planning working group formed as part of the Community Participation Program. This working group is responsible for target selection and observation planning of both science and calibration targets in the technology demonstration phase of the Roman Coronagraph. We will discuss the ongoing efforts to expand target and reference catalogs, and to model astrophysical targets (exoplanets and circumstellar disks) within the Coronagraph’s expected sensitivity. We will also present preparatory observations of high priority targets.
In preparation for the operational phase of the Nancy Grace Roman Space Telescope, NASA has created the Coronagraph Community Participation Program (CPP) to prepare for and execute Coronagraph Instrument technology demonstration observations. The CPP is composed of 7 small, US-based teams, selected competitively via the Nancy Grace Roman Space Telescope Research and Support Participation Opportunity, members of the Roman Project Team, and international partner teams from ESA, JAXA, CNES, and the Max Planck Institute for Astronomy. The primary goals of the CPP are to prepare simulation tools, target databases, and data reduction software for the execution of the Coronagraph Instrument observation phase. Here, we present the current status of the CPP and its working groups, along with plans for future CPP activities up through Roman’s launch. We also discuss plans to potentially enable future commissioning of currently-unsupported modes.
KEYWORDS: Calibration, Coronagraphy, Simulations, Observational astronomy, Data processing, Exoplanets, Equipment, Analog electronics, Space telescopes, Signal processing
The Nancy Grace Roman Space Telescope’s Coronagraph Instrument will for the first time demonstrate active wavefront sensing and control for a space-based coronagraph, and may image the first planet in reflected light. The Community Participation Program has been initiated to engage members of the broader scientific community in the preparation for its planned launch in late 2026/early 2027. Here we will present the on-going work of the Data Reduction and Simulations working group, one of the four working groups within the Community Participation Program. The working group is charged with the development of the data reduction and postprocessing pipeline for the on-sky data and the development of a simulation suite to aid in the preparation and planning of Roman Coronagraph observations.
Polarization differential imaging (PDI) is a key point-spread function subtraction technique that efficiently processes out starlight and reveals faint polarized structures, such as circumstellar disks and exoplanets. This technique operates by assuming that the signal measured from a star is unpolarized, and subtracts out of the measurement. However, in the presence of polarization aberrations the starlight will be slightly polarized by the telescope that observes it. This results in a spatially-varying polarized speckle field on the focal plane of high-contrast imaging polarimeters. Current high-contrast polarimeters are roughly an order of magnitude from achieving the photon noise limit, and polarization aberrations are a contributing factor. This effect will be stronger in the next-generation 30m Giant Segmented Mirror Telescopes (GSMTs) where polarization aberrations are stronger. In this work, we present a numerical model of a high-contrast imaging polarimeter behind an adaptive optics system subject to the polarization aberrations of an GSMT-class telescope. We use this model to understand the coupling of polarization aberrations into the adaptive optics residuals that leak through to the focal plane, and compare them to what has been observed on previous polarimetric instruments. We report on the fundamental limits imposed by polarization aberrations on PDI and discuss mitigation strategies to compensate for this effect.
Wide-Area Linear Optical Polarimeter (WALOP)-South is the first wide-field and survey-capacity polarimeter in the optical wavelengths. On schedule for commissioning in 2024, it will be mounted on the 1 m SAAO telescope in Sutherland Observatory, South Africa to undertake the PASIPHAE sky survey. PASIPHAE program will create the first polarimetric sky map in the optical wavelengths, spanning more than 2000 square degrees of the southern Galactic region. In a single exposure, WALOP-South’s innovative design will enable it to measure the linear polarization (Stokes parameters q and u) of all sources in a field of view (FoV) of 35 × 35 arc-minutes-squared in the SDSS-r broadband and narrowband filters between 500-750 nm with 0.1 % polarization accuracy. The unique goals of the instrument place very stringent systems engineering goals, including on the performance of the optical, polarimetric, optomechanical, and electronic subsystems. In particular, the major technical hurdles for the project included the development of: (a) an optical design to achieve imaging quality PSFs across the FoV, (b) an optomechanical design to obtain high accuracy optical alignment in conjugation with minimal instrument flexure and stress birefringence on optics (which can lead to variable instrumental polarization), and (c) an on-sky calibration routine to remove the strong polarimetric cross-talk induced instrumental polarization to obtain 0.1% across the FoV. All the subsystems have been designed carefully to meet the overall instrument performance goals. As of May 2024, all the instrument optical and mechanical subsystems have been assembled and are currently getting tested and integrated. The complete testing and characterization of the instrument in the lab is expected to be completed by August 2024. While the instrument was initially scheduled for commissioning in 2022, it got delayed due to various technical challenges; WALOP-South is now on schedule for commissioning in second half of 2024. In this paper, we will present (a) the design and development of the entire instrument and its major subsystems, focusing the instrument’s opto-mechanical design which has not been reported before, and (b) assembly and integration of the instrument in the lab and early results from lab characterization of the instrument’s optical performance.
SHARK-NIR is an instrument providing high-contrast coronagraphic imaging, dual band imaging and low resolution spectroscopy in Y, J and H bands, taking advantage of the high performance of the Large Binocular Telescope (LBT) AO systems. Its main scientific drivers is the detection and characterization of exoplanets, circumstellar disks, Solar System small bodies and acrive galactic nuclei. Binocular observations are provided by exploiting the synergy with SHARK-VIS (operating in V band) and LMIRCam of LBTI (operating from K to M band). SHARK-NIR was shipped to LBT in June 2022, and up to November 2022 was the subject of the three pre-commissioning runs: the first to install and test the instrument in the high-bay LBT clean tent, the second to install and align the instrument to the telescope and the third to perform daytime testing. This pre-commissioning phase resulted in the successful alignment of the instrument with a very good internal optical quality and the performance of additional tests using simulated turbulence injected through LBT Adaptive Secondary Mirror. In January 2023 we had the first commissioning run and the instrument first light, in which we successfully tested the imaging capabilities with and without the Gaussian Lyot coronagraph. In March 2023 we had the second commissioning run, in which despite the bad weather we successfully performed a variety of technical activities, as well as preliminary testing of the Shaped Pupil coronagraph. After two additional commissioning runs in May and October, our first early scientific run in October 2023 focused on the Taurus star formation region, a region populated by targets of considerable scientific interest. In this paper we will give an overview of commissioning and the early science phases running from October 2023 to May 2024, focusing on the technical challenges we overcame and future work needed to push the instrument to its very limit, as well as presenting the first preliminary scientific results.
Astronomical polarimetry is a powerful tool in the investigation of asymmetries, magnetic fields, and scattering phenomena in and around stars, planets, and galaxies and also in the study of interstellar and circumstellar media. As polarimetry is a photon-starved technique, a polarimetric capability on the next-generation giant segmented mirror telescopes (GSMTs) will enable observations of fainter targets, thereby opening up a wealth of science that cannot be achieved with the current generation of large telescopes. We are investigating the feasibility of adding a polarimetric capability to one of the planned first-generation instruments for the Giant Magellan Telescope (GMT). Here, we present a study of incorporating polarimetric optics into ComCam, the GMT Commissioning Camera. We analyze the performance of the instrument after the integration of a waveplate and Wollaston prism into the pseudo-collimated beam. We also present a design for new ComCam camera optics that are optimized to accommodate the polarimetric optics. Using the technical requirements from the polarimetric science cases, this ‘GMT-Pol’ version of the instrument delivers seeing-limited performance over a wavelength range of 0.5-0.9 μm with a ∼2 arcmin field-of-view.
A major goal of proposed future space observatories, such as the Habitable World Observatory, is to directly image and characterize Earth-like planets around Sun-like stars to search for habitability signatures requiring the starlight suppression (contrast) of 10−10. One of the significant aspects affecting this contrast is the polarization aberrations generated from the reflection from mirror surfaces. The polarization aberrations are the phase-dependent amplitude and phase patterns originating from the Fresnel reflections of the mirror surfaces. These aberrations depend on the angle of incidence and coating parameters of the surface. This paper simulates the polarization aberrations for an on-axis and off-axis TMA telescope of a 6.5 m monolithic primary mirror. We analyze the polarization aberrations and their effect on the coronagraphic performance for eight different recipes of mirror coatings for Astronomical filter bands g-I: three single-layer metal coatings and five recipes of protective coatings. First, the Jones pupils are estimated for each coating and filter band using the polarization ray tracing in Zemax. Then, we propagate these Jones pupils through a Vector Vortex Coronagraph and Perfect Coronagraphs using hcipy, a physical optics-based simulation framework. The analysis shows that the two main polarization aberrations generated from the four mirrors are the retardance-defocus and retardance-tilt. The simulations also show that the coating plays a significant role in determining the strength of the aberrations. The bare/oxi-aluminum and Al+18nm LiF coating outperforms all the other coatings by one order of magnitude.
We present a generalized non-negative factorization (NMF)-based data reduction pipeline for circumstellar disk and exoplanet detection. By using an adaptable pre-processing routine that applies algorithmic masks and corrections to improper data, we are able to easily offload the computationally-intensive NMF algorithm to a Graphics Processing Unit (GPU), significantly increasing computational efficiency. NMF has been shown to better preserve disk structural features compared to other post-processing approaches and has demonstrated improvements in the analysis of archival data. The adaptive pre-processing routine of this pipeline, which automatically aligns and applies image corrections to the raw data, is shown to significantly improve chromatic halo suppression. Utilizing HST-STIS and JWST-MIRI coronagraphic datasets, we demonstrate a factor of five increase in real-time computational efficiency by using GPUs to perform NMF compared to using CPUs. Additionally, we demonstrate the usefulness of higher numbers of NMF components with SNR and contrast improvements, which necessitates the use of a more computationally efficient approach for data reduction.
KEYWORDS: Space telescopes, Design and modelling, Telescopes, Observatories, Mirrors, James Webb Space Telescope, Space mirrors, Equipment, Astronomy, Coronagraphy
New development approaches, including launch vehicles and advances in sensors, computing, and software, have lowered the cost of entry into space, and have enabled a revolution in low-cost, high-risk Small Satellite (SmallSat) missions. To bring about a similar transformation in larger space telescopes, it is necessary to reconsider the full paradigm of space observatories. Here we will review the history of space telescope development and cost drivers, and describe an example conceptual design for a low cost 6.5 m optical telescope to enable new science when operated in space at room temperature. It uses a monolithic primary mirror of borosilicate glass, drawing on lessons and tools from decades of experience with ground-based observatories and instruments, as well as flagship space missions. It takes advantage, as do large launch vehicles, of increased computing power and space-worthy commercial electronics in low-cost active predictive control systems to maintain stability. We will describe an approach that incorporates science and trade study results that address driving requirements such as integration and testing costs, reliability, spacecraft jitter, and wavefront stability in this new risk-tolerant “LargeSat” context.
Polarization observations through the next-generation large telescopes will be invaluable for exploring the magnetic fields and composition of jets in AGN, multi-messenger transients follow-up, and understanding interstellar dust and magnetic fields. The 25m Giant Magellan Telescope (GMT) is one of the next-generation large telescopes and is expected to have its first light in 2029. The telescope consists of a primary mirror and an adaptive secondary mirror comprising seven circular segments. The telescope supports instruments at both Nasmyth as well as Gregorian focus. However, none of the first or second-generation instruments on GMT has the polarimetric capability. This paper presents a detailed polarimetric modeling of the GMT for both Gregorian and folded ports for astronomical B-K filter bands and a field of view of 5 arc minutes. At 500nm, The instrumental polarization is 0.1% and 3% for the Gregorian and folded port, respectively. The linear to circular crosstalk is 0.1% and 30% for the Gregorian and folded ports, respectively. The Gregorian focus gives the GMT a significant competitive advantage over TMT and ELT for sensitive polarimetry, as these telescopes support instruments only on the Nasmyth platform. We also discuss a list of polarimetric science cases and assess science case requirements vs. the modeling results. Finally, we discuss the possible routes for polarimetry with GMT and show the preliminary optical design of the GMT polarimeter.
Integrated optical models allow for accurate prediction of the as-built performance of an optical instrument. Optical models are typically composed of a separate ray trace and diffraction model to capture both the geometrical and physical regimes of light. These models are typically separated across both open-source and commercial software that don’t interface with each other directly. To bridge the gap between ray trace models and diffraction models, we have built an open-source optical analysis platform in Python called Poke that uses commercial ray tracing APIs and open-source physical optics engines to simultaneously model scalar wavefront error, diffraction, and polarization. Poke operates by storing ray data from a commercial ray tracing engine into a Python object, from which physical optics calculations can be made. We present an introduction to using Poke, and highlight the capabilities of two new propagation modules that add to the utility of existing scalar diffraction models. Gaussian Beamlet Decomposition is a ray-based approach to diffraction modeling that allows us to integrate physical optics models with ray trace models to directly capture the influence of ray aberrations in diffraction simulations. Polarization Ray Tracing is a ray-based method of vector field propagation that can diagnose the polarization aberrations in optical systems. Poke has been recently used to study the next generation of astronomical observatories, including the ground-based Extremely Large Telescopes (TMT, GMT, ELT) and a 6 meter space telescope (6MST) early concept for NASA’s Habitable Worlds Observatory.
Two unique wide-field and high-accuracy polarimeters named WALOP (Wide-Area Linear Optical Polarimeter)- North and WALOP-South are currently under development at the Inter-University Center for Astronomy and Astrophysics (IUCAA), India, to create a large area optical polarization map of the sky for the upcoming PASIPHAE sky survey. These instruments are designed to achieve a linear polarimetric measurement accuracy of 0.1% across a field of view (FoV) of 30×30 arcminutes. The WALOP-South instrument will be installed first on a 1 m telescope at the Sutherland Observatory, where the temperatures during the night can vary between 10 to -5°C. These temperature variations and the instrument’s pointing to various non-zenithal positions in the sky can introduce stress birefringence in the lenses, leading to time-varying instrumental polarization. This work estimates stress-induced birefringence due to thermal, and gravity stresses on WALOP-South lenses. Using the optomechanical model of the WALOP-South, we carried out Finite Element Analysis (FEA) simulations in SolidWorks software to estimate the stresses for various scenarios of temperature, telescope pointing airmass, and lens mount material (aluminum and titanium). Further, we use the stress tensor analysis to estimate the principal stresses and their directions and consequent birefringence and retardance introduced in the lenses. The stressinduced birefringence will change the optical path length for orthogonal polarization states of the beam passing through the lenses and introduce phase retardation. Overall, with the lens mount design of the instrument, we find that the retardation and consequent instrumental polarization will be within the instrumental accuracy requirements. Additionally, the stress birefringence is found to be higher for aluminum compared to titanium mounts. We further incorporated this retardance in the instrument Mueller matrix estimation to understand its effects on the polarization measurements.
Here we present modeling of the polarization aberrations of the Gemini telescope as seen by the Gemini Planet Imager’s dual-channel polarimetry mode. We first calculate the Jones pupil using a polarization sensitive ray-tracing program and theoretical properties for the Gemini mirror coatings. The predominant effect is a differential astigmatism between incident x and y polarization states, also known as retardance defocus. We use this Jones pupil as an input to a Fourier optics model of the Gemini Planet Imager to create model Stokes Q and U images, which we compare to on-sky data. We demonstrate that the effects of the telescope polarization aberrations can be detected in the on-sky data and speculate that they may be limiting the polarimetric contrasts at the smallest inner working angles. Finally, we explore how the polarization aberrations couple with residual atmospheric wavefront error from the adaptive optics system, and how they affect the polarimetric performance of new GPI 2.0 coronagraph designs.
The Roman coronagraph instrument will demonstrate high-contrast imaging technology, enabling the imaging of faint debris disks, the discovery of inner dust belts, and planets. Polarization studies of debris disks provide additional information on dust grains’ size, distribution, and shape. The Roman coronagraph uses a polarization module comprising two Wollaston prism assemblies to produce four orthogonally polarized images (I0, I90, I45, and I135), each measuring 3.2 arcsecs in diameter and separated by 7.5 arcsecs in the sky. The expected RMS error in the linear polarization fraction measurement is 1.66% per resolution element of 3 by 3 pixels. We present a mathematical model to simulate the polarized intensity images through the Roman CGI, including the instrumental polarization and other uncertainties. We use disk modeling software, MCFOST, to model q, u, and polarization intensity of the debris disk, Epsilon-Eridani. The polarization intensities are convolved with the coronagraph throughput incorporating the PSF morphology. We include model uncertainties, detector noise, speckle noise, and jitter. The final polarization fraction of 0.4±0.0251 is obtained after post-processing and speckle noise removal.
The 2020 Decadal Survey on Astronomy and Astrophysics endorsed space-based high contrast imaging for the detection and characterization of habitable exoplanets as a key priority for the upcoming decade. To advance the maturity of starlight suppression techniques in a space-like environment, we are developing the Space Coronagraph Optical Bench (SCoOB) at the University of Arizona, a new thermal vacuum (TVAC) testbed based on the Coronagraphic Debris Exoplanet Exploring Payload (CDEEP), a SmallSat mission concept for high contrast imaging of circumstellar disks in scattered light. When completed, the testbed will combine a vector vortex coronagraph (VVC) with a Kilo-C microelectromechanical systems (MEMS) deformable mirror from Boston Micromachines Corp (BMC) and a self-coherent camera (SCC) with a goal of raw contrast surpassing 10−8 at visible wavelengths. In this proceedings, we report on our wavefront sensing and control efforts on this testbed in air, including the as-built performance of the optical system and the implementation of algorithms for focalplane wavefront control and digging dark holes (regions of high contrast in the focal plane) using electric field conjugation (EFC) and related algorithms.
The development of spaceborne coronagraphic technology is of paramount importance to the detection of habitable exoplanets in visible light. In space, coronagraphs are able to bypass the limitations imposed by the atmosphere to reach deeper contrasts and detect faint companions close to their host star. To effectively test this technology in a flight-like environment, a high-contrast imaging testbed must be designed for operation in a thermal vacuum (TVAC) chamber. A TVAC-compatible high-contrast imaging testbed is undergoing development at the University of Arizona inspired by a previous mission concept: The Coronagraphic Debris and Exoplanet Exploring Payload (CDEEP). The testbed currently operates at visible wavelengths and features a Boston Micromachines Kilo-C DM for wavefront control. Both a vector vortex coronagraph and a knife-edge Lyot coronagraph operating mode are under test. The optics will be mounted to a 1 × 2 meter pneumatically isolated optical bench designed to operate at 10−8 torr and achieve raw contrasts of 10−8 or better. The validation of our optical surface quality, alignment procedure, and first light results are presented. We also report on the status of the testbed’s integration in the vaccum chamber.
The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument will be mounted on the 1-m South African Astronomical Observatory telescope in South Africa as part of the Polar-Areas Stellar Imaging Polarization High Accuracy Experiment (PASIPHAE) program to carry out a linear imaging polarization survey of the Galactic polar regions in the optical band. Designed to achieve polarimetric sensitivity of 0.05% across a 35 × 35 arc min field of view (FOV), it will be capable of measuring the Stokes parameters I, q, and u in a single exposure in the R broadband and narrowband filters between 0.5 to 0.7 μm. For each measurement, four images of the full field corresponding to linear polarization angles of 0 deg, 45 deg, 90 deg, and 135 deg in the instrument coordinate system will be created on four detectors from which the Stokes parameters can be found using differential photometry. In designing the optical system, major challenges included correcting for the dispersion introduced by large split angle Wollaston prisms used as analysers and other aberrations from the entire field to obtain imaging quality point spread function (PSF) at the detector. We present the optical design of the WALOP-South instrument which overcomes these challenges and delivers near seeing limited PSFs for the entire FOV.
WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around 106 stars in the SDSS-r broadband with 0.1 % polarimetric accuracy, covering 4000 square degrees in the Galactic polar regions. The combined capabilities of one-shot linear polarimetry, high polarimetric accuracy (< 0.1 %) and polarimetric sensitivity (< 0.05 %), and a large field of view (FOV) of
35 35 arcminutes make WALOP-South a unique astronomical instrument. In a single exposure, it is designed to measure the Stokes parameters I, q and u in the SDSS-r broadband and narrowband filters between 500-700 nm. During each measurement, four images of the full field corresponding to the polarization angles of 0°, 45°, 90° and 135° will be imaged on four detectors and carrying out differential photometry on these images will yield the Stokes parameters. Major challenges in designing WALOP-South instrument include- (a) in the optical design, correcting for the spectral dispersion introduced by large split angle Wollaston Prisms used as polarization analyzers as well as aberrations from the wide field, and (b) making an optomechanical design adherent to the tolerances required to obtain good imaging and polarimetric performance under all temperature conditions as well as telescope pointing positions. We present the optical and optomechanical design for WALOP-South which
overcomes these challenges.
Solar polarimetry aims at measuring the full set of Stokes vectors (I, Q, U, V) to extract the magnetic field information of the solar atmosphere. During the polarimetric observations, the oblique reflections from the telescope mirrors modify (crosstalk) or even produce polarization (instrumental polarization). For accurate polarimetric measurements of the source, it is important to correctly model and remove the instrumental polarization and crosstalk introduced by the telescope. The Multi-Application Solar Telescope (MAST) at the Udaipur Solar Observatory is a Gregorian-Coude telescope with a 50 cm off-axis parabolic primary mirror. It consists of nine mirrors that rotate as the telescope tracks the object and delivers a constant beam at the observing floor. Here, we present a formalism for an analytical estimation of the telescope's Mueller matrix using a polarization ray tracing algorithm. The model was experimentally verified at 6173 Å using the observations taken from the facility imaging spectro-polarimeter. The observations were split into two sets, during January and May 2018, to verify the model's consistency. The instrumental polarization was found to vary between 1.3% to 5.54%, and 3.5% to 4.3% throughout the observation from 9 AM to 4 PM during summer and winter respectively. The maximum value of the crosstalk (Q to V and U to V) was found to be 29.08% and 39.46% respectively. We obtained a reasonable match between the model and the observations with some offsets. We also discuss the possible reasons for the observed deviations and their effects.
The Thirty Meter Telescope (TMT) is a proposed future generation telescope which will be located on either Maunakea, Hawaii or La Palma in the Canary islands. A thermal-infrared (TIR) imager and spectrometer (MICHI) combined with an adaptive optics system is being investigated as a possible second-generation instrument for this telescope. MICHI has been designed to also have a polarimetry capability in both imaging and low dispersion spectroscopic modes. Using polarization ray tracing in Zemax, we have estimated the instrumental polarization (IP) and crosstalk introduced at the focus of the near- and mid-infrared imaging system. In our calculations, we find that the IP varies from 1.0-0.54% and 0.54-0.42%, whereas the polarization crosstalk varies between 25-4% and 4-0.7%, in the near and TIR regions respectively at the instrument port of MICHI. These values of IP and crosstalk may cause problems during the high absolute accuracy polarization observations. Here we present the polarization effects for the imaging system of MICHI and it impacts on the polarization observations.
The Thirty Meter Telescope (TMT) is a future generation telescope proposed to be located in Mauna Kea, Hawaii or in La Palma in the Canary Islands. The telescope will have a segmented primary and an inclined tertiary mirror. The segmentation of the primary mirror and the inclination in the tertiary mirror can introduce significant polarization aberrations. Typically, the polarization aberrations, introduced due to the mirror coating and the high incident angles cause small modifications to the Point Spread Function (PSF). Here, we perform the polarization ray tracing for TMT using the optical design software Zemax for different input polarizations for a point source (on-axis). We calculate the diattenuation and retardance aberration maps for all the three mirrors of TMT. The coating induced astigmatism obtained from the retardance of the primary and secondary mirror is found to be of the order of 0.048 radians, whereas, the polarization induced tilt by the retardance of the tertiary mirror is in the order of 0.29 radians. The Jones pupil maps are estimated at two of the instrument ports, Wide Field Optical Spectrograph (WFOS) and Narrow Field Infrared Adaptive Optics System (NFIRAOS).The Amplitude Response Matrix (ARM) estimated at the WFOS port show the presence of ghost PSF's. The magnitude of the ghost PSF components is of the order of 2.5 x 10-5 at 1μm at WFOS port. The ARM and the Point Spread Matrix (PSM) are estimated at the focus of the NFIRAOS instrument. The Stokes PSF is shown for horizontal and vertical polarization as inputs. The Huygen's point spread function obtained from Zemax shows the variations in FWHM for unpolarized and polarized inputs. These estimations would help in the design aspects of a high contrast imaging instrument for the TMT in the future.
An analytical model has been developed to estimate the polarization effects, such as instrumental polarization (IP), crosstalk (CT), and depolarization, due to the optics of the Thirty Meter Telescope. These are estimated for the unvignetted field-of-view and the wavelengths of interest. The model estimates an IP of 1.26% and a CT of 44% at the Nasmyth focus of the telescope at the wavelength of 0.6 μm at field angle zero with the telescope pointing to zenith. Mueller matrices have been estimated for the primary, secondary, and Nasmyth mirrors. It is found that some of the Mueller matrix elements of the primary and secondary mirrors show a fourfold azimuthal antisymmetry, which indicates that the polarization at the Cassegrain focus is negligible. At the inclined Nasmyth mirror, there is no azimuthal antisymmetry in the matrix elements, and this results in nonzero values for IP and CT, which would negatively impact the polarization measurements at the telescope focus. The averaged Mueller matrix is estimated at the Nasmyth focus at different instrument ports and various zenith angles of the telescope. The variation in the Mueller matrix elements for different coatings is also estimated. The impact of this polarization effect on the science case requirements has been discussed. This analysis will help in achieving precise requirements for future instruments with polarimetric capability.
The polarization introduced due to Thirty Meter Telescope (TMT) optics is calculated using an analytical model. Mueller matrices are also generated for each optical element using Zemax, based on which the instrumental polarization due to the entire system at the focal plane is estimated and compared with the analytical model. This study is significant in the estimation of the telescope sensitivity and also has great implications for future instruments.
The Thirty Meter Telescope (TMT) will be called upon to support a polarimetric observing capability. Many different observing programs covering a range of different science areas are being considered for the TMT and a model of the overall polarization characteristics is being developed. The instrument development program will provide a means for polarimetric instruments to be developed, however the telescope itself and the AO system must be able to support polarimetric instruments. As a first step to defining the necessary polarimetric technical requirements we have created an international working group to carry out a study in which technical and cost implications will be balanced with scientific impact; new requirements will be generated with supporting science cases. We present here initial results of the instrumental polarization sensitivity of TMT with NFIRAOS, the first-light adaptive optics system.
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