Whilst many algorithms exist for interferometric image reconstruction, there are not yet algorithms for polarimetric interferometric image reconstruction. The polarisation state of light contains critical information otherwise uncaptured by standard, unpolarised interferometry, and many major facilities are now looking towards fully leveraging this information to broaden the observational reach of new and existing instruments. Polarimetric image reconstruction has additional challenges compared to unpolarised image reconstruction, as reconstructions of polarised images (Stokes I, Q and U) are spatial maps of vector components. As such, they need to individually and collectively display physically realistic and mutually consistent scattering physics. Within the present work, we demonstrate that a two-stage machine learning framework (a convolutional neural network (CNN) + iterative fitting) can be used to successfully perform polarimetric image reconstruction, whilst satisfying these challenging regularisation requirements. Using a custom set of MCFOST radiative transfer models, we train a convolutional neural network to learn the mapping between polarised images and interferometric polarimetric observables. We then deploy an iterative fitting mechanism inspired by the Deep Image Prior, which iteratively improves the fit of polarimetric observables with cognisance of observational errors. In particular, the improvement provided by iterative fitting also results in the reconstruction of physically meaningful image structures that were missing from the original CNN image reconstruction. Our results suggest that this two-stage framework is a powerful tool for performing image reconstruction with complex regularisation constraints - in both polarimetric and non-polarimetric contexts. Here we briefly report our algorithm and initial results.
Speckle polarimeter (SPP) is a facility instrument of the 2.5-m telescope of the Caucasian Mountain Observatory of SAI MSU. By design it is a combination of a speckle interferometer and a dual–beam polarimeter. In 2022 we performed a major upgrade of the instrument. New version of the instrument features Hamamatsu ORCA-Quest qCMOS C15550-20UP, having subelectron readout noise, as a main detector, as opposed to EMCCD Andor iXon 897 used in previous version. Optical distortions present in the instrument are considered as they directly affect the accuracy of the speckle interferometric astrometric measurements of binary stars. We identified the Atmospheric Dispersion Compensator (ADC) as the main source of distortions which are not constant and depend on the rotational angles of ADCs prisms. Distortions are estimated using internal calibration light source and multiple binary stars measurements. Method for their correction is developed. Flux ratio estimates are subject to CMOS-specific negative factors: spatially correlated noise and flux–dependent pixel–to–pixel sensitivity difference. We suggest ways to mitigate these factors. The use of speckle transfer function measured using a reference star further improves flux ratio estimation performance. We discuss the precision of the estimates of position angle, separation and flux ratio of binary stars.
We present upgrades to the visible-light (600nm to 800 nm) high-contrast imaging polarimeter, VAMPIRES, installed on the Subaru Coronagraphic Extreme Adaptive Optics instrument (SCExAO). These upgrades included low read noise, high dynamic range qCMOS detectors, a novel dichroic-based multiband imaging optic for spectral differential imaging, a suite of classic Lyot coronagraphs, and an achromatic fast polarization modulator. VAMPIRES is capable of diffraction-limited imaging (17 mas to 22 mas angular resolution) and achieves Strehl ratios up to ∼60% with 5σ coronagraphic contrast of 10-4 at 0.1′′ and 10-6 at >0.4′′. We highlight commissioning results from HD 169142, R Aqr, Neptune, and HD 1160B.
The Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES) is a visible light instrument on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. In a previous work, the instrumental polarization (IP) and crosstalk of VAMPIRES was characterized from measurements with a polarized internal calibration source. In this work, we use unpolarized standard star observations to estimate the polarization of Subaru’s Nasmyth mirror (M3) and use polarized standard star observations to quantify the overall system Mueller matrix model’s accuracy. We present an instrument matrix inversion routine that has returned stable results for all tested on-sky sources. With the current 675nm Mueller matrix model of VAMPIRES, the median residuals when retrieving the on-sky degree of linear polarization (DoLP) and angle of linear polarization (AoLP) of three polarized standards were 0.27% and 3.57% respectively.
The Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES) is a visible light instrument on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. To characterize the Instrumental Polarization (IP) and crosstalk of the system, measurements with a polarized internal calibration source were taken to provide diattenuation and retardance estimates of components downstream of the telescope mirrors. Using Markov Chain Monte Carlo (MCMC) simulations, we developed a Mueller matrix model of VAMPIRES in five wavelength bands. The process of deriving the model revealed degeneracies involving multiple possible sets of values for highly correlated instrument parameters. It also revealed a significant difference in the EM gain ratio between VAMPIRES’ two cameras and the highly non-ideal behavior of the Ferroelectric Liquid Crystal (FLC). Observations of unpolarized and polarized standard stars from two separate nights have been collected and are currently being analyzed to provide an estimate of M3’s diattenuation and retardance as well, and the polarized standard stars in particular will be used as a final metric to quantify the instrument model’s accuracy. The current Mueller matrix model has already been applied to preliminary data analysis for the circumstellar disk HD34700 and has helped confirm that an observed 17° phase shift in differential visibilities for Mu Cephei is astrophysical in nature. The non-ideal behaviour of certain components highlights the need for a standard calibration procedure that should be performed regularly to track changes in VAMPIRES’ polarization qualities. Our laboratory and on-sky data collection and analysis procedures will be refined to be re-used to calibrate future versions of VAMPIRES, including its most recent upgrade in 2023.
The differential polarization visibilities RQ and RU of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. We demonstrate the estimation of both amplitude and phase of RQ and RU from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the differential speckle polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of RQ and RU measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 × 10 − 3.
The differential polarization visibilities RQ and RU of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used e.g. for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. Here we demonstrate the estimation of both amplitude and phase of RQ and RU from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the Differential Speckle Polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of RQ, RU measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 x 10-3.
Here we demonstrate how diffraction limited polarized flux imaging at single filled-aperture telescope can be implemented using an instrument combining properties of a speckle interferometer and dual—beam polarimeter. By processing of a series of short-exposure, seeing limited images of an object obtained at orthogonal polarizations simultaneously the ratio of its visibilities at orthogonal polarizations is estimated. Two such differential polarimetric visibilities can be defined: for Stokes Q and U; both the amplitude and argument of them are being estimated. Assuming that the object under study is dominated by unpolarized unresolved source, e.g. star, the image of polarized circumstellar envelope can be recovered from the described ratio of visibilities. With wpeckle polarimeter, an instrument implementing DSP at 2.5-m SAI MSU telescope, differential polarimetric visibility can be measured with precision of 2.2 10−3 for Rc = 6 star in 150 sec of accumulation. This precision is significantly higher than precision of visibility measurement using conventional speckle interferometry in the same conditions. Using laboratory measurements we show that in current design differential polarization aberrations of instrument and telescope do not limit the precision of instrument for targets fainter than Rc = 6. The latter is mainly defined by photon noise and detector noise.
On the basis of 95500 MASS/DIMM measurements of optical turbulence profile obtained on Mt. Shatdzhatmaz
in 2009-2011 we have constructed 2 atmosphere models for this summit: one consisting of 9 typical turbulence
profiles, and other of 300 randomly selected profiles. Profiles represent Cn2 and wind speed values estimated at
13 standard MASS altitudes. We discuss advantages and disadvantages of these models from the point of view of
AO simulation. We used these models as input parameters in analytical simulation (PAOLA tool) of AO system
of future 2.5-m telescope of SAI. This simulation was used to estimate optimal parameters of this system and
performance characteristics corresponding to these parameters. Second 300-profiles atmosphere model allowed
to evaluate the performance in terms of statistical distributions of metrics. E.g. for subaperture size 0.35 m and
optimizable exposure (lower limit 2 ms), the NGS AO system will deliver Strehl ratio more than 0.46 in R-band
using R=13 guide star for 10% of time.
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