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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