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.
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.
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