Speckle polarimeter is a facility instrument of 2.5-m telescope of CO SAI MSU which aims to investigate spatial structure and polarization of astrophysical objects at diffraction limited resolution between 400 and 1100 nm. By design it is a combination of two-beam polarimeter with half--wave plate and speckle interferometer. A fast Electron Multiplying CCD is used as a detector.

Recently published episodic spectral and high angular resolution observations of Betelgeuse suggest that the deep minimum of 2019-2020 was caused by an enhanced dust abundance in the stellar atmosphere. Detailed monitoring of such events may prove useful for constructing consistent physical models of mass loss by evolved stars. For such observations it is fundamentally important to employ methods resolving an inhomogeneous stellar atmosphere.

We present the differential speckle polarimetric observations of Betelgeuse at 2.5-m telescope of Caucasian Mountain Observatory of SAI MSU covering the period of 2019-2020 minimum...

Here we give computation of basic parameters of atmospheric dispersion compensator which are needed for manufacturing and further use. ADC consists of two non-deviating prisms independtly rotating around optical axis. Thus the angular dispersion of arbitrary value (up to a certain limit) and orientation can be produced. For magnification of 1000 (case of 2.5 m telescope) the compensator will work up to zenith distance of 71 degrees. Each prism consists of F1 glass and fused silica with geometry optimized for the best accordance with atmospheric dispersion between wavelenghts of 400 and 900 nm.

A method for measuring the difference between centroids of polarized flux and total flux of an astronomical object – polaroastrometry – is proposed. The deviation of the centroid of flux corresponding to Stokes parameter Q or U from the centroid of total flux multiplied by dimensionless Stokes parameter q or u, respectively, was used as a signal. The efficiency of the method is demonstrated on the basis of observations made in the V band by using an instrument combining features of a two-beam polarimeter with a rotating half-wave plate and a speckle interferometer. The polaroastrometric signal noise is 60-70 μas rms for a total number of accumulated photoelectrons Ne of 10^9 from a 70-cm telescope; this corresponds to a total integration time of 500 s and an object magnitude V = 6 mag. At smaller Ne, the noise increases as ≈ 1.7 arcsec/√Ne, while at larger Ne, it remains the same owing to imperfection of the half-wave plate. For main-sequence stars that are unpolarized and polarized by interstellar dust and the Mira-type variable R Tri the signal was undetectable. For the Mira-type variable chi Cyg the polaroastrometric signal is found to be 310±70 and 300±70 muas for Stokes Q and U, respectively; for o Cet these values are 490±100 and 1160±100 muas. The significant value of the polaroastrometric signal provides evidence of the asymmetry of the polarized flux distribution.

Prototype instrument is a two-beam polarimeter with rotating half-wave plate. It works in V band, polarization fraction and angle are measures. Double difference method is described. On basis of laboratory measurements we show that expected error in determination of dimensionless Stokes parameters is 2e-4 – 3e-4 for objects with polarization of several percents. This level of error was observed indeed for three polarization standards and three non-polarized stars. For other three polarization standards observed values of polarization differ significantly from expected ones, we discuss possible reasons. Instrumental polarization of AZT-2 telescope has been evaluated.

We propose the model adequately describing observed level of measurements uncertainties. Model includes photon noise, readout noise and additional noise of unknown nature which doesn't depend on object brightness. The latter limits accuracy of measurements at 1e-4 - 2e-4 and dominates for objects brighter than V=4. For fainter objects photon noise dominates.

Polarization measurements for three Mirae are given, we note significant change in polarization for two epochs separated by 2 months.

Slightly scattered description of design solutions applied in MFC prototype of stage 4. Optical scheme: selection of half-wave plate, selection of beam division element, evaluation of its dispersion effect, selection of optical elements arrangement. Description of optomechanics.

In this report we present results of first test speckle interferometric observations. We used 70-cm telescope AZT-2 located near SAI building on Vorobyevy Gory in Moscow as a feeding optics. Camera consisted of a EMCCD sensor Andor iXon+897, simple lens reducing focal ratio to F/64 and filter V or interferometric filter with center wavelength 643.4 nm and FWHM 7.9 nm. Angular scale was measured using aperture mask to be 69.98 mas/pix.

From observations of a bright single star we estimated: speckle transfer function, its dependece on wavelength and exposure, FWHM of long-exposure image, image motion spectrum and amplitude, Strehl number of short-exposure images.

Also we observed 20 double stars, which includes both binaries and optical pairs. We measured separation, position angle and magnitude difference. For stars with separation greater than 4" these parameters were estimated from long-exposure images. For closer pairs we performed approximation of averaged power spectrum, i.e. speckle interferometry. Results were compared to ephemeris.

In this report we present the results of first phase of investigation of EMCCD sensor iXon+897 (model DU-897E-CS0-#BV, serial X-5947). The sensor is intended for installation in Multimode Fast Camera (MFC).

Report consists of two parts. In first part basic parameters are given: sensitivity (preamp), readout noise, electron multiplication coefficient. Second part is dedicated for more specific characteristics of sensor: dark current, bias stability, flat field inhomogeneity, flat field stability, linearity, vertical clocking effiency, CIC noise. Measurements were made with no illumination and uniform illumination.