Differential speckle polarimetry at Cassegrain and Nasmyth foci

Polarimetric interferometry is a method that allows the study of the distribution of polarized flux at diffraction-limited resolution. Its basic observable is the ratio R of the visibilities of the object in two orthogonal polarizations. Here we demonstrate how this observable can be measured with the SPeckle Polarimeter (SPP) of the Sternberg Astronomical Institute 2.5-m telescope. The SPP is a combination of a dual-beam polarimeter and an EMCCD-based visible-range speckle interferometer. We propose a simple method for the correction of R for the instrumental polarization and differential polarization aberrations of the telescope. The polarized intensity image can be estimated from R under the assumption that the object is a point-like unpolarized source plus a faint extended polarized envelope. The phase of R can be used for measurement of the polaroastrometric signal - the difference between the photocentres of orthogonally polarized images of the object. We investigate both possibilities using observations of unpolarized stars and stars with a significant polarized circumstellar environment - μ Cep and RY Tau.