# Differential speckle polarimetry at Cassegrain and Nasmyth foci

## TL;DR

This paper demonstrates how differential speckle polarimetry can be performed at Cassegrain and Nasmyth foci using the SPeckle Polarimeter, enabling high-resolution polarization measurements and polaroastrometry of stars with polarized environments.

## Contribution

It introduces a method for measuring the polarization ratio with speckle interferometry and corrects for instrumental effects, expanding high-resolution polarimetric capabilities.

## Key findings

- Successful measurement of polarization ratio using speckle interferometry.
- Correction method for instrumental polarization and aberrations.
- Application to stars with polarized circumstellar environments.

## Abstract

Polarimetric interferometry is a method allowing the study of the distribution of polarized flux at diffraction-limited resolution. Its basic observable is the ratio $\mathcal{R}$ of the visibilities of the object in two orthogonal polarizations. Here, we demonstrate how this observables can be measured with the SPeckle Polarimeter (SPP) of the 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 $\mathcal{R}$ for the instrumental polarization and polarization differential aberrations of the telescope. The polarized intensity image can be estimated from $\mathcal{R}$ under the assumption that the object is a point-like unpolarized source plus a faint extended polarized envelope. The phase of $\mathcal{R}$ 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 - $\mu$~Cep and RY~Tau.