Calculated Ellipsometry of the VLTI AT mirror train

TL;DR

This paper calculates the polarization effects caused by reflections in the VLTI AT mirror train, providing insights into how pointing and rotator angles influence polarization states for interferometric observations.

Contribution

It introduces a method to compute polarization effects through the entire mirror train using Jones matrices and estimates the net polarization change based on the mirror properties.

Findings

Polarization ellipse axis ratio varies with pointing direction.

The model predicts polarization effects for different telescope orientations.

Results aid in calibrating polarization effects in VLTI observations.

Abstract

The polarization effect of the 31 reflections within the mirror train of an auxiliary telescope of the Very Large Telescope Interferometer is calculated as a function of pointing direction and rotator angle. With a rough estimate of the mean complex index of refraction of the reflecting surfaces, their Jones matrices are concatenated while tracing a ray from M1 up to the feeding mirror in front of an instrument. The net effect is summarized in terms of the axis ratio of the polarization ellipse of star light that was circularly polarized above the primary mirror.