Design of a Full-Stokes Polarimeter for VLT/X-shooter

TL;DR

This paper presents a novel full-Stokes polarimeter design for VLT/X-shooter, enabling comprehensive polarization measurements across a broad wavelength range with optimized efficiency and minimal sensitivity issues.

Contribution

It introduces a new polychromatic modulator and optical design for a full-Stokes polarimeter compatible with X-shooter, covering 300-2500 nm.

Findings

Achieves near-optimal polarimetric efficiencies across all wavelengths.

Analyzes temperature sensitivity and polarized fringes of the modulator.

Design supports diverse astrophysical polarization measurements.

Abstract

X-shooter is one of the most popular instruments at the VLT, offering instantaneous spectroscopy from 300 to 2500 nm. We present the design of a single polarimetric unit at the polarization-free Cassegrain focus that serves all three spectrograph arms of X-shooter. It consists of a calcite Savart plate as a polarizing beam-splitter and a rotatable crystal retarder stack as a "polychromatic modulator". Since even "superachromatic" wave plates have a wavelength range that is too limited for X-shooter, this novel modulator is designed to offer close-to-optimal polarimetric efficiencies for all Stokes parameters at all wavelengths. We analyze the modulator design in terms of its polarimetric performance, its temperature sensitivity, and its polarized fringes. Furthermore, we present the optical design of the polarimetric unit. The X-shooter polarimeter will furnish a myriad of science cases: from measuring stellar magnetic fields (e.g., Ap stars, white dwarfs, massive stars) to determining asymmetric structures around young stars and in supernova explosions.