Improving polarimetric accuracy of RoboPol to $<$ 0.05 % using a half-wave plate calibrator system

TL;DR

This paper presents a new half-wave plate calibrator system for RoboPol that significantly improves its polarimetric accuracy to below 0.05%, enabling more precise measurements across all broadband filters.

Contribution

Introduction of a rotating half-wave plate calibrator system that enhances RoboPol's polarimetric accuracy by a factor of two to three across all filters.

Findings

Achieved polarimetric accuracy better than 0.05% in $q$ and $u$

Improved polarization angle accuracy to 0.5 degrees

Enhanced instrument performance across all broadband filters

Abstract

RoboPol is a four-channel, one-shot linear optical polarimeter that has been successfully operating since 2013 on the 1.3 m telescope at Skinakas Observatory in Crete, Greece. Using its unique optical system, it measures the linear Stokes parameters $q$ and $u$ in a single exposure with high polarimetric accuracy of 0.1% - 0.15% and 1 degree in polarization angle in the R broadband filter. Its performance marginally degrades in other broadband filters. The source of the current instrumental performance limit has been identified as unaccounted and variable instrumental polarization, most likely originating from factors such as temperature and gravity-induced instrument flexure. To improve the performance of RoboPol in all broadband filters, including R, we have developed a rotating half-wave plate calibrator system. This calibrator system is placed at the beginning of the instrument and enables modulation of polarimetric measurements by beam swapping between all four channels of RoboPol. Using the new calibrator system, we observed multiple polarimetric standard stars over two annual observing seasons with RoboPol. This has enabled us to achieve a polarimetric accuracy of better than 0.05 % in both $q$ and $u$, and 0.5 degrees in polarization angle across all filters, enhancing the instrument's performance by a factor of two to three.