Achromatizing a liquid-crystal spectropolarimeter: Retardance vs Stokes-based calibration of HiVIS

TL;DR

This paper introduces a calibration method for high-resolution astronomical spectropolarimeters using liquid-crystal retarders, enabling achromatic polarization measurements despite chromatic instrument effects, and demonstrates its effectiveness in laboratory and telescope settings.

Contribution

The paper presents a novel Stokes-based calibration technique that achromatizes a liquid-crystal spectropolarimeter, improving measurement accuracy and reducing systematic errors.

Findings

Achieved achromatic polarization calibration with LCVRs.

Demonstrated calibration effectiveness in laboratory and telescope.

Enhanced spectropolarimeter sensitivity and accuracy.

Abstract

Astronomical spectropolarimeters can be subject to many sources of systematic error which limit the precision and accuracy of the instrument. We present a calibration method for observing high-resolution polarized spectra using chromatic liquid-crystal variable retarders (LCVRs). These LCVRs allow for polarimetric modulation of the incident light without any moving optics at frequencies >10Hz. We demonstrate a calibration method using pure Stokes input states that enables an achromatization of the system. This Stokes-based deprojection method reproduces input polarization even though highly chromatic instrument effects exist. This process is first demonstrated in a laboratory spectropolarimeter where we characterize the LCVRs and show example deprojections. The process is then implemented the a newly upgraded HiVIS spectropolarimeter on the 3.67m AEOS telescope. The HiVIS spectropolarimeter has also been expanded to include broad-band full-Stokes spectropolarimetry using achromatic wave-plates in addition to the tunable full-Stokes polarimetric mode using LCVRs. These two new polarimetric modes in combination with a new polarimetric calibration unit provide a much more sensitive polarimetric package with greatly reduced systematic error.