Development of a Precise Polarization Modulator for UV Spectropolarimetry

TL;DR

This paper presents a newly developed polarization modulation unit (PMU) for UV spectropolarimetry, demonstrating high precision in rotating waveplates with minimal errors, suitable for solar magnetic field observations.

Contribution

The paper introduces a novel PMU with high rotation uniformity, reducing measurement errors in UV spectropolarimetry for solar magnetic field studies.

Findings

Scale error and crosstalk <0.01% due to rotation non-uniformity

PMU successfully tested for the CLASP rocket experiment

Demonstrated high performance and suitability for spectropolarimetric measurements

Abstract

We developed a polarization modulation unit (PMU) to rotate a waveplate continuously in order to observe solar magnetic fields by spectropolarimetry. The non-uniformity of the PMU rotation may cause errors in the measurement of the degree of linear polarization (scale error) and its angle (crosstalk between Stokes-Q and -U), although it does not cause an artificial linear polarization signal (spurious polarization). We rotated a waveplate with the PMU to obtain a polarization modulation curve and estimated the scale error and crosstalk caused by the rotation non-uniformity. The estimated scale error and crosstalk were <0.01 % for both. This PMU will be used as a waveplate motor for the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) rocket experiment. We confirmed that the PMU has the sufficient performance and function for CLASP.