A multi-spectral band stellar photo-polarimeter

TL;DR

This paper presents a novel multi-spectral stellar photo-polarimeter capable of simultaneous polarization measurements in three spectral bands, with improved accuracy and efficiency for astronomical observations.

Contribution

The design introduces a beam-displacement prism analyzer and a high-speed chopper for quasi-simultaneous detection, enhancing measurement accuracy and reducing background polarization errors.

Findings

Performance tests show satisfactory results with polarized and unpolarized stars.

The instrument reduces sky background polarization effects during moonlit nights.

Efficiency is balanced by reduced sky observation time.

Abstract

We designed and built a new astronomical photo-polarimeter that can measure linear polarization simultaneously in three spectral bands. It has a Calcite beamdisplacement prism as the analyzer. The ordinary and extra-ordinary emerging beams in each spectral bands are quasi-simultaneously detected by the same photomultiplier by using a high speed rotating chopper. A rotating superachromatic Pancharatnam halfwave plate is used to modulate the light incident on the analyzer. The spectral bands are isolated using appropriate dichroic and glass filters. We show that the reduction of 50% in the efficiency of the polarimeter because of the fact that the intensities of the two beams are measured alternately is partly compensated by the reduced time to be spent on the observation of the sky background. The use of a beam-displacement prism as the analyzer completely removes the polarization of background skylight, which is a major source of error during moonlit nights, especially, in the case of faint stars. The field trials that were carried out by observing several polarized and unpolarized stars show the performance of the polarimeter to be satisfactory.