Variable-delay Polarization Modulators for the CLASS Telescopes

TL;DR

This paper discusses the development and implementation of variable-delay polarization modulators (VPMs) in the CLASS telescopes to improve polarization measurements of the CMB by reducing systematic noise and separating sky polarization from instrumental effects.

Contribution

It introduces the design, deployment, and operational use of VPMs in the CLASS telescopes for the first time in CMB polarization experiments.

Findings

VPMs enable rapid polarization modulation in the CLASS telescopes.

The Q-band VPM began full-time observations in Spring 2016.

The W-band VPM was installed in Spring 2018.

Abstract

The search for inflationary primordial gravitational waves and the measurement of the optical depth to reionization, both through their imprint on the large angular scale correlations in the polarization of the cosmic microwave background (CMB), has created the need for high sensitivity measurements of polarization across large fractions of the sky at millimeter wavelengths. These measurements are subject to instrumental and atmospheric $1/f$ noise, which has motivated the development of polarization modulators to facilitate the rejection of these large systematic effects. Variable-delay polarization modulators (VPMs) are used in the Cosmology Large Angular Scale Surveyor (CLASS) telescopes as the first element in the optical chain to rapidly modulate the incoming polarization. VPMs consist of a linearly polarizing wire grid in front of a movable flat mirror. Varying the distance between the grid and the mirror produces a changing phase shift between polarization states parallel and perpendicular to the grid which modulates Stokes U (linear polarization at $45^\circ$) and Stokes V (circular polarization). The CLASS telescopes have VPMs as the first optical element from the sky; this simultaneously allows a lock-in style polarization measurement and the separation of sky polarization from any instrumental polarization further along in the optical path. The Q-band CLASS VPM was the first VPM to begin observing the CMB full time, starting in the Spring of 2016. The first W-band CLASS VPM was installed in the Spring of 2018.