Polarized Beam Patterns from a Multi-Moded Feed For Observations of the Cosmic Microwave Background

TL;DR

This paper measures and models the polarized beam patterns of a multi-moded feed used in CMB observations, quantifies the systematic errors from cross-polar response, and demonstrates correction methods to reduce these errors.

Contribution

It provides the first detailed measurement and modeling of polarized beam patterns for a multi-moded CMB feed, along with correction techniques for systematic errors.

Findings

Measured typical cross-polar error amplitude of 3 nK

Convolved beam patterns with CMB maps to estimate systematic errors

Demonstrated correction reduces errors to negligible levels

Abstract

We measure linearly polarized beam patterns for a multi-moded concentrator and compare the results to a simple model based on geometric optics. We convolve the measured co-polar and cross-polar beams with simulated maps of CMB polarization to estimate the amplitude of the systematic error resulting from the cross-polar beam response. The un-corrected error signal has typical amplitude of 3 nK, corresponding to inflationary B-mode amplitude r ~ 10^{-3}. Convolving the measured cross-polar beam pattern with maps of the CMB E-mode polarization provides a template for correcting the cross-polar response, reducing it to negligible levels.