Optical Characterization of the Keck Array Polarimeter at the South Pole

TL;DR

This paper details the optical characterization of the Keck Array polarimeters at the South Pole, focusing on beam shape and mismatch analysis to improve CMB polarization measurements related to inflation.

Contribution

It presents the first comprehensive optical characterization of the Keck Array's beam properties and their impact on polarization measurements, enhancing systematic control.

Findings

Beam shape and mismatch characterized accurately

Differential beam parameters quantified and analyzed

Implications for temperature to polarization leakage discussed

Abstract

The Keck Array (SPUD) is a set of microwave polarimeters that observes from the South Pole at degree angular scales in search of a signature of Inflation imprinted as B-mode polarization in the Cosmic Microwave Background (CMB). The first three Keck Array receivers were deployed during the 2010-2011 Austral summer, followed by two new receivers in the 2011-2012 summer season, completing the full five-receiver array. All five receivers are currently observing at 150 GHz. The Keck Array employs the field-proven BICEP/BICEP2 strategy of using small, cold, on-axis refractive optics, providing excellent control of systematics while maintaining a large field of view. This design allows for full characterization of far-field optical performance using microwave sources on the ground. We describe our efforts to characterize the main beam shape and beam shape mismatch between co-located orthogonally-polarized detector pairs, and discuss the implications of measured differential beam parameters on temperature to polarization leakage in CMB analysis.