Characterizing Atacama B-mode Search Detectors with a Half-Wave Plate

TL;DR

The paper details the characterization of Atacama B-Mode Search detectors, focusing on the use of a rotating half-wave plate to improve measurement accuracy of CMB polarization at large scales.

Contribution

It introduces a method to use the second harmonic of the HWP rotation frequency for data selection and detector responsivity monitoring.

Findings

HWP rotation effectively mitigates systematic effects.

Second harmonic signal aids in data quality control.

Detectors successfully observed large-scale CMB polarization.

Abstract

The Atacama B-Mode Search (ABS) instrument is a cryogenic ($\sim$10 K) crossed-Dragone telescope located at an elevation of 5190 m in the Atacama Desert in Chile that observed for three seasons between February 2012 and October 2014. ABS observed the Cosmic Microwave Background (CMB) at large angular scales ($40<\ell<500$) to limit the B-mode polarization spectrum around the primordial B-mode peak from inflationary gravity waves at $\ell \sim100$. The ABS focal plane consists of 480 transition-edge sensor (TES) bolometers. They are coupled to orthogonal polarizations from a planar ortho-mode transducer (OMT) and observe at 145 GHz. ABS employs an ambient-temperature, rapidly rotating half-wave plate (HWP) to mitigate systematic effects and move the signal band away from atmospheric $1/f$ noise, allowing for the recovery of large angular scales. We discuss how the signal at the second harmonic of the HWP rotation frequency can be used for data selection and for monitoring the detector responsivities.