Calibration Measurements of the BICEP3 and BICEP Array CMB Polarimeters from 2017 to 2024

TL;DR

This paper details calibration procedures for BICEP3 and BICEP Array CMB polarimeters, focusing on beam response and systematic control essential for detecting primordial gravitational waves in the CMB.

Contribution

It introduces comprehensive calibration measurement techniques for BICEP instruments, improving systematic control and informing future CMB receiver designs.

Findings

Precise beam calibration measurements conducted annually.

Identification and mitigation of instrumental systematics.

Insights into sidelobe effects on CMB polarization measurements.

Abstract

The BICEP3 and BICEP Array polarimeters are small-aperture refracting telescopes located at the South Pole designed to measure primordial gravitational wave signatures in the Cosmic Microwave Background (CMB) polarization, predicted by inflation. Constraining the inflationary signal requires not only excellent sensitivity, but also careful control of instrumental systematics. Both instruments use antenna-coupled orthogonally polarized detector pairs, and the polarized sky signal is reconstructed by taking the difference in each detector pair. As a result, the differential response between detectors within a pair becomes an important systematic effect we must control. Additionally, mapping the intensity and polarization response in regions away from the main beam can inform how sidelobe levels affect CMB measurements. Extensive calibration measurements are taken in situ every austral summer for control of instrumental systematics and instrument characterisation. In this work, we detail the set of beam calibration measurements that we conduct on the BICEP receivers, from deep measurements of main beam response to polarized beam response and sidelobe mapping. We discuss the impact of these measurements for instrumental systematics studies and design choices for future CMB receivers.