PROTOCALC, A W-band polarized calibrator for CMB Telescopes: application to Simons Observatory and CLASS

TL;DR

PROTOCALC is a drone-based polarized calibration source designed for 90 GHz CMB experiments, enabling precise instrument calibration crucial for future high-sensitivity polarization measurements.

Contribution

The paper introduces PROTOCALC, a novel drone-borne calibration source for CMB telescopes, with thorough lab testing, simulation modeling, and field deployment demonstrating high accuracy.

Findings

Achieved a polarization angle accuracy of 0.045 degrees.

Successfully integrated PROTOCALC into the Simons Observatory pipeline.

Demonstrated field deployment feasibility in the Atacama desert.

Abstract

Current- and next-generation Cosmic Microwave Background (CMB) experiments will measure polarization anisotropies with unprecedented sensitivities. The need for high precision in these measurements underscores the importance of gaining a comprehensive understanding of instrument properties, with a particular emphasis on the study of the beam properties and, in particular, their polarization characteristics, and the measurement of the polarization angle. In this context, a major challenge lies in the scarcity of millimeter polarized astrophysical sources with sufficient brightness and calibration knowledge to meet the stringent accuracy requirements of future CMB missions. This led to the development of a drone-borne calibration source designed for frequency band centered on approximately 90 GHz band, matching a commonly used channel in ground based CMB measurements. The PROTOtype CALibrator for Cosmology, PROTOCALC, has undergone thorough in-lab testing, and its properties have been subsequently modeled through simulation software integrated into the standard Simons Observatory (SO) analysis pipeline. Moreover, the PROTOCALC system has been tested in the field, having been deployed twice on calibration campaigns with CMB telescopes in the Atacama desert. The data collected constrain the roll angle of the source with a statistical accuracy of $0.045^\circ$.