Cosmology Large Angular Scale Surveyor (CLASS): Pointing Stability and Beam Measurements at 90, 150, and 220 GHz

TL;DR

The paper reports on the on-sky beam measurements and pointing stability of the CLASS telescope array at 90, 150, and 220 GHz, crucial for precise CMB polarization observations aimed at understanding fundamental cosmological parameters.

Contribution

It provides the first on-sky beam and pointing stability measurements at multiple frequencies for the CLASS telescope array, enhancing the accuracy of CMB polarization data analysis.

Findings

Beam FWHM at 90 GHz: 0.615 degrees

Beam FWHM at 150 GHz: 0.378 degrees

Beam FWHM at 220 GHz: 0.266 degrees

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) telescope array surveys 75% of the sky from the Atacama desert in Chile at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the largest-angular-scale CMB polarization with the aim of constraining the tensor-to-scalar ratio, measuring the optical depth to reionization to near the cosmic variance limit, and more. The CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic high frequency (150/220 GHz) telescopes have been observing since June 2016, May 2018, and September 2019, respectively. On-sky optical characterization of the 40 GHz instrument has been published. Here, we present preliminary on-sky measurements of the beams at 90, 150, and 220 GHz, and pointing stability of the 90 and 150/220 GHz telescopes. The average 90, 150, and 220 GHz beams measured from dedicated observations of Jupiter have full width at half maximum (FWHM) of 0.615+/-0.019 deg, 0.378+/-0.005 deg, and 0.266+/-0.008 deg, respectively. Telescope pointing variations are within a few percent of the beam FWHM.