The Cosmology Large Angular Scale Surveyor Receiver Design

TL;DR

This paper details the design and performance of the receiver system for the CLASS CMB polarization survey, highlighting cryogenic cooling, efficiency, and optical alignment to optimize measurements.

Contribution

It presents the detailed design and operational performance of the CLASS receiver, including cryogenic cooling, optical efficiency, and system alignment improvements.

Findings

Achieved detector temperatures below 50 mK in the 40 GHz receiver.

Maintained high in-band efficiency consistent with estimates.

Less than 26% of in-band power lost to filters and lenses.

Abstract

The Cosmology Large Angular Scale Surveyor consists of four instruments performing a CMB polarization survey. Currently, the 40 GHz and first 90 GHz instruments are deployed and observing, with the second 90 GHz and a multichroic 150/220 GHz instrument to follow. The receiver is a central component of each instrument's design and functionality. This paper describes the CLASS receiver design, using the first 90 GHz receiver as a primary reference. Cryogenic cooling and filters maintain a cold, low-noise environment for the detectors. We have achieved receiver detector temperatures below 50 mK in the 40 GHz instrument for 85% of the initial 1.5 years of operation, and observed in-band efficiency that is consistent with pre-deployment estimates. At 90 GHz, less than 26% of in-band power is lost to the filters and lenses in the receiver, allowing for high optical efficiency. We discuss the mounting scheme for the filters and lenses, the alignment of the cold optics and detectors, stray light control, and magnetic shielding.