The Cosmology Large Angular Scale Surveyor

TL;DR

The CLASS project is a four-telescope array designed to measure the polarized cosmic microwave background on large scales to detect primordial gravitational waves and better understand reionization, utilizing multiple frequencies, high sensitivity detectors, and a high-altitude site.

Contribution

This paper introduces the design and capabilities of the CLASS instrument, highlighting its unique combination of broad frequency coverage, large sky coverage, and systematics control for CMB polarization measurements.

Findings

Will measure optical depth to reionization accurately

Aims to detect or constrain tensor-to-scalar ratio down to 0.01

Provides a new approach to large-scale CMB polarization observation

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) is a four telescope array designed to characterize relic primordial gravitational waves from inflation and the optical depth to reionization through a measurement of the polarized cosmic microwave background (CMB) on the largest angular scales. The frequencies of the four CLASS telescopes, one at 38 GHz, two at 93 GHz, and one dichroic system at 145/217 GHz, are chosen to avoid spectral regions of high atmospheric emission and span the minimum of the polarized Galactic foregrounds: synchrotron emission at lower frequencies and dust emission at higher frequencies. Low-noise transition edge sensor detectors and a rapid front-end polarization modulator provide a unique combination of high sensitivity, stability, and control of systematics. The CLASS site, at 5200 m in the Chilean Atacama desert, allows for daily mapping of up to 70\% of the sky and enables the characterization of CMB polarization at the largest angular scales. Using this combination of a broad frequency range, large sky coverage, control over systematics, and high sensitivity, CLASS will observe the reionization and recombination peaks of the CMB E- and B-mode power spectra. CLASS will make a cosmic variance limited measurement of the optical depth to reionization and will measure or place upper limits on the tensor-to-scalar ratio, $r$, down to a level of 0.01 (95\% C.L.).