The Cosmology Large Angular Scale Surveyor (CLASS): 40 GHz optical design

TL;DR

This paper details the optical design of the 40 GHz telescope for the CLASS experiment, which aims to measure cosmic microwave background polarization with high precision using a specialized diffraction-limited system.

Contribution

It introduces a novel 40 GHz optical system with a diffraction-limited catadioptric design, integrating advanced polarization modulation and detector coupling for CMB observations.

Findings

Designed a 40 GHz optical system with 1.5° beam resolution.

Implemented a polarization modulation system with VPM.

Optimized stray light control with a Lyot-stop.

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) instrument will measure the polarization of the cosmic microwave background at 40, 90, and 150 GHz from Cerro Toco in the Atacama desert of northern Chile. In this paper, we describe the optical design of the 40 GHz telescope system. The telescope is a diffraction limited catadioptric design consisting of a front-end Variable-delay Polarization Modulator (VPM), two ambient temperature mirrors, two cryogenic dielectric lenses, thermal blocking filters, and an array of 36 smooth-wall scalar feedhorn antennas. The feed horns guide the signal to antenna-coupled transition-edge sensor (TES) bolometers. Polarization diplexing and bandpass definition are handled on the same microchip as the TES. The feed horn beams are truncated with 10 dB edge taper by a 4 K Lyot-stop to limit detector loading from stray light and control the edge illumination of the front-end VPM. The field-of-view is 19deg x 14deg with a resolution for each beam on the sky of 1.5deg FWHM.