UC San Diego

Measurements of the polarization of the cosmic microwave background (CMB) provide powerful probes of early-universe physics and cosmology, including for evidence of cosmic inflation. The Simons Observatory (SO) is a ground-based survey experiment designed to measure the CMB polarization signal using three small aperture telescopes (SATs) and one large aperture telescope (LAT) from the Atacama Desert in Chile. In order to achieve the required sensitivity, SO will field over 60,000 transition edge sensor (TES) bolometers in six spectral bands ranging from 27 to 280 GHz. Before being installed into the receivers, the detectors' dark and optical properties must be characterized in in-lab testbeds. A testbed specifically focused on optical characterization of the detector modules was developed at UC San Diego, along with required test equipment to enable these measurements. Following this, detector modules were integrated into the full receiver, where we perform final validation. The first-light instrument for SO, the first SAT, was integrated at UC San Diego, where we characterized the instrument beam, spectral properties, and polarization performance, among other tests. The testing, as well as field operation, requires new software implementation for detector control and analysis, as well as metadata packaging for science-grade analysis. These software packages were developed and tested during the lab testing program described here, both in the optical testbed and the fully integrated SAT cryogenic receiver.