Development of an Optical Detector Testbed for the Simons Observatory

The Simons Observatory (SO) is a cosmic microwave background (CMB) survey experiment with three small-aperture telescopes and one large-aperture telescope, which will observe from the Atacama Desert in Chile. In total, SO will field over 60,000 transition edge sensor (TES) bolometers in six spectral bands centered between 27 and 280 GHz in order to achieve the sensitivity necessary to measure or constrain numerous cosmological quantities, as outlined in The Simons Observatory Collaboration et al. (2019). To verify consistency of fabrication and performance in line with our sensitivity requirements, we will perform in-lab optical tests1 on isolated SO detectors as well as full detector arrays. The tests include beam measurements, bandpass measurements, and polarization measurements, among others. Here, we will describe the development of a cryogenic testbed that enables optical characterization of SO's detectors. We include the infrared filtering strategy to allow suitable cryogenic performance, design and implementation of the test equipment used in characterization, and the preliminary results from our validation of the testbed's cryo-optical performance.