The Simons Observatory: Status of the Small-Aperture Telescope Program

The Simons Observatory (SO) is a cosmic microwave background (CMB) experiment consisting of a set of four telescopes sited at 5200 m elevation in the Atacama Desert of Chile. To pursue the key science goal of constraining the amplitude of primordial gravitational waves in the early universe, SO features three small-aperture telescopes (SATs) observing in atmospheric transmission windows centered at 90, 150, 220, and 280 GHz. Each instrument features a cryogenically-cooled rotating half-wave plate used as a polarization modulator, a refractive optical design featuring silicon lenses with metamaterial anti-reflection coatings, and a focal plane of seven close-packed arrays of horn-coupled transition-edge sensor bolometers. Each of the more than 3,000 pixels in the focal plane is sensitive to both linear polarizations in two frequency bands, with four bolometers per pixel for a total of 12,000 detectors. These devices are read out using GHz resonators in a microwave-multiplexing scheme. All three instruments are in the field and moving toward initial science observations of the CMB across roughly 10% of the sky. In this presentation, I will report on the as-deployed design and early performance results of the SATs, as well as the observing strategy planned for them and the ambitious science goals which they will enable when fully operational.