The Simons Observatory: calibration and characterization of the first small-aperture telescope

The Simons Observatory (SO) group of instruments are together pursuing a major step forward in the ground-based study of the Cosmic Microwave Background (CMB). With one 6 m large-aperture telescope and three 0.4 m small-aperture telescopes (SATs), SO will strive to recover faint CMB polarization signals at a wide range of angular scales and across six frequency bands inside of atmospheric transmission windows spanning the range 27 GHz to 280 GHz. The first instrument to record celestial light is the first of two mid-frequency SATs, SAT MF-1, with over 3,000 dichroic pixels sensitive to two frequency bands centered at 90 and 150 GHz. This instrument began observing in October 2023, and features a cryogenically-cooled polarization modulator consisting of a spinning half-wave plate, a set of three silicon lenses with metamaterial anti-reflection coating, and a focal plane of seven modules referred to as universal focal-plane modules (UFMs), each containing 1,720 AlMn transition-edge sensor (TES) bolometers coupled to a 100 mK bath. In this proceedings, we report on initial efforts to calibrate the TES bolometer response to electrical and optical signals and preliminary characterization of possible confounding signals like scan-synchronous pickup. We comment on how these elements pertain to the analysis of systematic errors relating to the ultimate goal of the SO SAT program: the further constraint of the tensor-to-scalar ratio, r, and the possibility of primordial gravitational waves generated in the early universe by a period of inflation.