The Simons Observatory: Laboratory Beam Characterization for the first Small Aperture Telescope

The Simons Observatory is a ground-based telescope array located at an elevation of 5200 meters, in the Atacama Desert in Chile, designed to measure the temperature and polarization of the cosmic microwave background. It comprises four telescopes: three 0.42-meter small aperture telescopes (SATs), focused on searching for primordial gravitational waves, and one 6-meter large aperture telescope, focused on studying small-scale perturbations. Each of the SATs will field over 12,000 TES bolometers, with two SATs sensitive to both 90 and 150 GHz frequency bands (SAT-MF1, and SAT-MF2), while the third SAT is sensitive to 220 and 280 GHz frequency bands. Prior to its deployment in 2023, the optical properties of SAT-MF1 were characterized in the laboratory. We report here on measurements of beam maps acquired using a thermal source on SAT-MF1, along with measurements of near-field beam maps using a holographic method that enables characterization of both the amplitude and phase of the beam response, yielding an estimate of the far-field radiation pattern received by the telescope. We find that the near-field half-width-half-maximum (HWHM) requirements are met across the focal plane array for the 90 GHz frequency band, and through most of the focal plane array for the 150 GHz frequency band. Namely, the mean of the bandpass averaged HWHM of the edge-detector universal focal plane modules match the simulated HWHM to 10.4 %, with the discrepancy caused by fringing in the simulation. The measured radial profile of the beams matches simulations to within 2 dB from the beam center to at least the -10 dB level. Holography estimates of the far-field 90 GHz beams match the full-width-half-maximum from simulation within 1%, and the beam radial profiles deviate by less than 2 dB inside the central lobe. The success of the holography and thermal beam map experiments confirmed the optical performance were sufficient to meet the science requirements. SAT-MF1 was deployed to Chile in June, 2023. On-site observations are currently underway.