Performance characterization of the transition-edge sensor array, readout system, and optical coupling of the Simons...

High-sensitivity measurements of the cosmic microwave background (CMB) polarization are critical for constraining cosmological parameters and probing the physics of the early universe. Simons Array is an experiment designed to observe the CMB polarization at 90 and 150 GHz from the Atacama Desert in Chile, at an altitude of 5200 m. Its deployed receivers, polarbear-2a and polarbear-2b, employed over 15 000 transition-edge sensors (TESs) with dichroic, dual-polarization pixels using lenslet-coupled sinuous antennas. The readout of thousands of cryogenic detectors was accomplished using 40× frequency-domain multiplexing. In this work, we characterize the achieved on-sky sensitivity of the Simons Array detectors. First, we characterize detector properties to model TES responsivity, estimate the receivers' optical throughput, and model the readout noise contributions to the total noise of operating detectors. Then, we report the calibrated detector sensitivity and compare it to predictions based on the evaluated detector, readout, and optical properties. We find that the on-sky noise equivalent temperature of the Simons Array detectors is higher than the design goal and show that this is a consequence of unexpectedly low optical throughput and elevated readout noise. Our results show that the performance of polarbear-2a can be predicted with an updated model of detector sensitivity, incorporating data from field measurements. However, we find that polarbear-2b had further degraded performance that may be primarily due to increased susceptibility to radio frequency interference. The methods described in this investigation can be directly applied to the characterization of other instruments that employ TES arrays and/or frequency-domain multiplexing.