BICEP/Keck XV: The BICEP3 Cosmic Microwave Background Polarimeter and the First Three-year Data Set

We report on the design and performance of the BICEP3 instrument and its first three-year data set collected from 2016 to 2018. BICEP3 is a 52 cm aperture refracting telescope designed to observe the polarization of the cosmic microwave background (CMB) on degree angular scales at 95 GHz. It started science observation at the South Pole in 2016 with 2400 antenna-coupled transition-edge sensor bolometers. The receiver first demonstrated new technologies such as large-diameter alumina optics, Zotefoam infrared filters, and flux-activated SQUIDs, allowing similar to 10x higher optical throughput compared to the Keck design. BICEP3 achieved instrument noise equivalent temperatures of 9.2, 6.8, and 7.1 mu K-CMB root s and reached Stokes Q and U map depths of 5.9, 4.4, and 4.4 mu K arcmin in 2016, 2017, and 2018, respectively. The combined three-year data set achieved a polarization map depth of 2.8 mu K arcmin over an effective area of 585 square degrees, which is the deepest CMB polarization map made to date at 95 GHz.