2017 upgrade and performance of BICEP3: a 95GHz refracting telescope for
  degree-scale CMB polarization
  BICEP3 is a 520mm aperture on-axis refracting telescope observing the
polarization of the cosmic microwave background (CMB) at 95GHz in search of the
B-mode signal originating from inflationary gravitational waves. BICEP3's focal
plane is populated with modularized tiles of antenna-coupled transition edge
sensor (TES) bolometers. BICEP3 was deployed to the South Pole during 2014-15
austral summer and has been operational since. During the 2016-17 austral
summer, we implemented changes to optical elements that lead to better noise
performance. We discuss this upgrade and show the performance of BICEP3 at its
full mapping speed from the 2017 and 2018 observing seasons. BICEP3 achieves an
order-of-magnitude improvement in mapping speed compared to a Keck 95GHz
receiver. We demonstrate $6.6\mu K\sqrt{s}$ noise performance of the BICEP3
receiver.
