BICEP2/Keck Array IV: Optical Characterization and Performance of the
  BICEP2 and Keck Array Experiments
  BICEP2 and the Keck Array are polarization-sensitive microwave telescopes
that observe the cosmic microwave background (CMB) from the South Pole at
degree angular scales in search of a signature of inflation imprinted as B-mode
polarization in the CMB. BICEP2 was deployed in late 2009, observed for three
years until the end of 2012 at 150 GHz with 512 antenna-coupled transition edge
sensor bolometers, and has reported a detection of B-mode polarization on
degree angular scales. The Keck Array was first deployed in late 2010 and will
observe through 2016 with five receivers at several frequencies (95, 150, and
220 GHz). BICEP2 and the Keck Array share a common optical design and employ
the field-proven BICEP1 strategy of using small-aperture, cold, on-axis
refractive optics, providing excellent control of systematics while maintaining
a large field of view. This design allows for full characterization of
far-field optical performance using microwave sources on the ground. Here we
describe the optical design of both instruments and report a full
characterization of the optical performance and beams of BICEP2 and the Keck
Array at 150 GHz.
