Abstract
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.
Highlights
Inflation is a theory that describes the entire observable universe as a microscopic volume that underwent violent, exponential expansion during the first tiny fraction of a second
Comparing the beam map simulations with regular simulations, where beam mismatch modes are derived from cosmic microwave background (CMB) temperature data itself, we find that the beam map simulations accurately predict the leakage of the beam mismatch modes, discussed fully in the Systematics Paper
We have fully described the optical system and characterized the optical performance of the BICEP2 experiment and the Keck Array 2012 and 2013 configurations
Summary
Inflation is a theory that describes the entire observable universe as a microscopic volume that underwent violent, exponential expansion during the first tiny fraction of a second (see Planck Collaboration 2014b for a review). BICEP2 and the Keck Array are microwave polarimeters that observe the CMB from the South Pole in search of a B-mode polarization signature from inflation (Ogburn et al 2010; Sheehy et al 2010; BICEP2 Collaboration 2014a). We report on beam characterization for BICEP2 and the 2012 and 2013 observing seasons of the Keck Array. Differential beam effects between colocated orthogonally polarized pairs of detectors can lead to leakage of the CMB temperature signal into the much smaller polarization signal. We discuss the construction of perdetector beam maps, which are inputs to simulations that are used to measure temperature to polarization leakage after removal of leading order contributions to beam mismatch between co-located orthogonally polarized detectors in a pair (Section 4)
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