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Published August 19, 2014 | Published
Book Section - Chapter Open

BICEP2 and Keck Array: upgrades and improved beam characterization


Searching for evidence of inflation by measuring B-modes in the cosmic microwave background (CMB) polarization at degree angular scales remains one of the most compelling experimental challenges in cosmology. BICEP2 and the Keck Array are part of a program of experiments at the South Pole whose main goal is to achieve the sensitivity and systematic control necessary for measurements of the tensor-to-scalar ratio at σ(r) ~0:01. Beam imperfections that are not sufficiently accounted for are a potential source of spurious polarization that could interfere with that goal. The strategy of BICEP2 and the Keck Array is to completely characterize their telescopes' polarized beam response with a combination of in-lab, pre-deployment, and on-site calibrations. We report the status of these experiments, focusing on continued improved understanding of their beams. Far-field measurements of the BICEP2 beam with a chopped thermal source, combined with analysis improvements, show that the level of residual beam-induced systematic errors is acceptable for the goal of σ(r)~ 0:01 measurements. Beam measurements of the Keck Array side lobes helped identify a way to reduce optical loading with interior cold baffles, which we installed in late 2013. These baffles reduced total optical loading, leading to a ~ 10% increase in mapping speed for the 2014 observing season. The sensitivity of the Keck Array continues to improve: for the 2013 season it was 9:5 μK _/s noise equivalent temperature (NET). In 2014 we converted two of the 150-GHz cameras to 100 GHz for foreground separation capability. We have shown that the BICEP2 and the Keck Array telescope technology is sufficient for the goal of σ(r) ~ 0:01 measurements. Furthermore, the program is continuing with BICEP3, a 100-GHz telescope with 2560 detectors.

Additional Information

© 2014 Society of Photo-Optical Instrumentation Engineers (SPIE). August 19, 2014. The Bicep2 and the Keck Array projects have been made possible through support from the National Science Foundation (grant Nos. ANT-0742818, ANT-0742592, ANT-1044978, ANT-1110087, and ANT-1145172), the W. M. Keck Foundation, the Canada Foundation for Innovation, and the British Columbia Development Fund. The development of antenna-coupled detector technology was supported by the JPL Research and Technology Development Fund and grants 06-ARPA206-0040 and 10-SAT10-0017 from the NASA APRA and SAT programs. The development and testing of focal planes were supported by the Gordon and Betty Moore Foundation at Caltech. The computations in these proceedings were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. Tireless administrative support was provided by Irene Coyle and Kathy Deniston. We are grateful to Steffen Richter as our 2010-2012 Bicep2 winter-over, and to Robert Schwarz as our 2011-2014 Keck Array winter-over. We thank the staff of the US Antarctic Program and in particular the South Pole Station without whose help this research would not have been possible. We thank our BICEP1 and SPIDER colleagues for useful discussions and shared expertise.

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