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Proceedings Paper

Optical modeling and polarization calibration for CMB measurements with ACTPol and Advanced ACTPol
Author(s): Brian Koopman; Jason Austermann; Hsiao-Mei Cho; Kevin P. Coughlin; Shannon M. Duff; Patricio A. Gallardo; Matthew Hasselfield; Shawn W. Henderson; Shuay-Pwu Patty Ho; Johannes Hubmayr; Kent D. Irwin; Dale Li; Jeff McMahon; Federico Nati; Michael D. Niemack; Laura Newburgh; Lyman A. Page; Maria Salatino; Alessandro Schillaci; Benjamin L. Schmitt; Sara M. Simon; Eve M. Vavagiakis; Jonathan T. Ward; Edward J. Wollack
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Paper Abstract

The Atacama Cosmology Telescope Polarimeter (ACTPol) is a polarization sensitive upgrade to the Atacama Cosmology Telescope, located at an elevation of 5190 m on Cerro Toco in Chile. ACTPol uses transition edge sensor bolometers coupled to orthomode transducers to measure both the temperature and polarization of the Cosmic Microwave Background (CMB). Calibration of the detector angles is a critical step in producing polarization maps of the CMB. Polarization angle offsets in the detector calibration can cause leakage in polarization from E to B modes and induce a spurious signal in the EB and TB cross correlations, which eliminates our ability to measure potential cosmological sources of EB and TB signals, such as cosmic birefringence. We calibrate the ACTPol detector angles by ray tracing the designed detector angle through the entire optical chain to determine the projection of each detector angle on the sky. The distribution of calibrated detector polarization angles are consistent with a global offset angle from zero when compared to the EB-nulling offset angle, the angle required to null the EB cross-correlation power spectrum. We present the optical modeling process. The detector angles can be cross checked through observations of known polarized sources, whether this be a galactic source or a laboratory reference standard. To cross check the ACTPol detector angles, we use a thin film polarization grid placed in front of the receiver of the telescope, between the receiver and the secondary reflector. Making use of a rapidly rotating half-wave plate (HWP) mount we spin the polarizing grid at a constant speed, polarizing and rotating the incoming atmospheric signal. The resulting sinusoidal signal is used to determine the detector angles. The optical modeling calibration was shown to be consistent with a global offset angle of zero when compared to EB nulling in the first ACTPol results and will continue to be a part of our calibration implementation. The first array of detectors for Advanced ACTPol, the next generation upgrade to ACTPol, will be deployed in 2016. We plan to continue using both techniques and compare them to astrophysical source measurements for the Advanced ACTPol polarization calibration.

Paper Details

Date Published: 20 July 2016
PDF: 12 pages
Proc. SPIE 9914, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, 99142T (20 July 2016); doi: 10.1117/12.2231912
Show Author Affiliations
Brian Koopman, Cornell Univ. (United States)
Jason Austermann, National Institute of Standards and Technology (United States)
Hsiao-Mei Cho, SLAC National Accelerator Lab. (United States)
Kevin P. Coughlin, Univ. of Michigan (United States)
Shannon M. Duff, National Institute of Standards and Technology (United States)
Patricio A. Gallardo, Cornell Univ. (United States)
Matthew Hasselfield, The Pennsylvania State Univ. (United States)
Shawn W. Henderson, Cornell Univ. (United States)
Shuay-Pwu Patty Ho, Princeton Univ. (United States)
Johannes Hubmayr, National Institute of Standards and Technology (United States)
Kent D. Irwin, SLAC National Accelerator Lab. (United States)
Stanford Univ. (United States)
Dale Li, SLAC National Accelerator Lab. (United States)
Jeff McMahon, Univ. of Michigan (United States)
Federico Nati, Univ. of Pennsylvania (United States)
Michael D. Niemack, Cornell Univ. (United States)
Laura Newburgh, Univ. of Toronto (Canada)
Lyman A. Page, Princeton Univ. (United States)
Maria Salatino, Princeton Univ. (United States)
Alessandro Schillaci, Ponticia Univ. Catolica de Chile (Chile)
Benjamin L. Schmitt, Univ. of Pennsylvania (United States)
Sara M. Simon, Princeton Univ. (United States)
Eve M. Vavagiakis, Cornell Univ. (United States)
Jonathan T. Ward, Univ. of Pennsylvania (United States)
Edward J. Wollack, NASA Goddard Space Flight Ctr. (United States)

Published in SPIE Proceedings Vol. 9914:
Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII
Wayne S. Holland; Jonas Zmuidzinas, Editor(s)

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