Intensity-coupled Polarization in Instruments with a Continuously Rotating Half-wave Plate
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Didier, Joy et al. Intensity-coupled Polarization in Instruments with a Continuously Rotating Half-wave Plate. The Astrophysical Journal, 876 (May 3, 2019) 1. https://doi.org/10.3847/1538-4357/ab0f36.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
We discuss a systematic effect associated with measuring polarization with a continuously rotating half-wave plate
(HWP). The effect was identified with the data from the E and B Experiment, which was a balloon-borne
instrument designed to measure the polarization of the cosmic microwave background (CMB) as well as that from
Galactic dust. The data show polarization fractions larger than 10%, while less than 3% were expected from
instrumental polarization. We give evidence that the excess polarization is due to detector nonlinearity in the
presence of a continuously rotating HWP. The nonlinearity couples intensity signals to polarization. We develop a
map-based method to remove the excess polarization. Applying this method to the 150 (250) GHz band data, we
find that 81% (92%) of the excess polarization was removed. Characterization and mitigation of this effect are
important for future experiments aiming to measure the CMB B-modes with a continuously rotating HWP.