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m Minor rewording of the section on the VPM giving sensitivity to circular polarization. |
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The CLASS instrument is specifically designed to measure polarization. As an [[electromagnetic wave]], light consists of oscillating electric and magnetic fields. These fields can have both an amplitude, or intensity, and a preferred direction in which they oscillate, or polarization. The polarized signal that CLASS will attempt to measure is incredibly small. It is expected to be only a few parts-per-billion change in the polarization of the already-cold 2.725 K CMB.<ref name=apj420_439/> To measure such a small signal, CLASS employs focal plane arrays with large numbers of [[horn antenna|feedhorn]]-coupled, [[transition-edge sensor|transition-edge-sensor]] [[bolometers]] cooled to just 0.1 °C above absolute zero by [[Dilution refrigerator|cryogenic helium refrigerators]]. This low temperature reduces the intrinsic thermal noise of the detectors.<ref name=2012SPIE_Eimer/><ref name=2013Eimer_Thesis/><ref name=2014SPIE_Appel/>
The other unique aspect of the CLASS telescopes is the use of a variable-delay polarization modulator (VPM) to allow a precise and stable measurement of polarization. The VPM modulates, or turns on and off, the polarized light going to the detector at a known frequency, approximately 10 Hz, while leaving unpolarized light unchanged. This allows for a clear separation of the tiny polarization of the CMB from the much larger unpolarized atmosphere by "[[lock-in amplifier|locking in]]" to the 10 [[Hertz|Hz]] signal. The VPM also modulates circular polarization out of phase with linear polarization, giving CLASS sensitivity to
Because water vapor in the atmosphere emits at microwave frequencies, CLASS observes from a very dry and high-altitude site in the Andes Mountains on the edge of the Atacama Desert of Chile. Nearby sites have been chosen by other observatories for the same reason, including [[Atacama Cosmology Telescope|ACT]], [[Atacama Pathfinder Experiment|APEX]], [[Atacama Large Millimeter Array|ALMA]], [[Atacama Submillimeter Telescope Experiment|ASTE]], [[Cosmic Background Imager|CBI]], [[Fred Young Submillimeter Telescope|CCAT-prime]], [[NANTEN2_Observatory|NANTEN2]], [[POLARBEAR]], [[Simons Observatory]], and [[University of Tokyo Atacama Observatory|TAO]].
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