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[[File:CLASS Experiment 40 GHz Focal Plane.png|thumbnail|250px|left|CLASS 40 GHz camera, showing the feedhorns that couple light onto the transition-edge sensor bolometers at a temperature of 0.1 [[Kelvin]].]]
The CLASS instrument is designed to survey 65% of the sky at millimeter wavelengths, in the microwave portion of the [[electromagnetic spectrum]], from a ground-based observatory with a resolution of about 1° — approximately twice the angular size of the sun and moon as viewed from Earth. The CLASS array consists of two [[altazimuth mount]]s that allow the telescopes to be pointed to observe different patches of sky. The four CLASS telescopes observe at a range of frequencies to separate emission from our [[Milky Way|galaxy]] from that of the CMB. One telescope observes at 40 [[Hertz|GHz]] (7.5 mm wavelength); one telescope observes at 90 GHz (3.3 mm wavelength) with a second 90 GHz telescope planned in the future; and the fourth telescope observes in two frequency bands centered at 150 GHz (2 mm wavelength) and 220 GHz (1.4 mm wavelength). Two separate telescopes, observing at different frequencies, are housed on each mount. The 90 GHz telescope detector array was upgraded in 2019 to significantly increase sensitivity. In 2024 the VPM for the CLASS 90 GHz telescope was replaced with a rotating reflective half-wave plate (HWP)<ref>{{Cite book |last1=Shi |first1=Rui |title=Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XII |last2=Brewer |first2=Michael |last3=Chan |first3=Carol |last4=Chuss |first4=David T. |last5=Couto |first5=Jullianna D. |last6=Eimer |first6=Joseph R. |last7=Karakla |first7=John |last8=Shukawa |first8=Koji |last9=Valle |first9=Deniz |date=2024-08-16 |publisher=SPIE |isbn=978-1-5106-7527-8 |editor-last=Zmuidzinas |editor-first=Jonas |pages=125 |chapter=Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHZ band telescope |doi=10.1117/12.3016346 |editor2-last=Gao |editor2-first=Jian-Rong |chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/13102/3016346/Design-and-characterization-of-a-60-cm-reflective-half-wave/10.1117/12.3016346.full |last10=Appel |first10=John W. |last11=Bennett |first11=Charles L. |last12=Dahal |first12=Sumit |last13=Essinger-Hileman |first13=Thomas |last14=Marriage |first14=Tobias T. |last15=Petroff |first15=Matthew A. |arxiv=2407.08912}}</ref> to concentrate on improved sensitivity for linear polarization.
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 circular polarization. There are many potential scenarios that could generate circular polarization in the early universe, and CLASS has now put very strong limits on these theories.
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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