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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 [[circular polarization]]. Because no circular polarization is expected in the CMB, the VPM allows for a valuable check for [[systematic errors]] in the data by looking at the circular polarization signal, which should be consistent with zero. In 2024 the VPM for the CLASS 90 GHz telescope was replaced with a rotating reflective [[half-wave plate]] (HWP)<ref>{{Cite
Because water vapor in the atmosphere emits at microwave frequencies, CLASS will observe 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 [[Cosmic Background Imager|CBI]], [[Atacama Submillimeter Telescope Experiment|ASTE]], [[NANTEN2 Observatory|Nanten]], [[Atacama Pathfinder Experiment|APEX]], [[Atacama Large Millimeter Array|ALMA]], [[Atacama Cosmology Telescope|ACT]], and [[POLARBEAR]].
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CLASS is currently observing the sky in all four frequency bands. The CLASS 40 GHz telescope achieved first light on 8 May 2016 and began a roughly five-year survey in September 2016 after initial commissioning observations were complete. In early 2018, a first 90 GHz telescope was installed on the same mount as the 40 GHz telescope, achieving first light on 30 May 2018. In 2019, the dual-frequency 150/220 GHz telescope was deployed, along with a second telescope mount, and achieved first light on 21 September 2019.
CLASS has released results from the first five years of observations with the 40 GHz telescope, through mid-2022, and has made the most sensitive maps covering approximately 74% of the sky in this frequency band at angular scales of approximately 2° to 20°.<ref name=":1">{{Cite journal |
CLASS has made a first detection of [[circular polarization]] from the atmosphere at a frequency of 40 GHz, which is in agreement with models of atmospheric circular polarization due to [[Zeeman effect|Zeeman splitting]] of [[Allotropes of oxygen|molecular oxygen]] in the presence of the Earth's magnetic field.<ref>{{Cite journal|last1=Petroff|first1=Matthew A.|last2=Eimer|first2=Joseph R.|last3=Harrington|first3=Kathleen|last4=Ali|first4=Aamir|last5=Appel|first5=John W.|last6=Bennett|first6=Charles L.|last7=Brewer|first7=Michael K.|last8=Bustos|first8=Ricardo|last9=Chan|first9=Manwei|last10=Chuss|first10=David T.|last11=Cleary|first11=Joseph|date=2020-01-30|title=Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q band|journal=The Astrophysical Journal|volume=889|issue=2|pages=120|doi=10.3847/1538-4357/ab64e2|issn=1538-4357|arxiv=1911.01016|bibcode=2020ApJ...889..120P |s2cid=207870198 |doi-access=free }}</ref> The atmospheric circular polarization is smoothly-varying over the sky, allowing it to be separated from celestial circular polarization. This has allowed CLASS to constrain celestial circular polarization at 40 GHz to be less than 0.1 μK at angular scales of 5 degrees and less than 1 μK at angular scales around 1 degree.<ref name=":1" /><ref>{{Cite journal|last1=Padilla|first1=Ivan L.|last2=Eimer|first2=Joseph R.|last3=Li|first3=Yunyang|last4=Addison|first4=Graeme E.|last5=Ali|first5=Aamir|last6=Appel|first6=John W.|last7=Bennett|first7=Charles L.|last8=Bustos|first8=Ricardo|last9=Brewer|first9=Michael K.|last10=Chan|first10=Manwei|last11=Chuss|first11=David T.|date=2020-01-29|title=Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz|journal=The Astrophysical Journal|volume=889|issue=2|pages=105|doi=10.3847/1538-4357/ab61f8|issn=1538-4357|arxiv=1911.00391|bibcode=2020ApJ...889..105P |s2cid=207870170 |doi-access=free }}</ref> This is an improvement upon previous limits on circular polarization in the CMB by more than a factor of 100.<ref>{{Cite journal|last1=Mainini|first1=R.|last2=Minelli|first2=D.|last3=Gervasi|first3=M.|last4=Boella|first4=G.|last5=Sironi|first5=G.|last6=Baú|first6=A.|last7=Banfi|first7=S.|last8=Passerini|first8=A.|last9=Lucia|first9=A. De|last10=Cavaliere|first10=F.|date=August 2013|title=An improved upper limit to the CMB circular polarization at large angular scales|journal=Journal of Cosmology and Astroparticle Physics|language=en|volume=2013|issue=8|pages=033|doi=10.1088/1475-7516/2013/08/033|issn=1475-7516|arxiv=1307.6090|bibcode=2013JCAP...08..033M |s2cid=119236025}}</ref><ref>{{Cite journal|last1=Nagy|first1=J. M.|last2=Ade|first2=P. A. R.|last3=Amiri|first3=M.|last4=Benton|first4=S. J.|last5=Bergman|first5=A. S.|last6=Bihary|first6=R.|last7=Bock|first7=J. J.|last8=Bond|first8=J. R.|last9=Bryan|first9=S. A.|last10=Chiang|first10=H. C.|last11=Contaldi|first11=C. R.|date=August 2017|title=A New Limit on CMB Circular Polarization from SPIDER|journal=The Astrophysical Journal|language=en|volume=844|issue=2|pages=151|doi=10.3847/1538-4357/aa7cfd|arxiv=1704.00215 |bibcode=2017ApJ...844..151N |issn=0004-637X|hdl=10852/60193|s2cid=13694135|hdl-access=free |doi-access=free }}</ref>
CLASS has also placed unique constraints on the disk-averaged microwave temperature of Venus in the 40 and 90 GHz frequency bands, which is sensitive to the composition of the Venusian atmosphere.<ref>{{Cite journal |
== See also ==
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{{Reflist|refs=
<ref name=CLASS_website>{{cite web |title=CLASS: Cosmology Large Angular Scale Surveyor |date=10 September 2013 |url=http://sites.krieger.jhu.edu/class/ |publisher=The Johns Hopkins University |accessdate=2015-08-12}}</ref>
<ref name=2014SPIE_EH>{{cite journal |doi=10.1117/12.2056701 |title=CLASS: the cosmology large angular scale surveyor |journal=Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series |year=2014 |author= Essinger-Hileman, T. E. |volume=9153 |series=Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII |editor1-last=Holland |editor1-first=Wayne S. |editor2-last=Zmuidzinas |editor2-first=Jonas |pages=91531I |arxiv = 1408.4788 |display-authors=etal|bibcode=2014SPIE.9153E..1IE |s2cid=13691600 }}</ref>
<ref name=2014SPIE_Appel>{{cite journal |doi=10.1117/12.2056530 |title=The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters |journal=Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series |year=2014 |author= Appel J. W. |volume=9153 |series=Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII |editor1-last=Holland |editor1-first=Wayne S. |editor2-last=Zmuidzinas |editor2-first=Jonas |pages=91531J |arxiv = 1408.4789 |display-authors=etal|bibcode=2014SPIE.9153E..1JA |s2cid=52233099 }}</ref>
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<ref name=Photonics-CLASS-Article>{{cite web |title=ARRA to Help Build Telescope |url=http://photonics.com/Article.aspx?AID=41508 |publisher=Photonics Media |accessdate=2014-01-15}}</ref>
<ref name=JHUGazette-CLASS-Article>{{cite web |title=Astrophysicist, team win stimulus grant to build telescope |url=http://archive.gazette.jhu.edu/2010/03/15/astrophysicist-team-win-stimulus-grant-to-build-telescope/ |publisher=The Johns Hopkins University |accessdate=2014-01-15 |archive-url=https://web.archive.org/web/20121214140753/http://archive.gazette.jhu.edu/2010/03/15/astrophysicist-team-win-stimulus-grant-to-build-telescope/ |archive-date=2012-12-14 |url-status=dead }}</ref>
<ref name=JHUGazette-CLASS-Article-2014>{{cite web |title=Johns Hopkins astrophysics team builds telescope to study origins of the universe |date=27 May 2014 |url=http://hub.jhu.edu/2014/05/27/class-telescope-bennett-marriage |publisher=The Johns Hopkins University |accessdate=2014-05-27}}</ref>
<ref name=Conicyt-Astronomy-Roadmap-CLASS>{{cite web |title=Astronomy, Technology, Industry: Roadmap for the Fostering of Technology Development and Innovation in the Field of Astronomy in Chile |url=http://www.conicyt.cl/astronomia/files/2012/10/Roadmap-Astronom%C3%ADa-25.10.12.pdf |publisher=Conicyt Ministry of Education, Government of Chile |accessdate=2014-02-10}}</ref>
<ref name=2014-Linde-Inflation-Overview>{{cite arXiv | title=Inflationary Cosmology after Planck 2013 |year=2014 |last1=Linde |first1=A. | eprint=1402.0526 |class=hep-th }}</ref>
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