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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
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/>
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CLASS is currently observing the sky in 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,<ref name=":1">{{Cite journal |last1=Eimer |first1=Joseph R. |last2=Li 李 |first2=Yunyang 云炀 |last3=Brewer |first3=Michael K. |last4=Shi 时 |first4=Rui 瑞 |last5=Ali |first5=Aamir |last6=Appel |first6=John W. |last7=Bennett |first7=Charles L. |last8=Bruno |first8=Sarah Marie |last9=Bustos |first9=Ricardo |last10=Chuss |first10=David T. |last11=Cleary |first11=Joseph |last12=Dahal |first12=Sumit |last13=Datta |first13=Rahul |last14=Denes Couto |first14=Jullianna |last15=Denis |first15=Kevin L. |date=2024-03-01 |title=CLASS Angular Power Spectra and Map-component Analysis for 40 GHz Observations through 2022 |journal=The Astrophysical Journal |volume=963 |issue=2 |pages=92 |arxiv=2309.00675 |bibcode=2024ApJ...963...92E |doi=10.3847/1538-4357/ad1abf |issn=0004-637X |doi-access=free}}</ref><ref>{{Cite journal |last1=Li 李 |first1=Yunyang 云炀 |last2=Eimer |first2=Joseph R. |last3=Osumi |first3=Keisuke |last4=Appel |first4=John W. |last5=Brewer |first5=Michael K. |last6=Ali |first6=Aamir |last7=Bennett |first7=Charles L. |last8=Bruno |first8=Sarah Marie |last9=Bustos |first9=Ricardo |last10=Chuss |first10=David T. |last11=Cleary |first11=Joseph |last12=Couto |first12=Jullianna Denes |last13=Dahal |first13=Sumit |last14=Datta |first14=Rahul |last15=Denis |first15=Kevin L. |date=2023-10-01 |title=CLASS Data Pipeline and Maps for 40 GHz Observations through 2022 |journal=The Astrophysical Journal |volume=956 |issue=2 |pages=77 |arxiv=2305.01045 |bibcode=2023ApJ...956...77L |doi=10.3847/1538-4357/acf293 |issn=0004-637X |doi-access=free}}</ref> and from observations with the 90 GHz telescope through 2024.<ref>{{
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>
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