Cosmology Large Angular Scale Surveyor: Difference between revisions

Revision as of 21:46, 21 April 2019

Revision as of 16:30, 3 June 2019

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[[File:CLASS Experiment 40 GHz Focal Plane.png|thumbnail|200px|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 will consist of two [[altazimuth mount]]s that will allow the telescopes to be pointed to observe different patches of sky. The four CLASS telescopes will observe at a range of frequencies to separate emission from our [[milkyMilky wayWay|galaxy]] from that of the CMB. One telescope will observe at 40 [[Hertz|GHz]] (7.5 mm wavelength); two telescopes will observe at 90 GHz (3.3 mm wavelength); and the fourth telescope will observe 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 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 will employ focal plane arrays with large numbers of [[horn antenna|feedhorn]]-coupled, [[transition-edge sensor|transition-edge-sensor]] [[bolometers]] cooled to just 0.1&nbsp;°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/>