The South Pole Telescope (SPT) is a 10-metre (390 in) diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electromagnetic spectrum, with the particular design goal of measuring the faint, diffuse emission from the cosmic microwave background (CMB). Key results include the first detection of B-mode polarization in the CMB, the discovery of over 1000 clusters of galaxies using the Sunyaev–Zel'dovich effect, the discovery of a population of high-redshift, strongly lensed dusty galaxies, and unprecedentedly sensitive measurements of the primary temperature and polarization power spectra of the CMB at small angular scales. The first major survey with the SPT—designed to find distant, massive, clusters of galaxies through their interaction with the CMB, with the goal of constraining the dark energy equation of state—was completed in October 2011. In early 2012, a new camera (SPTpol) was installed on the SPT with even greater sensitivity and the capability to measure the polarization of incoming light. This camera operated from 2012–2016 and was used to make deep, high-resolution maps of hundreds of square degrees of the Southern sky. In 2017, the third-generation camera SPT-3G was installed on the telescope, providing nearly an order-of-magnitude increase in detectors in the focal plane. The SPT collaboration is made up of over a dozen (mostly North American) institutions. It is led out of the University of Chicago by project director John Carlstrom. The SPT program is funded primarily by the National Science Foundation and the United States Department of Energy.
Microwave and millimeter-wave observations at the South Pole The South Pole region is among the premier observing sites in the world for millimeter-wavelength observations. The Pole's high altitude of 2.8 km (9,200 ft) above sea level means the atmosphere is thin, and the extreme cold keeps the amount of water vapor in the air low. This is particularly important for observing at millimeter wavelengths, where incoming signals can be absorbed by water vapor, and where water vapor emits radiation that can be confused with astronomical signals. Since the sun does not rise and set daily, the atmosphere at the pole is particularly stable. In addition, no interference exists from the sun in the millimeter range during the months of polar night.
The telescope The telescope is a 10-metre (394 in) diameter off-axis Gregorian telescope in an altazimuth mount (at the poles, an altazimuth mount is effectively identical to an equatorial mount). It was designed to allow a large field of view (over 1 square degree) while minimizing systematic uncertainties from ground spill-over and scattering off the telescope optics. The surface of the telescope mirror is smooth down to roughly 25 micrometres (0.98 thou), or about one-thousandth of an inch (i.e., one thou), which allows sub-millimeter wavelength observations. A key feature of the SPT design (now standard among modern CMB telescopes ) is that the entire telescope is scanned, so the beam does not move relative to the telescope mirrors. The fast scanning of the telescope and the large field of view make SPT efficient for surveying large areas of sky, which is required to achieve the SPT science goals.
The SPT-SZ camera The first camera installed on the SPT contained a 960-element bolometer array of superconducting transition edge sensors (TES), which made it one of the largest TES bolometer arrays ever built. The focal plane for this camera (referred to as the SPT-SZ camera because it was designed to conduct a survey of galaxy clusters through their Sunyaev–Zel'dovich effect signature) was split into six pie-shaped wedges, each with 160 detectors. These wedges observed at three different frequencies: 95 GHz, 150 GHz, and 220 GHz. The modularity of the focal plane allowed it to be broken into many different frequency configurations. For the majority of the life of the camera, the SPT-SZ focal plane had one wedge at 95 GHz, four at 150 GHz, and one at 220 GHz. The SPT-SZ camera was used primarily to conduct a survey of 2500 square degrees of the Southern sky (20h to 7h in right ascension, −65d to −40d declination) to a noise level of roughly 15 micro-Kelvin in a 1-arcminute pixel at 150 GHz.
The SPTpol camera The second camera installed on the SPT–also designed with superconducting TES arrays–was even more sensitive than the SPT-SZ camera and, crucially, had the ability to measure the polarization of the incoming light (hence the name SPTpol). The 780 polarization-sensitive pixels (each with two separate TES bolometers, each sensitive to one linear polarization direction) were divided between observing frequencies of 90 GHz and 150 GHz, and pixels at the two frequencies were designed with different detector architectures. The 150 GHz pixels were corrugated-feedhorn-coupled TES polarimeters fabricated in monolithic arrays at the National Institute of Standards and Technology. The 90 GHz pixels were individually packaged dual-polarization absorber-coupled polarimeters developed at Argonne National Laboratory. The 90 GHz pixels were coupled to the telescope optics through individually machined contoured feedhorns. The first year of SPTpol observing was used to survey a 100-square-degree field centered at R.A. 23h30m declination −55d. The next four years were primarily spent surveying a 500-square-degree region of which the original 100 square degrees is a subset.
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