The South Pole Telescope (SPT) is a 10 meter (394 in) diameter telescope located at the Amundsen-Scott South Pole Station, Antarctica. It is a microwave/millimeter-wave telescope that observes in a frequency range between 70 and 300 GHz. The primary science goal for SPT is to conduct a survey to find several thousand clusters of galaxies, which should allow interesting constraints on the Dark Energy equation of state.
The project is a collaboration between the University of Chicago, the University of California-Berkeley, Case Western Reserve University, the University of Illinois at Urbana-Champaign, the Smithsonian Astrophysical Observatory, the University of Colorado-Boulder, McGill University, and University of California at Davis. It is funded by the National Science Foundation.
 Microwave/millimeter-wave observations at the South Pole
The South Pole is the premier observing site in the world for millimeter-wavelength observations. The Pole's high altitude (2.8 km/1.7 mi 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. Another advantage of the South Pole site is that the sun sets in mid-March and is followed by six months of total darkness, during which time the atmospheric conditions become extremely stable without the added turbulence caused by the daily rising and setting sun.
 The telescope
The telescope is a 10 meter (394 in) 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 (about 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 micrometers (one one-thousandth of an inch), which allows sub-millimeter wavelength observations. A key advantage of the SPT observing strategy 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 its large field of view makes SPT efficient at surveying large areas of sky, which is required to achieve the science goals of the SPT cluster survey.
 The camera
The focal plane for SPT is a 960 element bolometer array of superconducting transition edge sensors (TES), which makes it one of the largest TES bolometer arrays ever built. The focal plane for SPT is split up into six pie-shaped wedges, each with 160 detectors. These wedges observe at three different frequencies: 95 GHz, 150 GHz, and 220 GHz. The modularity of the focal plane allows it to be broken into many different frequency configurations. Currently (2009 season), the SPT focal plane has one wedge at 95 GHz, four at 150 GHz, and one at 220 GHz.
 Science goals
The first key project for the SPT will be a >1000 square degree survey to search for clusters of galaxies using the Sunyaev-Zel'dovich effect, a distortion of the cosmic microwave background radiation (CMB) due to interactions between CMB photons and the hot, ionized gas in clusters. Given three years of observing the South Pole Telescope should find several thousand clusters of galaxies, which should allow interesting constraints on the Dark Energy equation of state.
The Atacama Cosmology Telescope has similar, but complementary, science objectives.
 Current status
The South Pole Telescope achieved first light on February 16, 2007, and began science observations in March 2007. Commissioning observations and an initial small survey was completed in austral winter 2007 with winter-overs Stephen Padin and Zak Staniszewski at its helm. Larger survey fields were completed in 2008 with winter-overs Keith Vanderlinde and Dana Hrubes, and in 2009 with winter-overs Erik Shirokoff and Ross Williamson. The camera will be upgraded again in December 2009 for the 2010 observing season.
 See also