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The Atacama Cosmology Telescope (ACT) is a six-metre telescope on Cerro Toco in the Atacama Desert in the north of Chile. It is designed to make high-resolution, microwave-wavelength surveys of the sky in order to study the cosmic microwave background radiation (CMB). At an altitude of 5190 metres (17030 feet), it is currently the highest permanent, ground-based telescope in the world.1
Erected in the (austral) autumn of 2007, ACT saw first light on 22 October 2007 with its science receiver, the Millimeter Bolometer Array Camera (MBAC), and completed its first season in December of 2007. In began its second season of observations in June of 2008.
The project is a collaboration between Princeton University, the University of Pennsylvania, NASA/GSFC, the University of British Columbia, NIST, the Pontificia Universidad Católica de Chile, the University of KwaZulu-Natal, Cardiff University, Rutgers University, the University of Pittsburgh, Columbia University, Haverford College, INAOE, LLNL, NASA/JPL, the University of Toronto, the University of Cape Town, the University of Massachusetts and York College, CUNY. It is funded by the US National Science Foundation.
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Design & Location
The ACT is an off-axis Gregorian telescope, with a six metre (20') primary mirror and a two metre (6.5') secondary mirror. Both mirrors are segmented, consisting of 71 (primary) and 11 (secondary) aluminum panels. Unlike most telescopes which track the rotating sky during observation, the ACT observes a strip of sky, typically five degrees wide, by scanning back and forth in azimuth at the relatively rapid rate of two degrees per second. The rotating portion of the telescope weighs approximately 32 tonnes (35 tons), creating a substantial engineering challenge. A ground screen surrounding the telescope minimises contamination from microwave radiation emitted by the ground. The design, manufacture and construction of the telescope were done by Dynamic Structures in Vancouver, British Columbia.
Observations are made at resolutions of about an arcminute (1/60th of a degree) in three frequencies: 145 GHz, 215 GHz and 280 GHz. Each frequency is measured by a 3 cm x 3 cm (1.2" x 1.2"), 1024 element array, for a total of 3072 detectors. The detectors are superconducting transition-edge sensors, a new technology whose high sensitivity should allow measurements of the temperature of the CMB to within a few millionths of a degree.2 A system of cryogenic helium refrigerators keep the detectors a third of a kelvin above absolute zero.
In its currently scheduled survey, the ACT will map about two hundred square degrees of the sky.3
Because water vapour in the atmosphere emits microwave radiation which contaminates measurements of the CMB, the telescope benefits from its arid, high-altitude site, located in the lofty — yet easily accessible — Chajnantor plain in the Andean mountains in the Atacama Desert. Several other observatories are located in the region, including CBI, ASTE, Nanten, APEX and ALMA.
Science Goals
Measurements of cosmic microwave background radiation (CMB) by experiments such as COBE, BOOMERanG, WMAP, CBI and many others, have greatly advanced our knowledge of cosmology, particularly the early evolution of the universe. It is expected that higher resolution CMB observations will not only improve the precision of current knowledge, but will also allow new types of measurements. At ACT resolutions, the Sunyaev-Zeldovich effect, by which galaxy clusters leave an imprint on the CMB, should be prominent. The power of this method of detection is that it is a redshift-independent measurement of the mass of the clusters, meaning that very distant, ancient clusters are as easy to detect as nearby clusters.
It is expected that ACT will detect on the order of 1000 such clusters.4 Together with follow-up measurements in visible and X-ray light, this would provide a picture of the evolution of structure in the universe since the Big Bang. Among other things, this would improve our understanding of the nature of the mysterious Dark Energy which seems to be a dominant component of the universe.
The South Pole Telescope has similar, but complementary, science objectives.
Notes
- ^ The Receiver Lab Telescope (RLT), an 80 cm instrument, is higher at 5525 m, but is not permanent as it is fixed to the roof of a movable shipping container. See Observations in the 1.3 and 1.5 THz Atmospheric Windows with the Receiver Lab Telescope.
- ^ J. Fowler et al. (2007). "Optical Design of the Atacama Cosmology Telescope and the Millimeter Bolometric Array Camera" (abstract). Appl. Optics 46 (17): 3444–54. doi:, http://xxx.lanl.gov/abs/astro-ph/0701020.
- ^ A. Kosowsky (2003). "The Atacama Cosmology Telescope" (abstract). New Astron. Rev. 47 (939), http://arxiv.org/abs/astro-ph/0402234.
- ^ Ibid.
External links
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- This page was last modified on 21 November 2008, at 22:58.
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