Atacama Cosmology Telescope - Design and Location

Design and Location

The ACT is an off-axis Gregorian telescope, with a six metre (236 in) primary mirror and a two metre (79 in) 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 short 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. A system of cryogenic helium refrigerators keep the detectors a third of a degree above absolute zero.

In its currently scheduled survey, the ACT will map about two hundred square degrees of the sky.

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.