The Atacama Cosmology Telescope (ACT) is a six meter Microwave telescope on the Atacama Desert, aimed a surveying the Cosmic Microwave Background. It is aimable only north-south, across 5 degrees, using the Earth's rotation for the other dimension in its coverage.
The original receiver was MBAC (Millimeter Bolometer array camera), three 32 x 32 grids, for 145 GHz, 215 GHz and 280 GHz. It identifies distant Galaxy Clusters using the Sunyaev-Zel'dovich Effect, e.g., for further study using Sloan Digital Sky Survey.
A third receiver, AdvACT, for Advanced ACTPol, will be polarization-sensitive and cover more Frequencies and a wider range.
The instruments are designed for surveying, i.e., coverage of fields of hundreds to thousands of square degrees, the surveys sometimes referred to as the ACT Survey, the ACTPol Survey, and the AdvACT Survey. As the telescope is located in Chile, the coverage is generally in the southern hemisphere. Locations were chosen so as to provide complimentary coverage of some of the same regions covered by Visible Light surveys.
CMB Stage-4 (CMB-S4)