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 , 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.