submillimeter astronomy(sub-mil -ă-mee-ter) A branch of astronomy covering the wavelength range from 0.3 to 1 millimeter approximately. It is the highest frequency range, 300–1000 gigahertz, in which radio astronomy can be carried out, and is particularly important because of the large number of molecular emission lines to be found in the range (see molecular-line radio astronomy). These originate primarily from giant molecular clouds.
Submillimeter astronomy usually involves a hybrid of radio and infrared techniques. A parabolic radio dish plus a line receiver is normally used. The receiver can be at the prime focus of the dish but is more usually at the Cassegrain focus behind the dish, to which radio waves are reflected by a small subreflector surface. The receiver is cooled to a very low temperature using liquid helium. To maintain sensitivity the dish must be more accurately shaped and smooth than dishes for longer-wavelength studies, and must keep its shape under different orientations and weather conditions. Observations have to be made at high-altitude sites where there is very little atmospheric attenuation and absorption by water vapor. Submillimeter telescopes include JCMT, CSO, SMTO, and SEST. See also FIRST; millimeter astronomy; SWAS.