  The Submillimeter Array (SMA) was conceived at the Smithsonian Astrophysical
Observatory in 1984 as a six element interferometer to operate in the major
atmospheric windows from about 200 to 900 GHz. In 1996, the Academica Sinica
Institute of Astronomy and Astrophysics of Taiwan joined the project and agreed
to provide additional hardware to expand the interferometer to eight elements.
All eight antennas are now operating at the observatory site on Mauna Kea, and
astronomical observations have been made in the 230, 345, and 650 GHz bands.
  The SMA antennas have a diameter of 6 m, a surface accuracy of better than 25
micron rms, and can be reconfigured to provide spatial resolutions down to
about 0.5" at 200 GHz and, eventually, 0.1" at 850 GHz. Coupling to the
receiver package within each antenna is achieved via a beam waveguide, in a
bent Nasmyth configuration, comprised of a flat tertiary mirror and two
ellipsoidal mirrors that form a secondary pupil used for receiver calibration.
An additional fixed mirror and a rotating wire grid polarizer are then used for
receiver selection. Each antenna houses a single cryostat, with an integrated
cryocooler capable of cooling up to eight receivers to 4 K. In the current
configuration only three receiver bands are available: 175-255 GHz, 250-350
GHz, and 600-720 GHz, and simultaneous operation of the 650 GHz receiver with
either of the lower frequency receivers is possible. Eventually dual
polarization will be available from 325-350 GHz, and dual frequency operation
will be possible, pairing either of the lower frequency receivers with any of
the high frequency units: 325-425 GHz, 425-510 GHz, 600-720 GHz, and 800-900
GHz. (Abridged)
