The Submillimeter Array (SMA) consists of eight 6-meter (20 ft) diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations. It is the first purpose-built submillimeter interferometer, constructed after successful interferometry experiments using the pre-existing 15-meter (49 ft) James Clerk Maxwell Telescope and 10.4-meter (34.1 ft) Caltech Submillimeter Observatory (now decommissioned) as an interferometer. All three of these observatories are located at Mauna Kea Observatory on Mauna Kea, Hawaii, and have been operated together as a ten element interferometer in the 230 and 345 GHz bands (eSMA, for extended Submillimeter Array). The baseline lengths presently in use range from 16 to 508 meters (52 to 1,667 ft). The radio frequencies accessible to this telescope range from 194–408 gigahertz (1.545–0.735 mm) which includes rotational transitions of dozens of molecular species as well as continuum emission from interstellar dust grains. Although the array is capable of operating both day and night, most of the observations take place at nighttime when the atmospheric phase stability is best. The SMA is jointly operated by the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA).
History The SMA project was begun in 1983 as part of a broad initiative by Irwin Shapiro, the then new director of the SAO, to produce high resolution astronomical instruments across the electromagnetic spectrum. Initially the design called for an array consisting of six antennas, but in 1996 ASIAA joined the project and funded the construction of two additional antennas and the expansion of the correlator to accommodate the near doubling of the number of interferometer baselines. Sites considered for the array included Mount Graham in Arizona, a location near the South Pole, and the Atacama Desert in Chile, but Mauna Kea was ultimately chosen due to its existing infrastructure, the availability of a fairly flat area for array construction, and the potential to include the JCMT and CSO in the array. A receiver laboratory was established at the SAO's Cambridge location in 1987.
The antennas were constructed at Haystack Observatory in Westford, Massachusetts, partially disassembled and trucked across the United States, then shipped by sea to Hawaii. The antennas were reassembled in a large hangar at the Mauna Kea summit site. The SMA was dedicated and began official operations on November 22, 2003.
Array Design
The SMA was built just northwest of the saddle between the cinder cones Pu'u Poli'ahu and Pu'u Hauoki, about 140 meters below the summit of Mauna Kea. A perennial issue for radio interferometers, especially those with a small number of antennas, is where the antennas should be placed relative to each other, in order to produce the best synthesized images. In 1996 Eric Keto studied this problem for the SMA. He found that the most uniform sampling of spatial frequencies, and thus the cleanest (lowest sidelobe) point spread function was obtained when the antennas were arranged in the shape of a Reuleaux triangle. Because of that study, pads upon which SMA antennas can be placed were arranged to form four Reuleaux triangles, with the easternmost pad forming a shared corner for all four triangles. However the SMA site is a lava field with many rocky ridges and depressions, so the pads could not be placed in exactly the optimal positions. In most cases all eight antennas are deployed on the pads forming one Reuleaux triangle, leading to four configurations named, in order of increasing size, subcompact, compact, extended and very extended. The schedule of antenna moves is determined by the requirements of the approved observing proposals, but tends to follow a roughly quarterly schedule. A custom-built transporter vehicle is used to lift an antenna off of a pad, drive it along one of the dirt access roads, and place it on a new pad while maintaining power to the cooling system for the cryogenic receivers.
Each antenna pad has a conduit connecting it to the central building, through which AC power cables, and optical fibers are pulled. Multi-mode optical fibers are used for low bandwidth digital signals, such as ethernet and phone service. Sumitomo LTCD single-mode fiber optic cables are used for the reference signals to generate the LO for the heterodyne receivers and the return of the IF signal from the antenna. The Sumitomo fibers have an extremely low coefficient of thermal expansion, which is nearly zero at the typical temperature below the surface of Mauna Kea. This allows the array to operate without closed-loop delay measurements.
Antennas
Each of the eight antennas has a 6 meter diameter primary mirror made of 72 machined cast aluminum panels. Machined aluminum was chosen over the lighter carbon fiber alternative, because of concerns that heavy snow accumulation, or windblown volcanic dust, might damage fragile carbon fiber panels. The panels, each about 1 meter wide, were machined to an accuracy of 6 microns. They are supported by a carbon fiber tube backup structure, which is enclosed by aluminum panels to protect it from windblown debris. The positions of the panels can be adjusted from the front of the dish. The initial adjustment of the surface panels in Hawaii was done in the service hangar, using a rotating template. After the antennas were deployed, the surfaces were measured using near-field holography with a 232.4 GHz beacon source mounted on the exterior cat-walk of the Subaru building, 67 meters above the SMA's subcompact pad ring. The panel positions were adjusted based on the holography results, and holography guided adjustments are repeated periodically, to maintain the surface quality. After several rounds of adjustment, the surface's error is typically about 15 microns RMS. Heating units are installed on the primary mirror, the stand supporting the secondary mirror, and the secondary mirror itself, in order to prevent ice formation in high humidity conditions. Each antenna has a cabin holding the electronics needed to control the antenna, as well as the Nasmyth focus receivers. This temperature-controlled cabin nearly encloses the antenna's steel mount to minimize pointing errors due to thermal changes.
Receivers
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