Editing Submillimeter Array (section)

== Array Design ==

[[File:SMA Array Map.png|thumb|The layout of the SMA is shown on a topographic map]]
The SMA was built just northwest of the saddle between the [[cinder cone]]s 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 frequency|spatial frequencies]], and thus the cleanest (lowest [[side lobe|sidelobe]]) [[point spread function]] was obtained when the antennas were arranged in the shape of a [[Reuleaux triangle]].<ref>{{cite journal |last1=Keto |first1=Eric |title=The shapes of cross-correlation interferometers |journal= The Astrophysical Journal|date=1997 |volume=475 |issue=2 |pages=843–852 |doi=10.1086/303545 |bibcode=1997ApJ...475..843K |s2cid=49578504 |doi-access=free }}</ref>   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.

[[File:SMATransporter.png|thumb|An SMA antenna in the observatory's transporter being moved to a new pad]]
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 fiber]]s are used for low bandwidth digital signals, such as [[ethernet]] and phone service.  Sumitomo LTCD [[Single-mode optical fiber|single-mode fiber optic cables]] are used for the reference signals to generate the [[Local oscillator|LO]] for the [[heterodyne]] receivers and the return of the [[intermediate frequency|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.<ref name="peck">{{cite book |last1=Peck |first1=A. |last2=Schinckel |first2=A. |last3=Team |first3=SMA |title=Exploring the Cosmic Frontier: Astrophysical Instruments for the 21st Century |date=2007 |publisher=Springer |isbn=978-3-540-39755-7 |pages=49–50}}</ref>