Submillimeter Array
 The Submillimeter Array | |
| Location(s) | Hawaii County, Hawaii |
|---|---|
| Coordinates | 19°49′27″N 155°28′41″W / 19.8243°N 155.478°W |
| Altitude | 4,080 m (13,390 ft) |
| Wavelength | 0.717 mm (418 GHz)–1.67 mm (180 GHz) |
| Telescope style | radio interferometer  |
| Number of telescopes | 8 |
| Diameter | 6 m (19 ft 8 in) |
| Website | www |
  Location of Submillimeter Array | |
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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 decomissioned) 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), and up to 783 meters (2,569 ft) for eSMA operations. The radio frequencies accessible to this telescope range from 180–418 gigahertz (1.666–0.717 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 and the Academia Sinica Institute of Astronomy and Astrophysics.
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.[1] Because of that study, pads upon which SMA antennas can be placed were arranged to form four Reuleaux trangles, 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.[2]
Science with the SMA
The SMA is a multi-purpose instrument which can be used to observe diverse celestial phenomena. The SMA excels at observations of dust and gas with temperatures only a few tens of kelvins above absolute zero. Objects with such temperatures typically emit the bulk of their radiation at wavelengths between a few hundred micrometers and a few millimeters, which is the wavelength range in which the SMA can observe. Commonly observed classes of objects include star-forming molecular clouds in our own and other galaxies, highly redshifted galaxies, evolved stars, and the Galactic Center. Occasionally, bodies in the Solar System, such as planets, asteroids, comets and moons, are observed.
The SMA has been used to discover that Pluto is 10 K (18 °F) cooler than expected.[3] It was the first radio telescope to resolve Pluto and Charon as separate objects.[4]
The SMA is a part of the Event Horizon Telescope, which observes nearby supermassive black holes with an angular resolution comparable to the size of the object's event horizon.
Gallery
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The Submillimeter Array at night in 2015, lit by flash -
The Array under construction in 2002
See also
- Atacama Large Millimeter Array, currently operating in Chile
References
- ^ Keto, Eric (1997). "The shapes of cross-correlation interferometers". ApJ. 475 (2): 843–852. doi:10.1086/303545. Retrieved 8 November 2020.
- ^ Peck, A.; Schinckel, A.; Team, SMA (2007). Exploring the Cosmic Frontier: Astrophysical Instruments for the 21st Century. Springer. pp. 49–50. ISBN 978-3-540-39755-7.
- ^ "A planet colder than it should be". Harvard.edu. 2006-01-03. Retrieved 2008-11-25.
- ^ Gurwell, Mark A; Butler, Bryan J (August 2005). "Sub-Arcsecond Scale Imaging of the Pluto/Charon Binary System at 1.4 mm". Bulletin of the American Astronomical Society. 37: 743. Bibcode:2005DPS....37.5501G.
External links
- Submillimeter Array website
- Smithsonian Astrophysical Observatory website
- Academia Sinica Institute of Astronomy and Astrophysics website
