Editing Submillimeter Array

{{short description|Astronomical radio interferometer in Hawaii, USA}}
{{Infobox telescope|logo = SubmillimeterArrayLogo.png|locmapin=USA Hawaii|}}

The '''Submillimeter Array''' ('''SMA''') consists of eight {{convert|6|m|adj=on|sp=us}} diameter [[radio telescope]]s arranged as an [[interferometer]] for [[submillimetre astronomy|submillimeter]] wavelength observations. It is the first purpose-built submillimeter interferometer, constructed after successful interferometry experiments using the pre-existing {{convert|15|m|0|adj=on|sp=us}} [[James Clerk Maxwell Telescope]] and {{convert|10.4|m|1|adj=on|sp=us}} [[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&nbsp;[[GHz]] bands (eSMA, for '''e'''xtended '''S'''ub'''m'''illimeter '''A'''rray). The baseline lengths presently in use range from {{convert|16|to|508|m|0|sp=us}}. The radio frequencies accessible to this telescope range from {{convert|194-408|GHz|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.
<ref name="ho">{{cite journal |last1=Ho |first1=T.P. |last2=Moran |first2=James M. |last3=Lo |first3=Kwok Yung |title=The Submillimeter Array |journal= The Astrophysical Journal|date=28 October 2004 |volume=616 |issue=1 |pages=L1–L6 |doi=10.1086/423245 |arxiv=astro-ph/0406352 |bibcode=2004ApJ...616L...1H |s2cid=115133614 |url=https://iopscience.iop.org/article/10.1086/423245 |access-date=9 November 2020}}</ref>

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 ==

[[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>

== Antennas ==
[[File:SMAAntenna1.png|thumb|An SMA antenna deployed on a pad]]
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.<ref>{{cite book |last1=Sridharan |first1=T.K. |last2=Saito |first2=Masao |last3=Patel |first3=Nimesh |title=Holographic Surface Quality Measurements of the Sub-Millimeter Array Antennas |date=August 2002 |publisher=URSI General Assembly |location=Maastricht |url=https://www.ursi.org/proceedings/procGA02/papers/p1387.pdf |access-date=11 November 2020}}</ref>

Heating units are installed on the primary mirror, the quadrupod 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 ==
[[File:SMA230GHzInsert.jpg|thumb|An SMA receiver insert, covering frequencies from 194 to 240&nbsp;GHz.   The large cryostat in each antenna can house up to eight inserts.]]
[[File:SMACryostatDiagram.png|thumb|Cutaway diagram of an SMA receiver cryostat showing the signal path]]
The SMA uses cryogenic [[Superconducting tunnel junction|SIS]] [[Superheterodyne receiver|heterodyne receivers]], at a bent [[Nasmyth telescope|Nasmyth]] focus.<ref>{{cite book |last1=Blundell |first1=Raymond |title=The Submillimeter Array – Antennas and Receivers |date=2004 |publisher=15th International Symposium on Space Terahertz Technology |location=Northhampton, MA |url=https://www.nrao.edu/meetings/isstt/papers/2004/2004003015.pdf |access-date=12 November 2020}}</ref>  All receivers are mounted in a single large [[cryostat]] within the antenna cabin.   The cryostat can accommodate up to eight receiver inserts, each of which holds a single receiver.   A rotating wire grid [[beam splitter]] followed by a rotating mirror directs the two linear polarizations of the incoming radiation to two of the receiver inserts.   This allows the array to observe either a single polarization of two different frequency bands simultaneously, or both polarizations of a single band simultaneously to improve sensitivity and measure [[Stokes parameters]].

Receivers are available to cover frequencies from 194 to 408&nbsp;GHz, without gaps.  However full polarization measurements can only be made around 230 and 345&nbsp;GHz, where pairs of receivers can be tuned to the same frequency, and [[Waveplate#Quarter-wave plate|quarter wave plates]] optimized for those frequencies can be inserted into the optical path.

The receivers are sensitive to both sidebands produced by the heterodyne mixing.   The sidebands are separated by introducing a [[Walsh function|Walsh pattern]] of 90 degree phase changes in the [[local oscillator|LO signal]], and demodulating that pattern within the correlator. A Walsh pattern of 180 degree phase changes, unique to each antenna, is also introduced to the LO, in order to suppress cross talk between the [[intermediate frequency|IFs]] arriving at the correlator from different antennas.

Thanks to the recent wideband update of the SMA receivers, with two receivers tuned to frequencies offset by 12&nbsp;GHz, the array can observe a 44&nbsp;GHz wide interval of sky frequencies without gaps.

== Correlator ==
The original SMA correlator was designed to correlate 2&nbsp;GHz of IF bandwidth per sideband from each of two active receivers in eight antennas, producing spectral data for 28 baselines.    Because the [[analog-to-digital converter]]s sampled at 208&nbsp;MHz, the IF was [[Heterodyne|downconverted]] into 24 partially overlapping "chunks", each 104&nbsp;MHz wide, before sampling.  After sampling, the data were sent to 90 large PC boards, each of which held 32 [[Application-specific integrated circuit|ASIC]] correlator chips.   The correlator was an XF design; in the default configuration 6144 lags were calculated for each of two receivers on 28 baselines, before an [[Fast Fourier transform|FFT]] was applied to convert the lag data to spectra.<ref name="ho" />   In the default configuration the spectral resolution was 812.5 kHz per channel, but the correlator could be reconfigured to increase the spectral resolution on certain chunks, at the expense of lower resolution elsewhere in the spectrum.  The correlator chips were designed at MIT Haystack, and funded by five institutions: SMA, [[United States Naval Observatory|USNO]], [[NASA]], [[Netherlands Foundation for Radio Astronomy|NRFA]] and [[Joint Institute for VLBI ERIC|JIVE]].<ref name="peck" />   The correlator could also be configured to correlate all 45 baselines produced by adding the CSO and JCMT to the array, but only for a single receiver per antenna.

[[File:SMASWARMCorrelatorOutput.png|thumb|A spectrum produced by the SWARM correlator when the SMA observed Orion BN/KL in 2016. This spectrum was produced when only four quadrants of SWARM were available. Six quadrants are available now.]]
In 2016 a new correlator called SWARM was brought online, allowing more total IF bandwidth to be correlated, increasing the array's sensitivity to continuum sources as well as its instantaneous spectral coverage.   The new correlator, an FX design, uses 4.576 GHz analog-to-digital converters<ref>{{cite journal |last1=Jiang |first1=H. |last2=Liu |first2=H. |last3=Guzzino |first3=K. |last4=Kubo |first4=Derek |title=A 5 Giga Samples Per Second 8-Bit Analog to Digital Printed Circuit Board for Radio Astronomy |journal=Publications of the Astronomical Society of the Pacific |date=July 2014 |volume=126 |issue=942 |pages=761–768 |doi=10.1086/677799 |bibcode=2014PASP..126..761J |s2cid=120387426 |url=https://www.researchgate.net/publication/271651990 |access-date=9 November 2020|doi-access=free }}</ref> and Xilinx Virtex-6 SX475T FPGAs rather than purpose-built correlator chips.   The FPGAs are housed with additional electronics on ROACH2 boards produced by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER).   The new correlator operates at only one spectral configuration, uniform 140&nbsp;kHz per channel resolution across the entire bandwidth.   The data are stored at this high spectral resolution even for projects that require only low resolution, so that the highest resolution will be retained in the observatory's data archive for use in later research.   Each quadrant of the correlator can process 2&nbsp;GHz of IF bandwidth per sideband for two active receivers in all eight antennas.   When the two receivers are tuned to the same frequency, full [[stokes parameters|Stokes]] polarization parameters are calculated.<ref>{{cite journal |last1=Primiani |first1=Rurik A. |last2=Young |first2=Kenneth H. |last3=Young |first3=Andre |last4=Patel |first4=Nimesh |last5=Wilson |first5=Robert W. |last6=Vertatschitsch |first6=Laura |last7=Chitwood |first7=Billie B. |last8=Srinivasan |first8=Ranjani |last9=MacMahon |first9=David |last10=Weintroub |first10=Jonathan |title=SWARM: A 32&nbsp;GHz Correlator and VLBI Beamformer for the Submillimeter Array |journal=Journal of Astronomical Instrumentation |date=2016 |volume=5 |issue=4 |pages=1641006–810 |doi=10.1142/S2251171716410063|arxiv=1611.02596 |bibcode=2016JAI.....541006P |s2cid=114780818 }}</ref>   Somewhat confusingly, there are now six SWARM "quadrants" in the full correlator, allowing 12&nbsp;GHz of bandwidth to be correlated for each sideband of two receivers on all baselines, allowing a 48&nbsp;GHz total sky frequency coverage.

SWARM can also operate as a phased array summer, making the SMA appear to be a single antenna for [[Very-long-baseline interferometry|VLBI]] operations.

== 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 [[kelvin]]s 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 [[redshift]]ed galaxies, evolved stars, and the [[Galactic Center]]. Occasionally, bodies in the Solar System, such as [[planet]]s, [[asteroid]]s, [[comet]]s and [[natural satellite|moons]], are observed.

The SMA has been used to discover that [[Pluto]] is {{convert|10|K-change|0|lk=in}} cooler than expected.<ref>{{cite web |url=http://www.cfa.harvard.edu/press/2006/pr200601.html |title=A planet colder than it should be |work=Harvard.edu |date=2006-01-03 |access-date=2008-11-25}}</ref> It was the first radio telescope to resolve Pluto and Charon as separate
objects.<ref>{{cite journal |title=Sub-Arcsecond Scale Imaging of the Pluto/Charon Binary System at 1.4 mm |journal=Bulletin of the American Astronomical Society |first1=Mark A |last1=Gurwell |first2=Bryan J |last2=Butler |volume=37 |page=743 |date=August 2005 |bibcode=2005DPS....37.5501G}}</ref>

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]] and which produced the [[Event Horizon Telescope#Messier 87*|first image of a black hole]].

== Gallery ==
{{Gallery
|width=300
|Image:Submillimeter Array Night.jpg|The Submillimeter Array at night in 2015, lit by [[Flash photography|flash]]
|Image:Mk submm array.jpg|The Array under construction in 2002
}}

== See also ==
* [[Atacama Large Millimeter Array]], currently operating in Chile

== References ==
{{reflist}}

== External links ==
* [http://www.cfa.harvard.edu/sma/ Submillimeter Array website]
*  [[Robert Pinsky]], [https://www.cfa.harvard.edu/~jmoran/PinskyPoem.pdf poem about the Submillimeter Array]
* [http://www.cfa.harvard.edu/saohome.html Smithsonian Astrophysical Observatory website]
* [http://www.asiaa.sinica.edu.tw/ Academia Sinica Institute of Astronomy and Astrophysics website]

{{Commons category|Submillimeter Array}}
{{radio-astronomy}}
{{Portal bar|Hawaii|Astronomy|Stars|Outer space|Solar System|Education|Science}}
[[Category:Radio telescopes]]
[[Category:Submillimetre telescopes]]
[[Category:Interferometric telescopes]]
[[Category:Astronomical observatories in Hawaii]]
[[Category:Buildings and structures in Hawaii County, Hawaii]]