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49-736: The SMA is jointly operated by the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). 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

98-514: A superconductor–insulator–superconductor tunnel junction ( SIS ) – is an electronic device consisting of two superconductors separated by a very thin layer of insulating material. Current passes through the junction via the process of quantum tunneling . The STJ is a type of Josephson junction , though not all the properties of the STJ are described by the Josephson effect. These devices have

147-472: A 48 GHz total sky frequency coverage. SWARM can also operate as a phased array summer, making the SMA appear to be a single antenna for VLBI operations. 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

196-532: 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. The SMA uses cryogenic SIS heterodyne receivers , at a bent Nasmyth focus. 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

245-406: A contract or grant. (Additional postdocs do research via Harvard fellowship programs or national/international fellowship awards); about 40% of the postdoctoral community are women and about 12% are from minority populations. SAO scientists can supervise Harvard Ph.D students, and in addition they typically supervise about 30 graduate students from other institutions who are pursuing Ph.D. theses at

294-585: 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

343-475: A high frequency current is applied to a Josephson junction, the ac Josephson current will synchronize with the applied frequency giving rise to regions of constant voltage in the I–V curve of the device (Shapiro steps). For the purpose of voltage standards, these steps occur at the voltages n f / K J {\displaystyle nf/K_{\text{J}}} where n {\displaystyle n}

392-606: A rapid expansion of its research program. Following the launch of Sputnik (the world's first human-made satellite) in 1957, SAO accepted a national challenge to create a worldwide satellite-tracking network, collaborating with the United States Air Force on Project Space Track . With the creation of NASA the following year and throughout the space race , SAO led major efforts in the development of orbiting observatories and large ground-based telescopes , laboratory and theoretical astrophysics , as well as

441-619: 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 GHz, without gaps. However full polarization measurements can only be made around 230 and 345 GHz, where pairs of receivers can be tuned to

490-423: A small quasiparticle tunneling current – called the subgap current – is present even for voltages less than twice the energy gap due to the thermal promotion of quasiparticles above the gap. If the STJ is irradiated with photons of frequency f {\displaystyle f} , the dc current-voltage curve will exhibit both Shapiro steps and steps due to photon-assisted tunneling. Shapiro steps arise from

539-509: A superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between cryogenic temperatures and room temperature, so lead is no longer widely used to make STJs. STJs are the most sensitive heterodyne receivers in the 100 GHz to 1000 GHz frequency range, and hence are used for radio astronomy at these frequencies. In this application,

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588-466: A well-defined overlap region, a procedure known as the Niemeyer-Dolan technique is commonly used. This technique uses a suspended bridge of resist with a double-angle deposition to define the junction. Aluminum is widely used for making superconducting tunnel junctions because of its unique ability to form a very thin (2–3 nm) insulating oxide layer with no defects that short-circuit

637-426: A wide range of applications, including high-sensitivity detectors of electromagnetic radiation , magnetometers , high speed digital circuit elements, and quantum computing circuits. All currents flowing through the STJ pass through the insulating layer via the process of quantum tunneling . There are two components to the tunneling current. The first is from the tunneling of Cooper pairs . This supercurrent

686-545: Is 10  K (18 °F) cooler than expected. It was the first radio telescope to resolve Pluto and Charon as separate objects. 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 first image of a black hole . Smithsonian Astrophysical Observatory The Smithsonian Astrophysical Observatory ( SAO )

735-540: Is a research institute of the Smithsonian Institution , concentrating on astrophysical studies including galactic and extragalactic astronomy , cosmology , solar , earth and planetary sciences , theory and instrumentation, using observations at wavelengths from the highest energy gamma rays to the radio , along with gravitational waves .  Established in Washington, D.C. , in 1890,

784-636: Is also introduced to the LO, in order to suppress cross talk between the 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 GHz, the array can observe a 44 GHz wide interval of sky frequencies without gaps. The original SMA correlator was designed to correlate 2 GHz of IF bandwidth per sideband from each of two active receivers in eight antennas, producing spectral data for 28 baselines. Because

833-470: Is an integer, f {\displaystyle f} is the applied frequency and the Josephson constant K J {\displaystyle K_{\text{J}}} = 483 597 .8484... × 10  Hz⋅V ‍ is a constant that is equal to 2 e / h {\displaystyle 2e/h} . These steps provide an exact conversion from frequency to voltage. Because frequency can be measured with very high precision, this effect

882-500: Is described by the ac and dc Josephson relations , first predicted by Brian David Josephson in 1962. For this prediction, Josephson received the Nobel prize in physics in 1973. The second is the quasiparticle current, which, in the limit of zero temperature, arises when the energy from the bias voltage e V {\displaystyle eV} exceeds twice the value of superconducting energy gap Δ. At finite temperature,

931-652: 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

980-518: Is suitable for making tunnel junctions. To form an insulating oxide, the first layer of niobium can be coated with a very thin layer (approximately 5 nm) of aluminum, which is then oxidized to form a high quality aluminum oxide tunnel barrier before the final layer of niobium is deposited. The thin aluminum layer is proximitized by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2 K. Early work used lead -lead oxide-lead tunnel junctions. Lead has

1029-402: Is then introduced into the chamber, resulting in the formation of an insulating layer of aluminum oxide (Al 2 {\displaystyle _{2}} O 3 {\displaystyle _{3}} ) with a typical thickness of several nanometres . After the vacuum is restored, an overlapping layer of superconducting metal is deposited, completing the STJ. To create

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1078-587: The Sun 's intensity in different regions of the optical electromagnetic spectrum . In doing so, the observatory enabled Abbot to make critical refinements to the Solar constant , as well as to serendipitously discover Solar variability . It is likely that SAO's early history as a solar observatory was part of the inspiration behind the Smithsonian's "sunburst" logo , designed in 1965 by Crimilda Pontes . In 1955,

1127-469: The analog-to-digital converters sampled at 208 MHz, the IF was downconverted into 24 partially overlapping "chunks", each 104 MHz wide, before sampling. After sampling, the data were sent to 90 large PC boards, each of which held 32 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 FFT

1176-496: The application of computers to astrophysical problems. SAO has operated a number of remote stations over the years. The current director of the SAO is Lisa Kewley (2022 to present). There are currently about 170 research staff working at the SAO, including affiliated research staff. In addition, the SAO has about 120 postdoctoral researchers/fellows working in five competitive, associated fellowship programs: CfA, Clay, SMA, ITAMP, and Leon Van Speybroeck , or in support of

1225-494: The infrared . The superconducting quantum interference device or SQUID is based on a superconducting loop containing Josephson junctions. SQUIDs are the world's most sensitive magnetometers , capable of measuring a single magnetic flux quantum . Superconducting quantum computing utilizes STJ-based circuits, including charge qubits , flux qubits and phase qubits . The STJ is the primary active element in rapid single flux quantum or RSFQ fast logic circuits. When

1274-575: The CSO and JCMT to the array, but only for a single receiver per antenna. 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 and Xilinx Virtex-6 SX475T FPGAs rather than purpose-built correlator chips. The FPGAs are housed with additional electronics on ROACH2 boards produced by

1323-521: The Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). The new correlator operates at only one spectral configuration, uniform 140 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

1372-494: The Mauna Kea summit site. The SMA was dedicated and began official operations on November 22, 2003. 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

1421-632: The SAO moved its headquarters in 1955 to Cambridge, Massachusetts , where its research is a collaboration with the Harvard College Observatory (HCO) and the Harvard University Department of Astronomy. In 1973, the Smithsonian and Harvard formalized the collaboration as the Center for Astrophysics | Harvard & Smithsonian (CfA) under a single Director. Samuel Pierpont Langley , the third Secretary of

1470-474: The SAO. About thirty undergraduate students intern at the SAO each year. All together there are about 950 staff (including administrative and management department employees) working at the Center. 42°22′53″N 71°07′42″W  /  42.38146°N 71.12837°W  / 42.38146; -71.12837 Superconducting tunnel junction The superconducting tunnel junction ( STJ ) – also known as

1519-452: The STJ is dc biased at a voltage just below the gap voltage ( | V | = 2 Δ / e {\displaystyle |V|=2\Delta /e} ). A high frequency signal from an astronomical object of interest is focused onto the STJ, along with a local oscillator source. Photons absorbed by the STJ allow quasiparticles to tunnel via the process of photon-assisted tunneling. This photon-assisted tunneling changes

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1568-423: The STJ is biased with a dc voltage less than the gap voltage. A photon absorbed in the superconductor breaks Cooper pairs and creates quasiparticles . The quasiparticles tunnel across the junction in the direction of the applied voltage, and the resulting tunneling current is proportional to the photon energy. STJ devices have been employed as single-photon detectors for photon frequencies ranging from X-rays to

1617-628: The Smithsonian , founded the Smithsonian Astrophysical Observatory on the south yard of the Smithsonian Castle (on the U.S. National Mall ) on March 1, 1890 . The Astrophysical Observatory's initial, primary purpose was to " record the amount and character of the Sun's heat ". Charles Greeley Abbot was named SAO's first director, and the observatory operated solar telescopes to take daily measurements of

1666-417: 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 trangles, with

1715-622: 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

1764-423: The correlator can process 2 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 polarization parameters are calculated. Somewhat confusingly, there are now six SWARM "quadrants" in the full correlator, allowing 12 GHz of bandwidth to be correlated for each sideband of two receivers on all baselines, allowing

1813-483: The current-voltage curve, creating a nonlinearity that produces an output at the difference frequency of the astronomical signal and the local oscillator. This output is a frequency down-converted version of the astronomical signal. These receivers are so sensitive that an accurate description of the device performance must take into account the effects of quantum noise . In addition to heterodyne detection, STJs can also be used as direct detectors. In this application,

1862-452: 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

1911-425: 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 quadrupod supporting the secondary mirror, and the secondary mirror itself, in order to prevent ice formation in high humidity conditions. Each antenna has

1960-510: The insulating layer. The superconducting critical temperature of aluminum is approximately 1.2  K . For many applications, it is convenient to have a device that is superconducting at a higher temperature, in particular at a temperature above the boiling point of liquid helium , which is 4.2 K at atmospheric pressure. One approach to achieving this is to use niobium , which has a superconducting critical temperature in bulk form of 9.3 K. Niobium, however, does not form an oxide that

2009-547: 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

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2058-635: 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

2107-580: 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. Each of the eight antennas has a 6 meter diameter primary mirror made of 72 machined cast aluminum panels. Machined aluminum

2156-417: The response of the quasiparticles and gives rise to steps displaced in voltage by n h f / e {\displaystyle nhf/e} relative to the gap voltage. The device is typically fabricated by first depositing a thin film of a superconducting metal such as aluminum on an insulating substrate such as silicon . The deposition is performed inside a vacuum chamber . Oxygen gas

2205-464: The response of the supercurrent and occur at voltages equal to n h f / ( 2 e ) {\displaystyle nhf/(2e)} , where h {\displaystyle h} is the Planck constant , e {\displaystyle e} is the electron charge, and n {\displaystyle n} is an integer . Photon-assisted tunneling arises from

2254-431: The same frequency, and 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 pattern of 90 degree phase changes in the LO signal , and demodulating that pattern within the correlator. A Walsh pattern of 180 degree phase changes, unique to each antenna,

2303-565: The scientific headquarters of SAO moved from Washington, D.C. to Cambridge, Massachusetts to affiliate with the Harvard College Observatory (HCO). Fred Lawrence Whipple , then the chairman of the Harvard Astronomy Department, was named the new director of SAO. The collaborative relationship between SAO and HCO therefore predates the official creation of the CfA by 18 years. SAO's move to Harvard's campus also resulted in

2352-504: Was applied to convert the lag data to spectra. 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, USNO , NASA , NRFA and JIVE . The correlator could also be configured to correlate all 45 baselines produced by adding

2401-410: 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

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