A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night.
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.
The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~50 miles (80 km) west of Socorro. The VLA comprises twenty-eight 25-meter radio telescopes deployed in a Y-shaped array and all the equipment, instrumentation, and computing power to function as an interferometer. Each of the massive telescopes is mounted on double parallel railroad tracks, so the radius and density of the array can be transformed to adjust the balance between its angular resolution and its surface brightness sensitivity. Astronomers using the VLA have made key observations of black holes and protoplanetary disks around young stars, discovered magnetic filaments and traced complex gas motions at the Milky Way's center, probed the Universe's cosmological parameters, and provided new knowledge about the physical mechanisms that produce radio emission.
The National Radio Astronomy Observatory (NRAO) is a federally funded research and development center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy. NRAO designs, builds, and operates its own high-sensitivity radio telescopes for use by scientists around the world.
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Radio Astronomy Observatory (NRAO) until September 30, 2016. Since October 1, 2016, the telescope has been operated by the independent Green Bank Observatory. The telescope's name honors the late Senator Robert C. Byrd who represented West Virginia and who pushed the funding of the telescope through Congress.
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation and orientation, the lobe-pattern of the interferometer produces an output which is one component of the Fourier transform of the spatial distribution of the brightness of the observed object. The image of the source is produced from these measurements. Astronomical interferometers are commonly used for high-resolution optical, infrared, submillimetre and radio astronomy observations. For example, the Event Horizon Telescope project derived the first image of a black hole using aperture synthesis.
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 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. The baseline lengths presently in use range from 16 to 508 meters. 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.
HALCA, also known for its project name VSOP, the code name MUSES-B, or just Haruka (はるか) was a Japanese 8 meter diameter radio telescope satellite which was used for Very Long Baseline Interferometry (VLBI). It was the first such space-borne dedicated VLBI mission.
The Low-Frequency Array, or LOFAR, is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Institute for Radio Astronomy, it was first opened by queen Beatrix of The Netherlands in 2010, and has since been operated on behalf of the International LOFAR Telescope (ILT) partnership by ASTRON.
The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources using very-long-baseline interferometry (VLBI). The EVN is the most sensitive VLBI array in the world, and the only one capable of real-time observations. The Joint Institute for VLBI ERIC (JIVE) acts as the central organisation in the EVN, providing both scientific user support and a correlator facility. Very Long Baseline Interferometry (VLBI) achieves ultra-high angular resolution and is a multi-disciplinary technique used in astronomy, geodesy and astrometry.
Haystack Observatory is a multidisciplinary radio science center, ionospheric observatory, and astronomical microwave observatory owned by Massachusetts Institute of Technology (MIT). It is located in Westford, Massachusetts (US), approximately 45 kilometers (28 mi) northwest of Boston. Haystack was initially built by MIT's Lincoln Laboratory for the United States Air Force and was known as Haystack Microwave Research Facility. Construction began in 1960, and the antenna began operating in 1964. In 1970 the facility was transferred to MIT, which then formed the Northeast Radio Observatory Corporation (NEROC) with a number of other universities to operate the site as the Haystack Observatory. As of January 2012, a total of nine institutions participated in NEROC.
Associated Universities, Inc. (AUI) is a research management corporation that builds and operates facilities for the research community. AUI is a not-for-profit 501(c)(3) corporation, headquartered in Washington, DC. The President is Dr. Adam Cohen. AUI's major current operating unit is the National Radio Astronomy Observatory (NRAO), which it operates under a Cooperative Agreement with the National Science Foundation.
An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, nebulas and galaxies by means of interferometry. The advantage of this technique is that it can theoretically produce images with the angular resolution of a huge telescope with an aperture equal to the separation between the component telescopes. The main drawback is that it does not collect as much light as the complete instrument's mirror. Thus it is mainly useful for fine resolution of more luminous astronomical objects, such as close binary stars. Another drawback is that the maximum angular size of a detectable emission source is limited by the minimum gap between detectors in the collector array.
The NRAO VLA Sky Survey (NVSS) was an astronomical survey of the Northern Hemisphere carried out by the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO), resulting in an astronomical catalogue. It was led by James J. Condon.
The Green Bank Interferometer (GBI) is a former radio astronomy telescope located at Green Bank, West Virginia (USA) and operated by the National Radio Astronomy Observatory (NRAO). It included three on-site radio telescopes of 85-foot (26m) diameter, designated 85-1, 85-3, and 85-2 and a portable telescope.
MeerKAT, originally the Karoo Array Telescope, is a radio telescope consisting of 64 antennas in the Meerkat National Park, in the Northern Cape of South Africa. In 2003, South Africa submitted an expression of interest to host the Square Kilometre Array (SKA) Radio Telescope in Africa, and the locally designed and built MeerKAT was incorporated into the first phase of the SKA. MeerKAT was launched in 2018.
The Array Operations Center (AOC) in Socorro, New Mexico, is the control and monitor center for the Very Long Baseline Array. From the AOC, National Radio Astronomy Observatory operators are able to remotely control and monitor the ten VLBA telescope stations over the internet. The operators aim the antennas, select radio frequencies for observation, control the hard drives, and monitor the weather and 'health' of the equipment at every site. A real-time display of the array's status is available for observers to also monitor their observations.
The OVRO 40 meter Telescope is a radio telescope at the Owens Valley Radio Observatory near Big Pine, California, US. It is owned and operated by Caltech. The telescope is easily visible from the section of US highway 395 just north of Big Pine.
Barry Gillespie Clark is an American astronomer who led the development of the world's first digitally recorded, software correlated Very Long Baseline Interferometry (VLBI) system for radio astronomy, the Green Bank Interferometer. He was also heavily involved in the development of the Very Large Array and the Very Long Baseline Array.
Related media on Wikimedia Commons 
