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The world’s most powerful radio telescope, in its combination of sensitivity, resolution, and versatility, is the Very Large Array (VLA) located on the plains of San Agustin near Socorro, in central New Mexico, U.S. The VLA consists of 27 parabolic antennas, each measuring 25 metres (82 feet) in diameter. The total collecting area is equivalent to a single 130-metre (430-foot) antenna. However, the angular resolution is equivalent to a single antenna 36 km (22 miles) in diameter. Each element of the VLA can be moved by a transporter along a Y-shaped railroad track; it is possible to change the length of the arms between 600 metres (2,000 feet) and 21 km (13 miles) to vary the resolution. Each antenna is equipped with receivers that operate in eight different wavelength bands from approximately 7 mm (0.3 inch) to 4 metres (13 feet). When used at the shorter wavelength in the largest antenna configuration, the angular resolution of the VLA is better than one-tenth of an arc second, or about the same as the Hubble Space Telescope at optical wavelengths. The VLA is operated by the U.S. National Radio Astronomy Observatory as a facility of the National Science Foundation and is used by nearly 1,500 astronomers each year for a wide variety of research programs devoted to the study of the solar system, the Milky Way Galaxy, radio stars, pulsars, atomic and molecular gas in the Milky Way Galaxy and in other galaxies, radio galaxies, quasars, and the radio afterglow of gamma-ray bursts.

In Europe the Netherlands Foundation for Research in Astronomy operates the Westerbork Synthesis Radio Telescope, which is an east-west array of 14 antennas, each 25 metres (82 feet) in diameter and extending over 2.7 km (1.7 miles). In Australia the Commonwealth Scientific and Industrial Research Organization maintains the six-element Australian Telescope Compact Array at Narrabri, New South Wales, for studies of the southern skies, including in particular the nearby Magellanic Clouds.

Indian radio astronomers built the Giant Metrewave Radio Telescope (GMRT), located near Pune, India. The GMRT contains 30 antennas extending some 25 km (16 miles) in diameter. Each antenna element is 45 metres (148 feet) in diameter and is constructed using a novel, inexpensive system of wire trusses to replace the conventional steel beam backup structure of the parabolic surface. The GMRT operates at relatively long wavelengths between 20 cm (8 inches) and 6 metres (20 feet).

The Multi-Element Radio Linked Interferometer Network (MERLIN), operated by the Nuffield Radio Astronomy Laboratories at Jodrell Bank, has been upgraded to use fibre-optic, instead of microwave radio, links to connect seven antennas separated by up to 217 km (135 miles) in the southern part of England. It is used primarily to study compact radio sources associated with quasars, AGN, and cosmic masers with a resolution of a few hundredths of an arc second.

The Very Long Baseline Array (VLBA) consists of ten 25-metre (82-foot) dishes spread across the United States from the Virgin Islands to Hawaii. The VLBA operates at wavelengths from 3 mm (0.1 inch) to 1 metre (3 feet) and is used to study quasars, galactic nuclei, cosmic masers, pulsars, and radio stars with a resolution as good as 0.0001 arc second, or more than 100 times better than that of the Hubble Space Telescope. The 10 individual antenna elements of the VLBA do not have any direct connection; instead, signals are recorded on high-density computer disk drives that are then shipped to a special processing centre in New Mexico where they are replayed and the signals analyzed to form images. Precise timing between the elements is maintained by a hydrogen maser atomic clock located at each antenna site. The control and analysis centre for the VLBA is located in central New Mexico along with the VLA Operations centre, and the two instruments are sometimes used together to obtain increased sensitivity and angular resolution.

In 1997 Japanese radio astronomers working at the Institute for Space Science near Tokyo launched an 8-metre (26-foot) dish, known as the VLBI Space Observatory Program (VSOP), in Earth orbit. Working with the VLBA and other ground-based radio telescopes, VSOP gave interferometer baselines up to 33,000 km (21,000 miles). (VSOP was also known as the Highly Advanced Laboratory for Communication and Astronomy [HALCA].) In 2003 the VSOP lost its ability to point accurately, and the program ended.

Interferometers and arrays are also used at millimetre and submillimetre wavelengths, where they are used to study the formation of stars and galaxies with resolution better than can be obtained with simple filled-aperture antennas. The operation of arrays at millimetre and submillimetre wavelengths is very difficult and requires that the instrument be at very high and dry locations to minimize the phase distortions of signals as they propagate through the atmosphere. Some prominent millimetre interferometers and arrays are the Combined Array for Research in Millimeter-wave Astronomy (CARMA) near Big Pine, California, the IRAM Plateau de Bure facility in France, and the Japanese Nobeyama Radio Observatory. In 2003 the Harvard-Smithsonian Center for Astrophysics, in collaboration with the Academia Sinica of Taiwan, completed the Submillimeter Array (SMA), located near the summit of Mauna Kea, Hawaii, at an elevation of 4,080 metres (13,385 feet). This is an eight-element array of 6-metre (20-foot) dishes designed to work at wavelengths as short as 0.3 mm (0.01 inch). A major new international facility—managed by the United States, Canada, Europe, and Japan in the Atacama Desert in northern Chile, at an elevation of more than 5,000 metres (16,000 feet)—was completed in 2013. The Atacama Large Millimeter Array (ALMA) consists of fifty 12-metre (39-foot) dishes operating at wavelengths as short as 0.3 mm (0.01 inch), as well as a more compact array of four 12-metre (40-foot) and sixteen 7-metre (23-foot) dishes.

Earth-orbiting radio telescopes

Most radio waves pass relatively undistorted through Earth’s atmosphere, and so there is little need to place radio telescopes in space. The exceptions are for observations at very long wavelengths that are distorted by Earth’s ionosphere, for observations at very short wavelengths that are affected by water vapour and oxygen in the atmosphere, and for precision observations at all wavelengths that might be affected by thermal radiation from the ground. The first radio astronomy satellite was the U.S.-British Ariel 2, launched in 1964, which studied long-wavelength radio noise from Earth’s ionosphere and the Milky Way Galaxy. Ariel 2 was followed by two more satellites in the Ariel series and by the U.S. satellites Radio Astronomy Explorers 1 and 2, launched in 1968 and 1973, respectively.

Subsequent radio astronomy satellites performed observations that were difficult to make from the ground or were enhanced by being made from space (such as VSOP). The U.S. Submillimeter Wave Astronomy Satellite (SWAS) and the Swedish-Canadian-French-Finnish Odin, launched in 1998 and 2001, respectively, observed at very short, submillimetre wavelengths. By observing the cosmic microwave background radiation left over from the big bang, the U.S. satellites Cosmic Background Explorer (launched in 1989) and Wilkinson Microwave Anistropy Probe (launched in 2001) and the European satellite Planck (launched in 2009) detected very small fluctuations in the cosmic microwave background radiation corresponding to the early structures from which galaxies would be formed and were able to accurately determine the age and composition of the universe.