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radio telescope:see radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes
Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
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radio telescopeAn instrument for recording and measuring the radio-frequency emissions from celestial radio sources. All radio telescopes consist of an antenna, or system of antennas, connected by feeders to one or more receivers. The antennas may be in the form of dishes, simple linear dipoles, or Yagi antennas. The receiver outputs may be displayed on suitable devices such as pen-recorders or, more usually, are passed directly to a computer for storage and analysis. Often the computer can then produce an image of the radio source similar to an optical photograph (see radio brightness).
The antenna system may consist of two separated units whose electrical signals are conveyed by feeders to a common receiver, forming an interferometer. The antenna units are often mounted on an east-west line and are arranged to point in the same direction. The Earth's rotation then causes a radio source to move through the antenna beam. Waves from the source interact when the identical signals are combined, alternately reinforcing each other (producing a signal) and canceling out (producing no signal) during the source's passage through the beam. The amplitude of the summed signal thus changes periodically, producing interference fringes at the output of the receiver. The fringes are sinusoidal variations whose maximum amplitude depends on the flux density of the source and whose period depends on both the radio wavelength and the spatial separation of the antenna units. Analysis of the changing interference fringes allows source position to be determined and source structure to be studied (see coherence). An interferometer is generally used to improve the angular resolution of the telescope: the longer the distance, or baseline, between the antennas, the finer the detail that can be resolved. As resolution increases, however, large-scale structure is lost. See also aperture synthesis; array; LBI; VLBI.
an astronomical instrument for the reception of radio emission from celestial objects in the solar system, the galaxy, and the metagalaxy and the study of the characteristics of such emission, including the coordinates and structure of the sources and the intensity, spectrum, and polarization of the radiation.
A radio telescope consists of an antenna system and a receiving device—a radiometer. Antennas exhibit major differences in design, which are caused by the very broad range of wavelengths used in radio astronomy (from 0.1 mm to 1,000 m). In order that the antenna may be pointed at the region of the sky under study, radio telescopes are usually set up on altazimuth mountings, which allow rotation in azimuth and altitude; such antennas are called fully steerable. Other antennas are only capable of limited motion or none at all; in antennas of this type, which are usually very large in diameter, the direction of reception is altered by moving the feed that receives the radio waves reflected from the dish. For shortwave observations, widespread use is made of parabolic dish antennas mounted on devices capable of pointing the telescope at the radio source; in principle, the operation of these radio telescopes is analogous to optical telescopes of the reflector type. Often a combination of a number of dish antennas is used, joined by cables into a single system, called an array. For long-wave observations, an array consisting of a large number of very small dipoles is used.
A radio telescope must have a high sensitivity, in order to guarantee the reliable detection of very low densities of radio fluxes, and a good resolving power (resolution), in order to permit the observation of very fine details of the object being studied. The minimum detectable radio flux density ΔP is determined by the relation
where P is the intensity of the internal noise, S is the effective area (collecting area) of the antenna, ΔP is the frequency bandwidth received, and τ is the time over which the signal is accumulated. In order to improve the sensitivity of a radio telescope, it is necessary to increase the collecting area and use low-noise receiving devices based on masers or parametric amplifiers. The resolving power of a radio telescope θ (in radians) is approximately λ/D), where λ is the wavelength and D is the linear dimension of the antenna aperture. The largest dish antennas (operating at centimeter wavelengths with diameters up to 100 m) have a resolution of about 1″, which is comparable to the resolution of the naked eye.
The difficulties of constructing large-diameter radio telescopes with a solid reflecting dish have brought about the widespread use of arrays and, to obtain two-dimensional resolution, of cross-shaped, ring-shaped, and similar antennas with apertures that are not entirely continuous. The most radical way of obtaining high resolution in radio astronomy is the creation of a large-aperture antenna using several comparatively small antennas, which are moved relative to each other in the course of observations in accordance with the specified motion of the large imaginary antenna they represent. The existence of aperture-synthesis radio telescopes makes it possible to obtain radio images with a resolution of approximately 1”. With the use of radio interferometers with very long base lines in aperture-synthesis systems, it should be possible to achieve resolving powers of the order of 10–2–10–4″ in obtaining images of objects.
Radio emissions of cosmic origin (from the Milky Way Galaxy) at a wavelength of 14.6 m were first recorded by K. Jansky (USA) in 1931 with an antenna designed to study static from thunderstorms. The first radio telescope for studying cosmic radio emissions, a reflector 9.5 m in diameter, was constructed by G. Reber (USA) in 1937; with the help of this instrument a number of successful sky surveys were conducted. The rapid development of radio astronomy began in the 1940’s. The first radio interferometer was constructed in Australia in 1948, and the first cross-shaped radio telescope was built there in 1953. The first large, fully steerable paraboloid (D = 76 m) was built in Great Britain in 1957. The principle of obtaining high-resolution images using aperture image synthesis has been developed since 1956 in Cambridge, England. In 1967, the USA and Canada conducted the first observations using interferometers with independent signal recording and very long base lines.
As of 1975, the best fully steerable paraboloids are located at radio observatories in Effelsburg, Federal Republic of Germany (D = 100 m, wavelengths to λ = 2 cm); Pushchino and Simeiz, USSR (D = 22 m, λ = 0.8 cm); and Kitt Peak, USA (D = 11 m, λ = 0.3 cm). A radio telescope with a fixed spherical dish has been built inside a volcanic crater at Arecibo, Puerto Rico (D = 300 m, λ = 10 cm); this telescope has a very large collecting surface and is used as a radar for mapping planetary surfaces. Cross-shaped and ring-shaped radio telescopes are operating at Hoskinstown, Australia (cross of two reticulated parabolic cylinders 1,600 × 13 m, λ = 75 cm and 3 m); Kharkov, USSR (T-shaped antenna 1,800 × 900 m, consisting of 2,040 broadband dipoles, λ = 10–30 m); Pushchino, USSR (cross of 2 cylinders, 1,000 × 1,000 m, λ = 2–10 m); and Cul-goora, Australia (96 paraboloids 13 m in diameter arranged in the form of a ring, D = 3 km, λ = 3.7 m). The RATAN-600 telescope in the USSR has a reflecting surface in the form of a ring with a 7.5-meter width; D = 600 m, and the wavelength band is 0.8–30 cm. The largest aperture-synthesis systems, located at Cambridge, England (λ = 5 cm), and Westerbork, the Netherlands (λ = 6 cm), have a resolving power of approximately 3”.
REFERENCESEsepkina, N. A., D. V. Korol’kov, and Iu. N. Pariiskii. Radiotekskopy i radiometry. Moscow, 1973.
Christiansen, W., and J. Hogbom. Radiotekskopy. Moscow, 1972. (Translated from English.)
IU. N. PARIISKII