Radio telescope 46656 221655114 2008-06-25T14:04:06Z 88.100.134.220 + interwiki [[Image:parkes.arp.750pix.jpg|thumb|right|250px|The 64 meter radio telescope at [[Parkes Observatory]]]] A '''radio telescope''' is a form of [[Directional antenna|directional]] [[radio]] [[Antenna (radio)|antenna]] used in [[radio astronomy]] and in tracking and collecting data from [[satellite]]s and [[space probe]]s. In their [[Astronomy|astronomical]] role they differ from [[optical telescope]]s in that they operate in the [[radio frequency]] portion of the [[electromagnetic spectrum]] where they can detect and collect data on [[radio source]]s. Radio telescopes are typically large [[parabola|parabolic]] ("dish") antenna used singularly or in an array. Radio [[observatory|observatories]] are located far from major centers of population in order to avoid [[electromagnetic interference]] (EMI) from [[radio]], [[TV]], [[radar]], and other EMI emitting devices. This is similar to the locating of [[optical telescopes]] to avoid [[light pollution]], with the difference being that radio observatories will be placed in [[valley]]s to further shield them from EMI as opposed to clear air mountain tops for optical observatories. ==Early radio telescopes== [[Image:Grote Antenna Wheaton.gif|thumb|right|175px|Reber's first "dish" radio telescope - Wheaton, IL 1937]] The first radio antenna used to identify an astronomical radio source was one built by [[Karl Guthe Jansky]], an engineer with [[Bell Labs|Bell Telephone Laboratories]], in 1931. Jansky was assigned the job of identifying sources of [[static (radio)|static]] that might interfere with [[radio telephone]] service. Jansky's antenna was designed to receive [[short wave]] radio signals at a [[frequency]] of 20.5 [[megaHertz|MHz]] (wavelength about 14.6 m). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "''Jansky's merry-go-round''". It had a diameter of approximately {{convert|100|ft|m|-1|abbr=on}}. and stood {{convert|20|ft|m|0|abbr=on}}. tall. By rotating the antenna on a set of four Ford [[Model-T]] tires, the direction of the received interfering radio source (static) could be pinpointed. A small shed to the side of the antenna housed an [[analog signal|analog]] pen-and-paper recording system. After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky finally determined that the "faint hiss" repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is typical of an astronomical [[sidereal day]], the time it takes any "fixed" object located on the [[celestial sphere]] to pass overhead twice. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the [[Milky Way]] and was strongest in the direction of the center of the galaxy, in the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]]. [[Grote Reber]] was one of the pioneers of what became known as '''[[radio astronomy]]''' when he built the first parabolic "dish" radio telescope (9 m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies. After [[World War II]], substantial improvements in radio astronomy technology were made by astronomers in Europe, Australia and the United States, and the field of radio astronomy began to blossom. ==Radio telescope types== [[Image:Molonglotele.jpg|thumb|left|120px|A ''cylindrical paraboloid'' antenna.]] The range of frequencies in the [[electromagnetic spectrum]] that makes up the [[Radio frequency|radio spectrum]] is very large. This means the variety and types of antennas that are used as radio telescopes vary in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz - 100 MHz), they are generally [[directional antenna]] arrays similar to "TV antennas" or large stationary reflectors with moveable focal points. Since the wave length being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wire mesh. At shorter wavelengths “dish” style radio telescopes predominate. The [[angular resolution]] of a dish style antenna is a function of the diameter of the dish in proportion to the wavelength of the electromagnetic radiation being observed. This dictates the size of the dish a radio telescope needs to have a useful resolution. Radio telescopes operating at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths above 30 cm (1 GHz) range in size from 3 to 90 meters in diameter. ===Big dishes=== In the late 1950s and early 1960s saw the development of large single-dish radio telescopes. The largest individual radio telescope is the [[RATAN-600]] (built in 1977 in the [[USSR]], belongs to [[Russia]] since 1991) with 576 [[meter]] diameter of circular antenna ([http://www.sao.ru/ratan/technic/desc.html.en RATAN-600 description]). Other two individual radio telescopes at [http://www.prao.ru Pushchino Radio Astronomy Observatory], Russia, designed specially for the low frequency observations, are between the largest in their class. LPA ([http://www.prao.ru/radiotelescopes/bsa.html LPA description (in Russian)]) is 187 x 384 m size phased array meridional radio telescope, and DKR-1000 is 1000 x 1000 m cross radio telescope ([http://www.prao.ru/radiotelescopes/dkr1000.html DKR-1000 description (in Russian)] ). The largest radio telescope in Europe is the 100 meter diameter antenna in [[Effelsberg]], [[Germany]], which also was the largest fully steerable telecope for 30 years until the [[Green Bank Telescope]] was opened in 2000. The largest radio telescope in the United States until 1998 was [[Ohio State University]]'s [[The Big Ear]]. [[Image:Arecibo ObservatoryX.jpg|thumb|left|200px|World's largest single-aperture radio telescope at [[Arecibo Observatory]] in [[Puerto Rico]]]] Other well known disk radio telescopes include the [[Arecibo radio telescope]] located in [[Arecibo, Puerto Rico]], which is steerable within about 20° of the zenith and is the largest single-aperture telescope (cf. multiple aperture telescope) ever to be constructed, and the fully steerable [[Bernard Lovell|Lovell]] telescope at [[Jodrell Bank]] in the United Kingdom. A typical size of the single antenna of a radio telescope is 25 metre, dozens of radio telescopes with comparable sizes are operated in [[observatory|radio observatories]] all over the world. ==Radio interferometry== [[Image:USA.NM.VeryLargeArray.02.jpg|thumb|right|250px|The [[Very Large Array]], an [[interferometer|interferometric array]] formed from many smaller telescopes, like many larger radio telescopes.]] One of the most notable developments came in 1946 with the introduction of the technique called [[Astronomical interferometer|astronomical interferometry]]. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the [[One-Mile Telescope]]), arrays of one-dimensional antennas (e.g. the [[Molonglo Observatory Synthesis Telescope]]) or two-dimensional arrays of omni-directional [[Dipole antenna|dipoles]] (e.g. [[Antony Hewish|Tony Hewish's]] [[Interplanetary Scintillation Array|Pulsar Array]]). All of the telescopes in the array are widely separated and are connected together using [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission line]]. This not only increases the total signal collected, it can also be used in a process called [[Aperture synthesis]] to vastly increase resolution. This technique works by superposing ('''[[Interference|interfering]]''') the signal [[wave]]s from the different telescopes on the principle that [[wave]]s that coincide with the same [[phase (waves)|phase]] will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is the size of the antennas furthest apart in the array. In order to produce a high quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a '''baseline''') - as many different baselines as possible are required in order to get a good quality image (For example the [[Very Large Array]] (VLA) in [[Socorro, New Mexico]] has 27 telescopes giving 351 independent baselines at once to achieve resolution of 0.2 [[arc seconds]] at 3 cm wavelengths<ref>[http://www.gps.caltech.edu/faculty/muhleman/muhleman.html gps.caltech.edu - ''Microwave Probing of the Invisible'' by Duane O. Muhleman]</ref>). [[Martin Ryle]]'s [[Cavendish Astrophysics Group|group in Cambridge]] obtained a [[Nobel Prize]] for interferometry and aperture synthesis<ref>[[Nature (journal)|Nature]] 158 pp 339 1946</ref>. The [[Lloyd's mirror]] interferometer was also developed independently in 1946 by [[Joseph Pawsey]]'s group at the [[University of Sydney]]<ref>[[Nature (journal)|Nature]] 157 pp 158 1946</ref>. In the early 1950s the [[Cambridge Interferometer]] mapped the radio sky to produce the famous [[Second Cambridge Catalogue of Radio Sources|2C]] and [[Third Cambridge Catalogue of Radio Sources|3C]] surveys of radio sources. The largest existing radio telescope array is the [[Giant Metrewave Radio Telescope]], located in [[Pune]], [[India]]. A larger array, [[LOFAR]] (the 'LOw Frequency ARray') is currently being constructed in western Europe, consisting of 25 000 small antennas over an area several hundreds of kilometres in diameter. ==Astronomical observations== {{main|Radio astronomy}} Many astronomical objects are not only observable in [[Visible spectrum|visible light]] but also emit [[radiation]] at [[Radio frequency|radio wavelegths]]. Besides observing energetic objects such as [[pulsar]]s and [[quasar]]s, radio telescopes are able to "image" most astronomical objects such as, [[Galaxy|galaxies]], [[nebula]]e, and even radio emissions from [[planets]]. == See also == * [[List of radio telescopes]] * [[Aperture synthesis]] * [[History of astronomical interferometry]] * [[Radio astronomy]] * [[SETI]] - Search for Extra-Terrestrial Intelligence using Radio telescopes * [[Telescope]] ===Category=== * [[:category:Radio telescopes|Complete list of radio telescopes]] ==Notes== <references/> ==References== * [http://www.astronomytoday.com/astronomy/radioastro.html astronomytoday.com - "''Radio Astronomy''" by Sancar J Fredsti] * Rohlfs, K., & Wilson, T. L. (2004). Tools of radio astronomy. Astronomy and astrophysics library. Berlin: Springer. * [[Isaac Asimov|Asimov, I.]] (1979). Isaac Asimov's Book of facts; ''Sky Watchers''. New York: Grosset & Dunlap. Page 390 - 399. 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