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A radio telescope is a form of Directional Radio Antenna used in Radio Astronomy and in tracking and collecting data from Satellites and Space Probe s. In their 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 Parabolic ("dish") Antenna used singularly or in an array. Radio 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 The first radio antenna used to identify an astronomical radio source was one built by , 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 . Grote Reber was one of the pioneers of what became known as Radio Astronomy when he built the first parabolic "dish" radio telescope (9m 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 The range of frequencies in the Electromagnetic Spectrum that makes up the 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 which, at the time of its construction, was the largest stearable dish radio telescope in the world.]] In the late 1950s and early 1960s saw the development of large single-dish radio telescopes. The largest individual radio telescope is the , 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 . 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 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 Radio Observatories all over the world. RADIO INTERFEROMETRY , an 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 's Group In Cambridge obtained a Nobel Prize for interferometry and aperture synthesis Nature 158 pp 339 1946. The Lloyd's Mirror interferometer was also developed independently in 1946 by Joseph Pawsey 's group at the University Of Sydney Nature 157 pp 158 1946. In the early 1950s the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 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 See Also: Radio astronomy Many astronomical objects are not only observable in Visible Light but also emit Radiation at Radio Wavelegths . Besides observing energetic objects such as Pulsar s and Quasar s, radio telescopes are able to "image" most astronomical objects such as, Galaxies , Nebulae , and even radio emissions from Planets . SEE ALSO
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