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The Dish: Fact vs fiction - CSIROscope
Date All. Keyword All. Intro Astronomy Jansky was assigned the job of identifying sources of static that might interfere with radio telephone service. Jansky's antenna was an array of dipoles and reflectors designed to receive short wave radio signals at a frequency of It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". By rotating the antenna, the direction of the received interfering radio source static could be pinpointed. A small shed to the side of the antenna housed an 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 period is the length of an astronomical sidereal day , the time it takes any "fixed" object located on the celestial sphere to come back to the same location in the sky. Thus Jansky suspected that the hiss originated outside of the Solar System , and by comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way Galaxy and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.
An amateur radio operator, Grote Reber , was one of the pioneers of what became known as radio astronomy. He repeated Jansky's pioneering work, identifying the Milky Way as the first off-world radio source, and he went on to conduct the first sky survey at very high radio frequencies, discovering other radio sources.
The rapid development of radar during World War II created technology which was applied to radio astronomy after the war, and radio astronomy became a branch of astronomy, with universities and research institutes constructing large radio telescopes. The range of frequencies in the electromagnetic spectrum that makes up the radio spectrum is very large.
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This means that the types of antennas that are used as radio telescopes vary widely in design, size, and configuration. They are generally either directional antenna arrays similar to "TV antennas" or large stationary reflectors with moveable focal points. Since the wavelengths being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wire mesh such as chicken wire. The angular resolution of a dish antenna is determined by the ratio of the diameter of the dish to the wavelength of the radio waves being observed.
This dictates the dish size a radio telescope needs for a useful resolution. The increasing use of radio frequencies for communication makes astronomical observations more and more difficult see Open spectrum. Negotiations to defend the frequency allocation for parts of the spectrum most useful for observing the universe are coordinated in the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science.
The world's largest filled-aperture i. The active dish is composed of moveable panels controlled by a computer. Construction was begun in and completed July  and the telescope became operational September 25, The world's second largest filled-aperture telescope is the Arecibo radio telescope located in Arecibo, Puerto Rico.
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The largest individual radio telescope of any kind is the RATAN located near Nizhny Arkhyz , Russia , which consists of a meter circle of rectangular radio reflectors, each of which can be pointed towards a central conical receiver. The above stationary dishes are not fully "steerable"; they can only be aimed at points in an area of the sky near the zenith , and cannot receive from sources near the horizon.
The largest fully steerable dish radio telescope is the meter Green Bank Telescope in West Virginia , United States, constructed in The largest fully steerable radio telescope in Europe is the Effelsberg m Radio Telescope near Bonn , Germany, operated by the Max Planck Institute for Radio Astronomy , which also was the world's largest fully steerable telescope for 30 years until the Green Bank antenna was constructed. A typical size of the single antenna of a radio telescope is 25 meters.
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Dozens of radio telescopes with comparable sizes are operated in radio observatories all over the world. Retrieved 14 August Australia Telescope National Facility Wiki. Archived from the original on 5 July Retrieved 10 August The Telegraph. Archived from the original on 16 February Retrieved 4 April The Guardian. Archived from the original on 3 March Retrieved 16 December Archived from the original on 7 May Retrieved 7 May International Business Times.
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