The 64-meter radio telescope atParkes Observatory as seen in 1969, when it was used to receive live televised video fromApollo 11Antenna ofUTR-2 low frequency radio telescope,Kharkiv region,Ukraine. Consists of an array of 2040cage dipole elements.
Since astronomical radio sources such asplanets,stars,nebulas andgalaxies are very far away, the radio waves coming from them are extremely weak, so radio telescopes require very large antennas to collect enough radio energy to study them, and extremely sensitive receiving equipment. Radio telescopes are typically largeparabolic ("dish") antennas similar to those employed in tracking and communicating withsatellites and space probes. They may be used individually or linked together electronically in an array. Radioobservatories are preferentially located far from major centers of population to avoidelectromagnetic interference (EMI) from radio,television,radar, motor vehicles, and other man-made electronic devices.
Radio waves from space were first detected by engineerKarl Guthe Jansky in 1932 atBell Telephone Laboratories inHolmdel, New Jersey using an antenna built to study radio receiver noise. The first purpose-built radio telescope was a 9-meter parabolic dish constructed by radio amateurGrote Reber in his back yard inWheaton, Illinois in 1937. The sky survey he performed is often considered the beginning of the field of radio astronomy.
Full-size replica of the first radio telescope, Jansky'sdipole array of 1932, preserved at the USGreen Bank Observatory in Green Bank, West Virginia.
Reber's "dish" radio telescope, Wheaton, Illinois, 1937
The first radio antenna used to identify an astronomical radio source was built byKarl Guthe Jansky, an engineer withBell Telephone Laboratories, in 1932. Jansky was assigned the task of identifying sources ofstatic that might interfere withradiotelephone service. Jansky's antenna was an array ofdipoles andreflectors designed to receiveshort wave radio signals at afrequency of 20.5MHz (wavelength about 14.6 meters). 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 100 ft (30 m) and stood 20 ft (6 m) tall. 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 ananalog 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 aboveshot noise, 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 astronomicalsidereal day, the time it takes any "fixed" object located on thecelestial sphere to come back to the same location in the sky. Thus Jansky suspected that the hiss originated outside of theSolar System, and by comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from theMilky Way Galaxy and was strongest in the direction of the center of the galaxy, in theconstellation ofSagittarius.
An amateur radio operator,Grote Reber, was one of the pioneers of what became known asradio astronomy. He built the first parabolic "dish" radio telescope, 9 metres (30 ft) in diameter, in his back yard in Wheaton, Illinois in 1937. 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 atvery high radio frequencies, discovering other radio sources. The rapiddevelopment of radar duringWorld 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.[4]
The range of frequencies in theelectromagnetic spectrum that makes up theradio spectrum is very large. As a consequence, the types of antennas that are used as radio telescopes vary widely in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10–100 MHz), they are generally eitherdirectional antenna arrays similar to "TV antennas" or large stationary reflectors with movable focal points. Since the wavelengths being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wiremesh such aschicken wire.[5][6] At shorter wavelengthsparabolic "dish" antennas predominate. Theangular resolution of a dish antenna is determined by the ratio of the diameter of the dish to thewavelength of the radio waves being observed. This dictates the dish size a radio telescope needs for a useful resolution. Radio telescopes that operate 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 shorter than 30 cm (above 1 GHz) range in size from 3 to 90 meters in diameter.[citation needed]
The increasing use of radio frequencies for communication makes astronomical observations more and more difficult (seeOpen spectrum).Negotiations to defend thefrequency 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 "Hydrogen line", also known as the "21 centimeter line": 1,420.40575177 MHz, used by many radio telescopes includingThe Big Ear in its discovery of theWow! signal
Comparison of theArecibo (top),FAST (middle) andRATAN-600 (bottom) radio telescopes at the same scale
The world's largest filled-aperture (i.e. full dish) radio telescope is theFive-hundred-meter Aperture Spherical Telescope (FAST) completed in 2016 byChina.[8] The 500-meter-diameter (1,600 ft) dish with an area as large as 30 football fields is built into a naturalkarst depression in the landscape inGuizhou province and cannot move; thefeed antenna is in a cabin suspended above the dish on cables. The active dish is composed of 4,450 moveable panels controlled by a computer. By changing the shape of the dish and moving the feed cabin on its cables, the telescope can be steered to point to any region of the sky up to 40° from the zenith. Although the dish is 500 meters in diameter, only a 300-meter circular area on the dish is illuminated by the feed antenna at any given time, so the actual effective aperture is 300 meters. Construction began in 2007 and was completed July 2016[9] and the telescope became operational September 25, 2016.[10]
The world's second largest filled-aperture telescope was theArecibo radio telescope located inArecibo, Puerto Rico, though it suffered catastrophic collapse on 1 December 2020. Arecibo was one of the world's few radio telescope also capable of active (i.e., transmitting)radar imaging of near-Earth objects (see:radar astronomy); most other telescopes employ passive detection, i.e., receiving only. Arecibo was another stationary dish telescope like FAST. Arecibo's 305 m (1,001 ft) dish was built into a natural depression in the landscape, the antenna was steerable within an angle of about 20° of thezenith by moving the suspendedfeed antenna, giving use of a 270-meter diameter portion of the dish for any individual observation.
The largest individual radio telescope of any kind is theRATAN-600 located nearNizhny Arkhyz,Russia, which consists of a 576-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 thezenith, and cannot receive from sources near the horizon. The largest fully steerable dish radio telescope is the 100 meterGreen Bank Telescope inWest Virginia, United States, constructed in 2000. The largest fully steerable radio telescope in Europe is theEffelsberg 100-m Radio Telescope nearBonn, Germany, operated by theMax 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.[11] The third-largest fully steerable radio telescope is the 76-meterLovell Telescope atJodrell Bank Observatory inCheshire, England, completed in 1957. The fourth-largest fully steerable radio telescopes are six 70-meter dishes: three RussianRT-70, and three in theNASA Deep Space Network. The plannedQitai Radio Telescope, at a diameter of 110 m (360 ft), is expected to become the world's largest fully steerable single-dish radio telescope when completed in 2028.
A more typical radio telescope has a single antenna of about 25 meters diameter. Dozens of radio telescopes of about this size are operated in radio observatories all over the world.
This section needs to beupdated. Please help update this article to reflect recent events or newly available information.(October 2024)
Since 1965, humans have launched four space-based radio telescopes. The aim of these projects are to take measurements in places with less interference than that on Earth.
The first telescope, KRT-10, was attached to Salyut 6 orbital space station in 1979.
One of the most notable developments came in 1946 with the introduction of the technique calledastronomical interferometry, which means combining the signals from multiple antennas so that they simulate a larger antenna, in order to achieve greater resolution. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g., theOne-Mile Telescope), arrays of one-dimensional antennas (e.g., theMolonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectionaldipoles (e.g.,Tony Hewish'sPulsar Array). All of the telescopes in the array are widely separated and are usually connected usingcoaxial cable,waveguide,optical fiber, or other type oftransmission line. Recent advances in the stability of electronic oscillators also now permit interferometry to be carried out by independent recording of the signals at the various antennas, and then later correlating the recordings at some central processing facility. This process is known asVery Long Baseline Interferometry (VLBI). Interferometry does increase the total signal collected, but its primary purpose is to vastly increase the resolution through a process calledaperture synthesis. This technique works by superposing (interfering) the signalwaves from the different telescopes on the principle thatwaves that coincide with the samephase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is equivalent in resolution (though not in sensitivity) to a single antenna whose diameter is equal to the spacing of the antennas furthest apart in the array.
A high-quality image requires a large number of different separations between telescopes. Projected separation between any two telescopes, as seen from the radio source, is called a baseline. For example, theVery Large Array (VLA) nearSocorro, New Mexico has 27 telescopes with 351 independent baselines at once, which achieves a resolution of 0.2arc seconds at 3 cm wavelengths.[12]Martin Ryle'sgroup in Cambridge obtained aNobel Prize for interferometry and aperture synthesis.[13] TheLloyd's mirror interferometer was also developed independently in 1946 byJoseph Pawsey's group at theUniversity of Sydney.[14] In the early 1950s, theCambridge Interferometer mapped the radio sky to produce the famous2C and3C surveys of radio sources. An example of a large physically connected radio telescope array is theGiant Metrewave Radio Telescope, located inPune,India. The largest array, theLow-Frequency Array (LOFAR), finished in 2012, is located in western Europe and consists of about 81,000 small antennas in 48 stations distributed over an area several hundreds of kilometers in diameter and operates between 1.25 and 30 m wavelengths. VLBI systems using post-observation processing have been constructed with antennas thousands of miles apart. Radio interferometers have also been used to obtain detailed images of the anisotropies and the polarization of theCosmic Microwave Background, like theCBI interferometer in 2004.
The world's largest physically connected telescope, theSquare Kilometre Array (SKA), is planned to start operations in 2027,[15] although the first stations had "first fringes" in 2024.[16]
^Sullivan, W.T. (1984).The Early Years of Radio Astronomy. Cambridge University Press.ISBN0-521-25485-X
^Ley, Willy; Menzel, Donald H.; Richardson, Robert S. (June 1965)."The Observatory on the Moon". For Your Information.Galaxy Science Fiction. pp. 132–150.
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