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{{short description|Directional radio antenna used in radio astronomy}}
[[File:CSIRO ScienceImage 4350 CSIROs Parkes Radio Telescope with moon in the background.jpg|thumb|upright=1.2|The 64-meter radio telescope at [[Parkes Observatory]] as seen in 1969, when it was used to receive live televised video from [[Apollo 11]]]]
[[File:UTR-2 - P3094042 (wiki).jpg|thumb|upright=1.0|Antenna of [[UTR-2]] low frequency radio telescope, [[Kharkiv]] region, [[Ukraine]]. Consists of an array of 2040 [[dipole antenna|cage dipole]] elements. ]]

A '''radio telescope''' is a specialized [[antenna (radio)|antenna]] and [[radio receiver]] used to detect [[radio wave]]s from [[astronomical radio source]]s in the sky.<ref name="Marr">{{cite book
 | last1  = Marr
 | first1 = Jonathan M.
 | last2  = Snell
 | first2 = Ronald L.
 | last3  = Kurtz
 | first3 = Stanley E.
 | title  = Fundamentals of Radio Astronomy: Observational Methods
 | publisher = CRC Press
 | date   = 2015
 | pages  = 21–24
 | url    = https://books.google.com/books?id=T54oCwAAQBAJ&pg=PA21
 | isbn   = 978-1498770194
 }}</ref><ref name="Britannica">{{cite book
 | title = Britannica Concise Encyclopedia
 | publisher = Encyclopædia Britannica, Inc.
 | date   = 2008
 | pages  = 1583
 | url    = https://books.google.com/books?id=ea-bAAAAQBAJ&q=%22radio+telescope%22&pg=PA1583
 | isbn   = 978-1593394929
 }}</ref><ref name="Verschuur">{{cite book
 | last1  = Verschuur
 | first1 = Gerrit
 | title  =  The Invisible Universe: The Story of Radio Astronomy
 | publisher = Springer Science & Business Media
 | edition = 2
 | date   = 2007
 | pages  = 8–10
 | url    = https://books.google.com/books?id=bUVQM_BAFlMC&q=%22radio+telescope%22+%22radio+receiver%22&pg=PA8
 | isbn   = 978-0387683607
 }}</ref> Radio telescopes are the main observing instrument used in [[radio astronomy]], which studies the [[radio frequency]] portion of the [[electromagnetic spectrum]], just as [[optical telescope]]s are used to make observations in the [[visible light|visible]] portion of the spectrum in traditional [[optical astronomy]]. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night.

Since astronomical radio sources such as [[planet]]s, [[star]]s, [[nebula]]s and [[galaxy|galaxies]] 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 large [[Parabolic antenna|parabolic ("dish") antennas]] similar to those employed in tracking and communicating with [[satellite]]s and space probes. They may be used individually or linked together electronically in an array. Radio [[observatory|observatories]] are preferentially located far from major centers of population to avoid [[electromagnetic interference]] (EMI) from radio, [[television]], [[radar]], motor vehicles, and other man-made electronic devices.

Radio waves from space were first detected by engineer [[Karl Guthe Jansky]] in 1932 at [[Bell Telephone Laboratories]] in [[Holmdel, 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 amateur [[Grote Reber]] in his back yard in [[Wheaton, Illinois]] in 1937. The sky survey he performed is often considered the beginning of the field of radio astronomy.

==Early radio telescopes==
{{multiple image
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| header   =
| image1   = Janksy Karl radio telescope.jpg
| caption1 = Full-size replica of the first radio telescope, Jansky's [[dipole antenna|dipole]] array of 1932, preserved at the US [[Green Bank Observatory]] in Green Bank, West Virginia.
| width1   = 330
| image2   = Grote Antenna Wheaton.gif
| caption2 = [[Grote Reber|Reber]]'s "dish" radio telescope, Wheaton, Illinois, 1937
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}}
The first radio antenna used to identify an astronomical radio source was built by [[Karl Guthe Jansky]], an engineer with [[Bell Labs|Bell Telephone Laboratories]], in 1932. Jansky was assigned the task of identifying sources of [[static (radio)|static]] that might interfere with [[radiotelephone]] service. Jansky's antenna was an array of [[dipole]]s and [[Reflector (antenna)|reflectors]] designed to receive [[short wave]] radio signals at a [[frequency]] of 20.5 [[Megahertz|MHz]] (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 {{convert|100|ft|m|-1|abbr=on}} and stood {{convert|20|ft|m|0|abbr=on}} 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 an [[Analog device|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 above [[shot 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 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 (constellation)|Sagittarius]].

An amateur radio operator, [[Grote Reber]], was one of the pioneers of what became known as [[radio astronomy]]. He built the first parabolic "dish" radio telescope, {{convert|9|m|ft|0}} 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 at [[VHF|very high]] radio frequencies, discovering other radio sources. The rapid [[History of radar|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.<ref>Sullivan, W.T. (1984). ''The Early Years of Radio Astronomy''. Cambridge University Press. {{ISBN|0-521-25485-X}}</ref>

==Types==
[[File:Ooty_Radio_Telescope.jpg|thumb|upright=0.8|[[Ooty Radio Telescope|Ooty radio telescope]], a 326.5&nbsp;MHz dipole array in [[Ooty]], India ]] 
The range of frequencies in the [[electromagnetic spectrum]] that makes up the [[radio 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&nbsp;MHz), they are generally either [[directional 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 wire [[mesh]] such as [[chicken wire]].<ref name="galaxy196506">{{Cite magazine
 |last1=Ley
 |first1=Willy
 |last2=Menzel
 |first2=Donald H.
 |last3=Richardson
 |first3=Robert S.
 |date=June 1965
 |title=The Observatory on the Moon
 |department=For Your Information
 |url=https://archive.org/stream/Galaxy_v23n05_1965-06#page/n131/mode/2up
 |magazine=Galaxy Science Fiction
 |pages=132–150
}}</ref>
<ref>{{cite web|title=The Dish turns 45|url=http://www.csiro.au/files/files/pbhq.rtf|author=CSIRO|access-date=October 16, 2008|publisher=[[Commonwealth Scientific and Industrial Research Organisation]]|url-status=dead |archive-url=https://web.archive.org/web/20080824003225/http://www.csiro.au/files/files/pbhq.rtf |archive-date=August 24, 2008 }}</ref> At shorter wavelengths [[parabolic antenna|parabolic "dish" antennas]] predominate. 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. Radio telescopes that operate at wavelengths of 3 meters to 30&nbsp;cm (100&nbsp;MHz to 1&nbsp;GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths shorter than 30&nbsp;cm (above 1&nbsp;GHz) range in size from 3 to 90 meters in diameter.{{citation needed|date=August 2016}}<!-- unclear if this is original research or not; how many radio telescopes are there that are >90m in diameter?  If only one, or a few, then the prose statement here does not make sense.  "usually"? -->

===Frequencies===
The increasing use of radio frequencies for communication makes astronomical observations more and more difficult (see [[open spectrum#Radio astronomy needs|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.
[[File:Atmospheric electromagnetic opacity.svg|thumb|upright=1.8|Plot of Earth's atmospheric [[transmittance]] (or opacity) to various wavelengths of [[electromagnetic radiation]].]]
Some of the more notable frequency bands used by radio telescopes include:
* Every frequency in the [[United States National Radio Quiet Zone]]
* [[Channel 37]]: 608 to 614&nbsp;MHz
* The "[[Hydrogen line]]", also known as the "21 centimeter line": 1,420.40575177&nbsp;MHz, used by many radio telescopes including [[The Big Ear]] in its discovery of the [[Wow! signal]]
* 1,406&nbsp;MHz and 430&nbsp;MHz <ref>{{cite web|url=http://www.jb.man.ac.uk/~pulsar/Education/Tutorial/tut/node115.html |title=Microstructure |website=Jb.man.ac.uk |date=1996-02-05 |access-date=2016-02-24}}</ref>
* The [[Waterhole (radio)|Waterhole]]: 1,420 to 1,666&nbsp;MHz
* The [[Arecibo Observatory]] had several receivers that together covered the whole 1–10&nbsp;GHz range.
* The [[Wilkinson Microwave Anisotropy Probe]] mapped the [[cosmic microwave background radiation]] in 5 different frequency bands, centered on 23&nbsp;GHz, 33&nbsp;GHz, 41&nbsp;GHz, 61&nbsp;GHz, and 94&nbsp;GHz.

===Big dishes===
{{comparison_FAST_Arecibo_Observatory_profiles.svg}}

The world's largest filled-aperture (i.e. full dish) radio telescope is the [[Five-hundred-meter Aperture Spherical Telescope]] (FAST) completed in 2016 by [[China]].<ref>{{cite web|url=http://english.peopledaily.com.cn/90001/90776/90881/6562884.html |title=China Exclusive: China starts building world's largest radio telescope |website=English.peopledaily.com.cn |date=2008-12-26 |access-date=2016-02-24}}</ref> The {{convert|500|m|ft|sp=us|adj=mid|-diameter}} dish with an area as large as 30 football fields is built into a natural [[karst]] depression in the landscape in [[Guizhou province]] and cannot move; the [[Antenna feed|feed 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&nbsp;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&nbsp;meters. Construction began in 2007 and was completed July 2016<ref>{{cite web|url=http://www.space.com/33357-china-largest-radio-telescope-alien-life.html |title=China Finishes Building World's Largest Radio Telescope |website=[[Space.com]] |date=2016-07-06 |access-date=2016-07-06}}</ref> and the telescope became operational September 25, 2016.<ref>{{citation|last=Wong|first=Gillian|title=China Begins Operating World's Largest Radio Telescope|publisher=ABC News|date=25 September 2016|url=https://abcnews.go.com/Technology/wireStory/china-begins-operating-worlds-largest-radio-telescope-42339475}}</ref>

The world's second largest filled-aperture telescope was the [[Arecibo radio telescope]] located in [[Arecibo, 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 {{convert|305|m|ft|abbr=on}} dish was built into a natural depression in the landscape, the antenna was steerable within an angle of about 20° of the [[zenith]] by moving the suspended [[antenna feed|feed 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 the [[RATAN-600]] located near [[Nizhny 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 the [[zenith]], and cannot receive from sources near the horizon. The largest fully steerable dish radio telescope is the 100 meter [[Green Bank Telescope]] in [[West Virginia]], United States, constructed in 2000. The largest fully steerable radio telescope in Europe is the [[Effelsberg 100-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.<ref name="Ridpath2012">{{cite book|last=Ridpath|first=Ian|title=A Dictionary of Astronomy|url=https://books.google.com/books?id=O31j9UJ3U4oC&pg=PA139|year=2012|publisher=OUP Oxford|isbn=978-0-19-960905-5|page=139}}</ref> The third-largest fully steerable radio telescope is the 76-meter [[Lovell Telescope]] at [[Jodrell Bank Observatory]] in [[Cheshire]], England, completed in 1957. The fourth-largest fully steerable radio telescopes are six 70-meter dishes: three Russian [[RT-70]], and three in the [[NASA Deep Space Network]]. The planned [[Qitai Radio Telescope]], at a diameter of {{convert|110|m|ft|abbr=on}}, 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.

====Gallery of big dishes====
<gallery mode="packed" heights="200">
File:FAST Radio Telescope (captured from video).jpg|The 500 meter [[Five-hundred-meter Aperture Spherical Telescope]] (FAST), China, as seen from above in 2020
File:GBT.png|alt=Green Bank Telescope|The 100 meter<!--by 110m--> [[Green Bank Telescope]], Green Bank, West Virginia, US, the largest fully steerable radio telescope dish (2002)
File:DSCN6149 Effelsberg totale.jpg|alt=Effelsberg 100-m Radio Telescope|The 100 meter [[Effelsberg 100-m Radio Telescope|Effelsberg]], in Bad Münstereifel, Germany (1971)
File:Lovell Telescope 5.jpg|alt=Lovell Telescope|The 76 meter [[Lovell Telescope|Lovell]], Jodrell Bank Observatory, England (1957)
File:Goldstone DSN antenna.jpg|alt=DSS 14 "Mars" antenna at Goldstone Deep Space Communications Complex|The 70 meter DSS 14 "Mars" antenna at [[Goldstone Deep Space Communications Complex]], Mojave Desert, California, US (1958)
File:70-м антенна П-2500 (РТ-70).jpg|alt=Yevpatoria RT-70 radio telescope|The 70 meter [[Yevpatoria RT-70 radio telescope|Yevpatoria RT-70]], Crimea, first of three [[RT-70]] in the former Soviet Union, (1978)<!--remove RT-70 list when image of third used-->
File:Антенна П-2500 (РТ-70) ВЦДКС - panoramio (2).jpg|The 70 meter [[Galenki RT-70 radio telescope|Galenki RT-70]], Galenki, Russia, second of three [[RT-70]] in the former Soviet Union, (1984)<!--remove RT-70 list when image of third used-->
</gallery>

===Radio Telescopes in space===
{{Update section|date=October 2024}}
Since 1965, humans have launched three space-based radio telescopes. The first one, KRT-10, was attached to Salyut 6 orbital space station in 1979. In 1997, [[Japan]] sent the second, [[HALCA]]. The last one was sent by [[Russia]] in 2011 called [[Spektr-R]].
{{gallery|mode=packed
| title    = Space radiotelescopes
|File:Rus Stamp GSS-Lyahov-Rumin.jpg|KRT-10 dish of a [[Salyut-6]] on a stamp
|File:Haruka HALCA VSOP MUSES-B deployment test.jpg|Japanese [[HALCA]] dish
|File:RIAN archive 930415 Russian Spektr R space-born radio telescope.jpg|Assembled [[Spektr-R]] dish (left)
}}

==Radio interferometry==
{{main|Astronomical interferometer}}
{{see also|Radio astronomy#Radio interferometry}}
[[File:USA.NM.VeryLargeArray.02.jpg|thumb|upright=1.2|The [[Very Large Array]] in Socorro, New Mexico, an [[interferometer|interferometric array]] formed of 27 parabolic dish telescopes.]]
One of the most notable developments came in 1946 with the introduction of the technique called [[astronomical 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., the [[One-Mile Telescope]]), arrays of one-dimensional antennas (e.g., the [[Molonglo Observatory Synthesis Telescope]]) or two-dimensional arrays of omnidirectional [[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 usually connected using [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission 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 as [[Very Long Baseline Interferometry|Very Long Baseline Interferometry (VLBI)]]. Interferometry does increase the total signal collected, but its primary purpose is to vastly increase the resolution through a process called [[aperture synthesis]]. This technique works by superposing ([[Interference (wave propagation)|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 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.
[[File:The Atacama Compact Array.jpg|thumb|upright=1.2|[[Atacama Large Millimeter Array]] in the [[Atacama Desert]] consisting of 66 12-metre (39 ft), and 7-metre (23 ft) diameter radio telescopes designed to work at [[submillimeter astronomy|sub-millimeter wavelengths]]]]
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, the [[Very Large Array]] (VLA) near [[Socorro, New Mexico]] has 27 telescopes with 351 independent baselines at once, which achieves a resolution of 0.2 [[arc seconds]] at 3&nbsp;cm wavelengths.<ref>{{cite web|title=Microwave Probing of the Invisible|url=http://www.gps.caltech.edu/faculty/muhleman/muhleman.html |access-date=June 13, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070831223606/http://www.gps.caltech.edu/faculty/muhleman/muhleman.html |archive-date=August 31, 2007 }}</ref> [[Martin Ryle]]'s [[Cavendish Astrophysics Group|group in Cambridge]] obtained a [[Nobel Prize]] for interferometry and aperture synthesis.<ref>''[[Nature (journal)|Nature]]'' vol.158, p. 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]]'' vol.157, p. 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. An example of a large physically connected radio telescope array is the [[Giant Metrewave Radio Telescope]], located in [[Pune]], [[India]]. The largest array, the [[Low-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&nbsp;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 the [[Cosmic Microwave Background]], like the [[Cosmic Background Imager|CBI]] interferometer in 2004.

The world's largest physically connected telescope, the [[Square Kilometre Array]] (SKA), is planned to start operations in 2027,<ref name=physicsworld201907>{{cite news |url=https://physicsworld.com/a/new-zealand-pulls-out-of-the-square-kilometre-array-after-benefits-questioned |title=New Zealand pulls out of the Square Kilometre Array after benefits questioned |publisher=IOP Publishing |work=Physics World |date=4 July 2019 |access-date=5 July 2019 |archive-url=https://web.archive.org/web/20190704174649/https://physicsworld.com/a/new-zealand-pulls-out-of-the-square-kilometre-array-after-benefits-questioned/ |archive-date=4 July 2019 |url-status=live }}</ref> Although the first stations had "first fringes" in 2024.<ref>{{Cite web |last=Wiegert |first=Theresa |date=2024-09-24 |title=SKA telescope gets its '1st fringes' |url=https://earthsky.org/space/ska-telescope-radio-south-africa-australia/ |access-date=2025-02-22 |website=earthsky.org |language=en-US}}</ref>

==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 wavelengths]]. 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]].<ref>{{Cite web|url=https://www.skatelescope.org/radio-astronomy/|title=What is Radio Astronomy?|website=Public Website}}</ref><ref>{{Cite web|url=https://public.nrao.edu/telescopes/radio-telescopes/|title=What are Radio Telescopes?}}</ref>

==See also==
* [[Aperture synthesis]]
* [[Astropulse]] – distributed computing to search data tapes for primordial black holes, pulsars, and ETI
* [[Ground station]]
* [[List of astronomical observatories]]
* [[List of radio telescopes]]
* [[List of telescope types]]
* [[Search for extraterrestrial intelligence]]
* [[Telescope]]
* [[Radar telescope]]

==References==
{{Reflist|30em}}

==Further reading==
{{Commons category|Radio telescopes}}
* Rohlfs, K., & Wilson, T. L. (2004). Tools of radio astronomy. Astronomy and astrophysics library. Berlin, Germany: Springer.
* [[Isaac Asimov|Asimov, I.]] (1979). Isaac Asimov's Book of facts; ''Sky Watchers''. New York: Grosset & Dunlap. pp.&nbsp;390–399. {{ISBN|0-8038-9347-7}}.

== External links ==
* [https://www.pictortelescope.com/ PICTOR: A free-to-use radio telescope]
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