Editing Radio telescope (section)

==Radio interferometry==
{{main article|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>