Editing Radio astronomy (section)

==Techniques==
[[File:Atmospheric electromagnetic opacity.svg|thumb|upright=2.00|Window of radio waves observable from Earth, on rough plot of Earth's atmospheric absorption and scattering (or [[opacity (optics)|opacity]]) of various [[wavelength]]s of electromagnetic radiation]]

Radio astronomers use different techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze its emission. To "image" a region of the sky in more detail, multiple overlapping scans can be recorded and pieced together in a [[mosaic]] image. The type of instrument used depends on the strength of the signal and the amount of detail needed.

Observations from the [[Earth]]'s surface are limited to wavelengths that can pass through the atmosphere. At low frequencies or long wavelengths, transmission is limited by the [[ionosphere]], which reflects waves with frequencies less than its characteristic [[plasma frequency]]. [[Water]] [[vapor]] interferes with radio astronomy at higher frequencies, which has led to building radio observatories that conduct observations at [[millimeter]] wavelengths at very high and dry sites to minimize the water vapor content in the line of sight. Finally, transmitting devices on Earth may cause [[radio-frequency interference]]. Because of this, many radio observatories are built at remote places.

===Radio telescopes===
{{Main|Radio telescope}}

Radio telescopes may need to be extremely large in order to receive signals with low [[signal-to-noise ratio]]. Also since [[angular resolution]] is a function of the diameter of the "[[Objective (optics)|objective]]" in proportion to the wavelength of the electromagnetic radiation being observed, ''[[radio telescope]]s'' have to be much larger in comparison to their [[Optical telescope|optical]] counterparts. For example, a 1-meter diameter optical telescope is two million times bigger than the wavelength of light observed giving it a resolution of roughly 0.3 [[arc second]]s, whereas a radio telescope "dish" many times that size may, depending on the wavelength observed, only be able to resolve an object the size of the full moon (30 minutes of arc).

===Radio interferometry===
{{main|Astronomical interferometry}}
{{see also|Radio telescope#Radio interferometry}}
[[File:The Atacama Compact Array.jpg|thumb|The [[Atacama Large Millimeter Array]] (ALMA), many antennas linked together in a radio interferometer]]
[[File:M87 optical image.jpg|thumb|300px]] [[File:M87 VLA VLBA radio astronomy.jpg|thumb|300px| An optical image of the galaxy [[Messier 87|M87]] ([[Hubble Space Telescope|HST]]), a radio image of same galaxy using interferometry ([[Very Large Array]], VLA), and an image of the center section (VLBA) using a Very Long Baseline Array (Global VLBI) consisting of antennas in the US, Germany, Italy, Finland, Sweden and Spain. The jet of particles is suspected to be powered by a [[black hole]] in the center of the galaxy.]]

The difficulty in achieving high resolutions with single radio telescopes led to radio [[interferometry]], developed by British radio astronomer [[Martin Ryle]] and Australian engineer, radiophysicist, and radio astronomer [[Joseph Lade Pawsey]] and [[Ruby Payne-Scott]] in 1946. The first use of a radio interferometer for an astronomical observation was carried out by Payne-Scott, Pawsey and [[Lindsay McCready]] on 26 January 1946 using a ''single'' converted radar antenna (broadside array) at [[Very high frequency|200 MHz]] near [[Sydney, Australia]]. This group used the principle of a sea-cliff interferometer in which the antenna (formerly a World War II radar) observed the Sun at sunrise with interference arising from the direct radiation from the Sun and the reflected radiation from the sea. With this baseline of almost 200 meters, the authors determined that the solar radiation during the burst phase was much smaller than the solar disk and arose from a region associated with a large [[sunspot]] group. The Australia group laid out the principles of [[aperture synthesis]] in a groundbreaking paper published in 1947. The use of a sea-cliff [[interferometer]] had been demonstrated by numerous groups in Australia, Iran and the UK during World War II, who had observed interference fringes (the direct radar return radiation and the reflected signal from the sea) from incoming aircraft.

The Cambridge group of Ryle and Vonberg observed the Sun at 175&nbsp;MHz for the first time in mid-July 1946 with a Michelson interferometer consisting of two radio antennas with spacings of some tens of meters up to 240 meters. They showed that the radio radiation was smaller than 10 [[Minute of arc|arc minutes]] in size and also detected circular polarization in the Type&nbsp;I bursts. Two other groups had also detected circular polarization at about the same time ([[David Martyn (scientist)|David Martyn]] in Australia and [[Edward Victor Appleton|Edward Appleton]] with [[James Stanley Hey]] in the UK).

Modern [[radio telescope#radio interferometry|radio interferometers]] consist of widely separated radio telescopes observing the same object that are connected together using [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission line]]. This not only increases the total signal collected, but it can also be used in a process called [[aperture synthesis]] to vastly increase resolution. 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 the size of the antennas furthest apart in the array. To produce a high-quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a "baseline") – as many different baselines as possible are required in order to get a good quality image. For example, the [[Very Large Array]] has 27 telescopes giving 351 independent baselines at once.

====Very-long-baseline interferometry====
{{main|Very-long-baseline interferometry}}

Beginning in the 1970s, improvements in the stability of radio telescope receivers permitted telescopes from all over the world (and even in Earth orbit) to be combined to perform [[very-long-baseline interferometry]]. Instead of physically connecting the antennas, data received at each antenna is paired with timing information, usually from a local [[atomic clock]], and then stored for later analysis on magnetic tape or hard disk. At that later time, the data is correlated with data from other antennas similarly recorded, to produce the resulting image. Using this method, it is possible to synthesise an antenna that is effectively the size of the Earth. The large distances between the telescopes enable very high angular resolutions to be achieved, much greater in fact than in any other field of astronomy. At the highest frequencies, synthesised beams less than 1 [[Minute of arc|milliarcsecond]] are possible.

The pre-eminent VLBI arrays operating today are the [[Very Long Baseline Array]] (with telescopes located across North America) and the [[European VLBI Network]] (telescopes in Europe, China, South Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed together producing increased sensitivity. This is referred to as Global VLBI. There are also a VLBI networks, operating in Australia and New Zealand called the LBA (Long Baseline Array),<ref>{{Cite web| url=http://www.atnf.csiro.au/vlbi/| title=VLBI at the ATNF| date=7 December 2016| access-date=16 June 2015| archive-date=1 May 2021| archive-url=https://web.archive.org/web/20210501051105/https://www.atnf.csiro.au/vlbi/| url-status=live}}</ref> and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).<ref>{{Cite web | url=http://www.astro.sci.yamaguchi-u.ac.jp/eavn/index.html | title=East Asia VLBI Network and Asia Pacific Telescope | access-date=2015-06-16 | archive-date=2021-04-28 | archive-url=https://web.archive.org/web/20210428080543/http://astro.sci.yamaguchi-u.ac.jp/eavn/index.html | url-status=live }}</ref>

Since its inception, recording data onto hard media was the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was originally pioneered in Japan, and more recently adopted in Australia and in Europe by the EVN (European VLBI Network) who perform an increasing number of scientific e-VLBI projects per year.<ref>{{Cite web |url=http://www.innovations-report.com/html/reports/physics_astronomy/report-25117.html |title=A technological breakthrough for radio astronomy – Astronomical observations via high-speed data link<!-- Bot generated title --> |date=26 January 2004 |access-date=2008-07-22 |archive-date=2008-12-03 |archive-url=https://web.archive.org/web/20081203145055/http://www.innovations-report.com/html/reports/physics_astronomy/report-25117.html |url-status=live }}</ref>