View source for Astronomy:Aperture synthesis

{{short description|Mixing signals from many telescopes to produce images with high angular resolution}}

'''Aperture synthesis''' or '''synthesis imaging''' is a type of [[Astronomy:Interferometry|interferometry]] that mixes signals from a collection of [[Astronomy:Telescope|telescope]]s to produce images having the same [[Angular resolution|angular resolution]] as an instrument the size of the entire collection.<ref name="Jennison">{{cite journal |author=R. C. Jennison |title=A Phase Sensitive Interferometer Technique for the Measurement of the Fourier Transforms of Spatial Brightness Distributions of Small Angular Extent |date=1958 |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=119 |pages=276–284 |number=3 |doi=10.1093/mnras/118.3.276 |bibcode=1958MNRAS.118..276J |bibcode-access=free |doi-access=free }}</ref><ref name="BurkeGraham-Smith2010">{{cite book|author1=Bernard F. Burke|author2=Francis Graham-Smith|title=An Introduction to Radio Astronomy|url=https://books.google.com/books?id=4dI6isxCmEcC|year=2010|publisher=Cambridge University Press|isbn=978-0-521-87808-1}}</ref><ref name = "KraussBook">{{cite book | author = John D. Krauss | title = Radio Astronomy | chapter = Chapter 6: Radio-Telescope Antennas | location = New York, NY | year = 1966 | publisher = McGraw Hill}}</ref> At each separation and orientation, the lobe-pattern of the interferometer produces an output which is one component of the [[Fourier transform]] of the spatial distribution of the brightness of the observed object. The image (or "map") of the source is produced from these measurements. [[Astronomy:Astronomical interferometer|Astronomical interferometer]]s are commonly used for high-resolution optical, [[Astronomy:Infrared astronomy|infrared]], [[Astronomy:Submillimetre astronomy|submillimetre]] and [[Astronomy:Radio astronomy|radio astronomy]] observations. For example, the [[Astronomy:Event Horizon Telescope|Event Horizon Telescope]] project derived the first image of a black hole using aperture synthesis.<ref name="APJL-20190410-2">{{cite journal |author=The Event Horizon Telescope Collaboration |title=First M87 Event Horizon Telescope Results. II. Array and Instrumentation |date=April 10, 2019 |journal=The Astrophysical Journal Letters |volume=87 |pages=L2 |number=1 |doi=10.3847/2041-8213/ab0c96 |arxiv=1906.11239 |bibcode=2019ApJ...875L...2E |doi-access=free }}</ref>

== Technical issues ==
Aperture synthesis is possible only if both the [[Physics:Amplitude|amplitude]] and the [[Physics:Phase (waves)|phase]] of the incoming signal are measured by each telescope. For radio frequencies, this is possible by electronics, while for optical frequencies, the electromagnetic field cannot be measured directly and correlated in software, but must be propagated by sensitive optics and interfered optically. Accurate optical delay and atmospheric wavefront aberration correction are required, a very demanding technology that became possible only in the 1990s. This is why imaging with aperture synthesis has been used successfully in radio astronomy since the 1950s and in optical/infrared astronomy only since the turn of the millennium. See [[Astronomy:Astronomical interferometer|astronomical interferometer]] for more information.

In order to produce a high quality image, a large number of different separations between different telescopes is 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. The number of baselines (''n''<sub>b</sub>) for an array of ''n'' telescopes is given by ''n''<sub>b</sub>=(''n''<sup>2</sup>&nbsp;−&nbsp;''n'')/2. (This is <math>\binom{n}{2}</math> or [[Binomial coefficient|<sup>''n''</sup>C<sub>2</sub>]]).  For example, the [[Astronomy:Very Large Array|Very Large Array]] has 27 telescopes giving 351 independent baselines at once, and can give high quality images.

[[File:Earth rotation aperture synthesis.jpg|thumb|upright=1.2|Most aperture synthesis interferometers use the rotation of the Earth to increase the number of baseline orientations included in an observation. In this example with the Earth represented as a grey sphere, the baseline between telescope A and telescope B changes angle with time as viewed from the radio source as the Earth rotates. Taking data at different times thus provides measurements with different telescope separations.]]

In contrast to radio arrays, the largest optical arrays currently have only 6 telescopes, giving poorer image quality from the 15 baselines between the telescopes.

Most aperture synthesis interferometers use the rotation of the Earth to increase the number of different baselines included in an observation (see diagram on right). Taking data at different times provides measurements with different telescope separations and angles without the need for buying additional telescopes or moving the telescopes manually, as the rotation of the Earth moves the telescopes to new baselines.

The use of Earth rotation was discussed in detail in the 1950 paper ''A preliminary survey of the radio stars in the Northern Hemisphere''.<ref>[http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1950MNRAS.110..508R&amp;db_key=AST A preliminary survey of the radio stars in the Northern Hemisphere]</ref> Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in aperture masking interferometry.

== History ==

The concept of aperture synthesis was first formulated in 1946 by Australian [[Astronomy:Radio astronomy|radio astronomers]] [[Biography:Ruby Payne-Scott|Ruby Payne-Scott]] and Joseph Pawsey. Working from Dover Heights in [[Place:Sydney|Sydney]], Payne-Scott carried out the earliest [[Astronomy:Interferometry|interferometer]] observations in radio astronomy on 26 January 1946 using an Australian Army radar as a radio telescope.<ref>{{Cite web |title=National Radio Astronomy Observatory |url=https://www.nrao.edu/news/newsletters/enews/enews_2_11/ruby.shtml |access-date=2022-11-02 |website=[[Astronomy:National Radio Astronomy Observatory|National Radio Astronomy Observatory]]}}</ref>

Aperture synthesis imaging was later developed at radio wavelengths by [[Biography:Martin Ryle|Martin Ryle]] and coworkers from the [[Physics:Cavendish Astrophysics Group|Radio Astronomy Group]] at [[Organization:University of Cambridge|Cambridge University]]. Martin Ryle and [[Biography:Antony Hewish|Tony Hewish]] jointly received a Nobel Prize for this and other contributions to the development of radio interferometry.

The radio astronomy group in Cambridge went on to found the [[Astronomy:Mullard Radio Astronomy Observatory|Mullard Radio Astronomy Observatory]] near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the [[Engineering:Titan (1963 computer)|Titan]]) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5&nbsp;km' effective aperture using the [[Astronomy:One-Mile Telescope|One-Mile]] and [[Astronomy:Ryle Telescope|Ryle]] telescopes, respectively.

The technique was subsequently further developed in [[Astronomy:Very-long-baseline interferometry|very-long-baseline interferometry]] to obtain baselines of thousands of kilometers and even in optical telescopes.  The term ''aperture synthesis'' can also refer to a type of [[Engineering:Radar|radar]] system known as synthetic aperture radar, but this is technically unrelated to the radio astronomy method and developed independently.

Originally it was thought necessary to make measurements at essentially every baseline length and orientation out to some maximum: such a fully sampled Fourier transform formally contains the information exactly equivalent to the image from a conventional telescope with an aperture diameter equal to the maximum baseline, hence the name ''aperture synthesis''.

It was rapidly discovered that in many cases, useful images could be made with a relatively sparse and irregular set of baselines, especially with the help of non-linear [[Deconvolution|deconvolution]] algorithms such as the maximum entropy method. The alternative name ''synthesis imaging'' acknowledges the shift in emphasis from trying to synthesize the complete aperture (allowing image reconstruction by Fourier transform) to trying to synthesize the image from whatever data is available, using powerful but computationally expensive algorithms.

== See also ==
* [[Beamforming]]
* [[Physics:Interferometric synthetic-aperture radar|Interferometric synthetic-aperture radar]] (IfSAR or InSAR)
* [[Light field]]
* [[Optical heterodyne detection]] (SAHD)
* [[Engineering:Synthetic-aperture magnetometry|Synthetic-aperture magnetometry]]
* [[Engineering:Synthetic aperture sonar|Synthetic aperture sonar]]
* [[Engineering:Synthetic-aperture radar|Synthetic-aperture radar]] (SAR) and [[Engineering:Inverse synthetic-aperture radar|Inverse synthetic-aperture radar]] (ISAR)
* [[Astronomy:Van Cittert–Zernike theorem#Aperture synthesis|Van Cittert–Zernike theorem]]

== References ==
<references />

== External links ==
* [https://web.archive.org/web/20161005081151/http://www.lchr.org/a/36/il/page3.html Development of radio interferometry], from ''Astronomical Optical Interferometry, A Literature Review'' by Bob Tubbs, Cambridge, 2002
* [http://www.mrao.cam.ac.uk/telescopes/coast Cambridge Optical Aperture Synthesis Telescope]
* [https://launchpad.net/apsynsim APerture SYNthesis SIMulator (an interactive tool to learn the concepts of Aperture Synthesis)]

{{radio-astronomy}}

[[Category:Radio astronomy]]
[[Category:Interferometry]]
[[Category:Astronomical imaging]]

{{Sourceattribution|Aperture synthesis}}