Aperture synthesis is a type of interferometry that mixes signals from a collection of instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. 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. In order 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 good quality results. 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 (the rotation of the Earth moves the telescopes for you!). Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in Aperture Masking Interferometry
This process is used in radio astronomy, where it was first developed by Martin Ryle and coworkers from the Cavendish Laboratory, Cambridge University. Martin Ryle and 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 Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 60s and early 70s, as computers became available capable of handling the computationally-intensive Fourier Transform inversions required, they used aperture synthesis to synthesis first a 'One-Mile' and later a '5km' effective aperture using the One-Mile and Ryle telescopes respectively. The technique was subsequently further developed in Very Long Baseline Interferometry. It is also used by a type of radar system known as synthetic aperture radar, and even in optical telescopes.
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