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. 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 radio astronomers Ruby Payne-Scott and Joseph Pawsey. Working from Dover Heights in Sydney, Payne-Scott carried out the earliest interferometer observations in radio astronomy on 26 January 1946 using an Australian Army radar as a radio telescope. Aperture synthesis imaging was later developed at radio wavelengths by Martin Ryle and coworkers from the Radio Astronomy Group at 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 1960s and early 1970s, as computers (such as the 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 km' effective aperture using the One-Mile and Ryle telescopes, respectively. The technique was subsequently further developed in 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 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 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 Interferometric synthetic-aperture radar (IfSAR or InSAR) Light field Optical heterodyne detection (SAHD)