Focal Plane Arrays are arrays of receivers placed at the focus of a radio-telescope. Traditional radio-telescopes have only one receiver at the focus of the telescope, but radio-telescopes are now starting to be equipped with focal plane arrays, which are of three different types: multi-beam feed arrays, bolometer arrays, and the experimental phased-array feeds.
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Multi-beam feed arrays incorporate several distinct Superheterodyne receiver receivers, each with its own feed horn, in a small array at the focus of the radio-telescope. A feed array with n receivers will increase the survey speed of the telescope by a factor n, making them very powerful survey instruments. Because radio wavelengths are large, the resulting feed arrays are amongst the largest radio-astronomy receivers ever built. Examples include the multi-beam arrays [1][2] on the Parkes Observatory, and the ALFA [3] array at Arecibo Observatory, both of which have been used for major pulsar and Hydrogen line studies, such as HIPASS.
Bolometer arrays are arrays of bolometer receivers which measure the energy of incoming radio photons. They are typically used for astronomy at millimeter wavelengths. Examples include the SCUBA receiver on the James Clerk Maxwell Telescope and the LABOCA [4] instrument on the APEX telescope.
Phased Array Feeds are an experimental type of focal plane array using Phased array technology in which heterodyne receivers are closely spaced so that they do not act independently, but instead act as sensors of the electromagnetic field across the array of the incoming radio waves. The outputs of the receivers can then be combined to synthesise the effect of several discrete beams. They are currently being developed for the Apertif [5] upgrade to the Westerbork Synthesis Radio Telescope, and for the Australian Square Kilometre Array Pathfinder radio telescope. Phased array systems are widely used in medical ultrasound where a phased array digitally focuses on one axis, and a conventional cylindrical acoustic lens focuses on the other.