FPAs are arrays of receivers placed at the focus of the optical system in a radio-telescope.
Because radio wavelengths are large, the resulting feed arrays are amongst the largest radio-astronomy receivers ever built.
Examples include the multi-beam arrays on the Parkes Observatory,[1][2] and the ALFA array at Arecibo Observatory,[3] both of which have been used for major pulsar and Hydrogen line studies, such as HIPASS.
Phased Array Feeds are an experimental type of focal plane array using phased array technology in which antenna elements are closely spaced so that they do not act independently, but instead act as sensors of the electromagnetic field across the focal plane of the telescope.
The outputs of the receivers are then coherently combined in a beamformer with appropriate weights to synthesise several discrete beams.