Discussion
What is a focal plane array used for on a reflector antenna?
Answer: C
Placing several feeds around the focus produces several beams, each squinted slightly from the axis, so one dish can serve several coverage areas or survey the sky faster. The spacing of the beams is set by how far each feed can be displaced before coma becomes serious. Aperture phase measurement is done by holography, surface correction requires an active surface or adaptive optics, and efficiency is a feed design matter. Getting more than one beam from one expensive aperture is often the whole justification.
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