Theory of imaging in Cassegrainian and Gregorian antennas.

The formation of an image by an ellipsoidal reflector under illumination from one of its foci is discussed. The theory applies in general to any multireflector arrangement derived from quadric surfaces of revolution and, in particular, to Cassegrainian and Gregorian antennas. As an application, a simple solution to the classical problem of illuminating efficiently the aperture of a reflector antenna is proposed. We combine a horn of relatively small aperture with an imaging reflector. The imaging reflector, an ellipsoid, transforms the horn aperture distribution into a magnified image illuminating efficiently the main reflector, with negligible spill-over over a wide frequency range.