Abstract :
The illumination of large paraboloidal reflectors is increasingly being effected by indirect systems such as the so-called Cassegrainian system. A short review of wave optics shows how the range of validity of geometrical optics, and therefore of the Cassegrainian system, is restricted at radio wavelengths. A system is therefore proposed employing a secondary paraboloidal reflector exactly on the focus of the main reflector and illuminated by a primary feed exactly at its focus. The theory of its operation is then given, yielding the required illumination through two successive Fourier transforms. The practical design problems are then dealt with, including the size of the secondary reflector, which in the lowest limit can be taken to be roughly the size of the central lobe of the main focal diffraction pattern; the aperture of the primary feed has to be similar in shape to the main reflector and in position with respect to the main focus. Some examples of applications dealt with include computations for various types of reflectors and illuminations, and possibilities for space problems, utilisation and performances, including with the large radiotelescope at Nancay.
