The zone plate as a focussing element (abstract)

Certain corrections to the Fresnel theory are necessary for zone plates with focal lengths of only a few wave-lengths. Formulas are derived for the gain of these zone plates as a function of the number of zones and the focal length. Theoretical values are compared with experimental results. The advantages of the zone-plate relative to a parabolic reflector/are illustrated by a proposed design for a large radio telescope.