Characterization of near-field focusing with application to the Arecibo tri-reflector system

The radio telescope of the Arecibo Observatory has a large primary reflector of 300 m diameter and with spherical shape. The old line feeds are now being replaced by a dual-reflector feed, referred to as the Gregorian feed, consisting of two sub reflectors (the tertiary and the secondary reflector) and several feed horns. The present paper presents a study in which the analysis techniques have been extended to study focusing of the beam at low altitudes in the ionosphere. The study is of general interest as it contains definitions of near-field gain and efficiency. Also, an algorithm is described from which the focus location can be calculated directly from the aperture field. This is used to study the displacement of the feed, or the tertiary or both at the same time to obtain as good focusing as possible at a certain altitude.