The Arecibo upgrading: electrical design and expected performance of the dual-reflector feed system

The paper describes the electrical design and predicted performance of a dual-reflector feed for the fixed spherical reflector of the Arecibo radio telescope. This dual-reflector feed, dubbed the Gregorian has much larger bandwidth, lower losses, and provides better aperture illumination than the old line feeds. The paper begins with a historical review of the line feeds and the first conceptual designs of the dual-reflector feed. Following is a description of the mini-Gregorian, a small test version of the dual-reflector feed concept which is in use on the Arecibo telescope. The selection of an optimized geometry for the full-size Gregorian is detailed. The primary feeds must be a practical size at lower frequencies, the selected ±60° illumination angle from the focal point provides this attribute. The main body of the paper describes the expected performance of the final Gregorian design in the frequency range of 250 MHz to 8 GHz. The high-frequency performance is limited by the surface tolerance of the spherical main reflector and the alignment tolerances of the Gregorian feed. The latter effects are simulated and discussed. The low-frequency performance is limited by edge diffraction on both reflectors and by an unusual diffraction effect created by a very nonuniform field distribution near the center of the larger feed reflector. At low frequencies the exit pupil of the surrounding enclosure blocks some of the edge diffracted fields. This blockage causes extra losses and potential multi-path problems, problems which are both discussed. A cluster of seven feed horns providing multiple beams at 1.42 GHz is discussed