A deformable subreflector for the Haystack radio telescope

A deformable subreflector was designed and implemented to compensate for part of the gravity deformations of the primary reflector of Haystack, a 37-m-(120-ft-)diameter Cassegrain radio telescope. This was done to allow it to operate at 100+ GHz, as compared to the 1-to-10 GHz range for which it was originally designed. The design, analysis, construction, testing, and the results of preliminary measurements of performance are presented. The deformable subreflector consists of a fiberglass shell, supported on an aluminum back structure. The homologous components of deformations are compensated for by optimal positioning of the subreflector, which can be displaced axially and laterally, and tilted. The deformation modes of the subreflector compensate for astigmatic deformations of the back structure of the primary, and for part of the symmetric and anti-symmetric components of gravity sag of the panels of the primary reflector. Analyses show that, due to the deformable subreflector, the surface RMS due to gravity has been reduced from 494 mm (19.4 mil) down to 146 mm (5.7 mil), as the antenna travels over its operating range of 15 to 70 degrees elevation. Combining the reduced gravity effects with surface adjustment and thermal errors results in a predicted combined surface error of 250 mm (9.8 mil), at the extremes of the operating range.<>