Improved efficiency with a mechanically deformable subreflector

Like most conventional radio telescopes, the 140-ft at the National Radio Observatory (NRAO), Green Bank, WV, is limited in its short-wave performance by gravitational deformations, whose main part is of an astigmatic shape with elevation-dependant amplitude. This surface degradation could be corrected if a subreflector were deformed in a similar shape by the same amount. For testing this possibility an experimental Cassegrain subreflector was built which can be deformed in an astigmatic mode by simple means: two stiff diagonals and four points in-between where a motor pushes or pulls normal to the surface, servo-controlled by the on-line computer. Although the amount of deformation is too limited in the present setup, good results have already been obtained. Seven unresolved radio sources were observed at the water vapor line (22.3 GHz, cm), at various telescope pointing, and with different amounts of subreflector deformation, scanning in both directions for complete beam maps. The astigmatic deformation of the subreflector gave considerable improvements, especially fat south and east: fairly strong secondary beams disappeared; the beamshape became more symmetrical, narrow, and round; and at elevation the aperture efficiency increased by factors between two and three.