ASTIGMATISM IN REFLECTOR ANTENNAS - MEASUREMENT AND CORRECTION

We investigate astigmatism is Cassegrain antennas from cross scans through in-and-out-of-focus diffraction images of extended radio sources. The use of extended sources is of interest for large (mm-wavelength) telescopes where the bright planets subtend a substantial fraction of the beam. The experimental and theoretical results presented here refer to the IRAM 30-m telescope and measurements at 7.3 mm-1.3 mm wavelength; however, extrapolation to other telescopes is evident. We report on an experiment to correct the astigmatism (phase) at a position close to the focal plane of the telescope, i.e., the flat surface of the corotating Nasmyth mirror.