Coma correction of a wobbling subreflector

Radio astronomical observations at millimeter wavelengths with Cassegrain telescopes often use on-source/off-source measurements with a wobbling subreflector to eliminate the influence of the Earth's atmosphere. The tilt of the subreflector produces the required beam throw, but also a coma-like wavefront deformation noticeable as a degradation of the beam pattern and a reduction of the main beam power. We demonstrate, with measurements made with the IRAM 30-m telescope at 1.3 mm wavelength (230 GHz) of the quasi-point-like source Mars and the extended source Saturn, that it is possible to eliminate (to a large extent) this wavefront deformation when using a near-focus corrector, on the 30-m telescope conveniently located on the co-rotating Nasmyth mirror.