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Coma correction of a wobbling subreflector

A. Greve, B. LeFloch, J. Peñalver (IRAM)
Abstract: 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 large extent this wavefront deformation when using a near-focus corrector, in this telescope conveniently located on the co-rotating Nasmyth mirror.