Out-of-focus holography at the Green Bank Telescope

Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge CB3 0HE, UKthe date of receipt and acceptance should be inserted laterAbstract. We describe phase-retrieval holography measurements of the 100-m diameter Green Bank Telescope using astro-nomical sources and an astronomical receiver operating at a wavelength of 7mm. We use the technique with parameterizationof the aperture in terms of Zernike polynomials and employing a large defocus, as described by Nikolic et al. (2006). Individualmeasurements take around 25minutes and from the resulting beam maps (which have peak signal to noise ratios of 200:1) weshow that it is possible to produce low-resolution maps of the wavefront errors with accuracy around λ/100.Using such measurements over a wide range of elevations, we have calculated a model for the wavefront-errors due to theuncompensated gravitational deformation of the telescope. This model produces a significant improvement at low elevat ions,where these errors are expected to be the largest; after applying the model, the aperture efficiency is largely independent ofelevation. We have also demonstrated that the technique can be used to measure and largely correct for thermal deformationsof the antenna, which often exceed the uncompensated gravitational deformations during daytime observing.We conclude that the aberrations induced by gravity and thermal effects are large-scale and the technique used here is particu-larly suitable for measuring such deformations in large millimetre wave radio telescopes.Key words. telescopes