Out-of-focus holography at the Effelsberg telescope.

Out-of-focus (OOF) holography can be used to determine aperture deformations of radio telescopes that lead to errors in the phase of the complex aperture distribution. In contrast to traditional holography, OOF can be done without a reference antenna, which has a number of practical advantages. The aim of this work is to develop a standard procedure for OOF holography at the Effelsberg telescope. This includes performing OOF holography observations and the development of a software, the pyoof package, to compute aberrations of the telescope's optical system. Based on the OOF holography method developed by Nikolic et al. (2007a), we adapted the approach to the Effelsberg 100-m telescope in order to determine the aberrations of the aperture phase distribution (phase-error maps). The developed OOF holography software is presented as well as the results from observations performed at Effelsberg. Early results reveal a possible gravitationally-caused residual deformation not contained in the previously existing aperture and pointing model, and hence we propose to make changes to the model to counteract aberrations in the telescope's surface. The OOF holography method (observations and pyoof package) works as expected at the Effelsberg 100-m telescope and is able to validate the good performance of the existing finite element model. Test measurements show that slight improvements of the aperture efficiency and gain elevation dependence are possible with a more extensive OOF holography campaign.