  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 methods, OOF holography can be
performed 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 dedicated software, the pyoof package, to
compute aberrations of the telescope's optical system. Based on the OOF
holography method developed at the Green Bank telescope, 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 at Effelsberg. Early results reveal 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 but limited in the current
configuration.
