Out-of-focus holography at the Effelsberg telescope

TL;DR

This paper develops and applies an out-of-focus holography method and software for the Effelsberg telescope to identify and correct aperture deformations, improving telescope performance and surface accuracy.

Contribution

It adapts the OOF holography technique and software from Green Bank to Effelsberg, enabling aperture aberration detection without a reference antenna.

Findings

Detected gravitational deformations not in existing models

Validated the finite element model with OOF holography

Potential for slight improvements in aperture efficiency

Abstract

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.