The antenna in the valley

The construction of the 100-metre radio telescope four decades ago at Effelsberg bears witness to the fine art of engineering

When Galileo Galilei turned his modest “spy glass” towards the stars in the summer of 1609, he opened up new skies. He observed things which no one had ever seen before: mountains and craters on the moon, the phases of Venus, individual stars of the Milky Way. Galileo Galilei had pushed the window into space wide open. He had no way of knowing that his telescope observed only a tiny octave in the cosmic keyboard of light, because the electromagnetic spectrum we receive from space stretches across twelve orders of magnitude: at one end are the high-energy gamma rays with wavelengths of 0.01 nanometres (one billionth of a metre); at the other, the radio region with wavelengths of several metres.

In 1962 the Denkschrift zur Lage der Astronomie (memorandum on the situation of astronomy) submitted a proposal to build a large-scale instrument for radio astronomy. Otto Hachenberg, Head of the Institute for Radio Astronomy at the University of Bonn, then started to plan an 80-metre telescope. The government of the Federal State of North Rhine-Westphalia funded the planning stage and preliminary work. The company Krupp provided a draft design which involved a flexible solution instead of a rigid steel construction so that the reflector’s parabolic form was maintained as the telescope tilted. This made it possible to increase the antenna’s diameter to 90 metres. In 1964 Hachenberg, together with his colleagues Friedrich Becker and Wolfgang Priester, submitted an application to the Volkswagen Foundation for funds to build this large-scale instrument.

At around the same time, the Foundation received a second application: Sebastian von Hoerner from Tübingen was planning a 160-metre reflector. Both projects were initially recognized as being worthy of support; however, the high operating costs of such instruments meant a suitable sponsoring organization had to be found. The Max Planck Society declared its willingness to create an appropriate institute. Thanks to the preliminary work which had already been done and the support of the State of North Rhine-Westphalia, the Max Planck Institute for Radio Astronomy (MPIfR) was established in Bonn. Otto Hachenberg was appointed Director.

As the plans for the Tübingen project had been abandoned, more funds were available for the project in Bonn. Calculations had shown that a surface with sufficient accuracy could also be achieved for a 100-metre reflector – and so it was decided to build this.

The search for a suitable location started in 1966. It was clear that only a valley which provided shielding against interfering radiation was an option. The final choice was a valley running north-south, close to the village of Effelsberg near Bad Münstereifel. Most of the 15.4 hectare site was just within the borders of the State of North Rhine-Westphalia.

The engineers had to tread completely new paths for the construction. They had to break with the conventional way of designing a reflector, as had been realized at the time with the Mark 1 telescope at Jodrell Bank, England, and the Parkes telescope in Australia. After all, the 100-metre dish was to have a surface accuracy of one millimetre.  One team at Krupp achieved its aim of eventually producing a radially symmetric structure for the reflector. A separate tilting frame was to hold the reflector in the axial direction. This design allowed particularly simple computations of the elastic deformation as it was tilted.

The tilting frame and the focal support legs together formed an octahedron which was stabilized by a diagonal bracing structure in the interior. In addition, this construction also contained the counterweight for the reflector, which the tilting frame was to hold in the axial direction like an umbrella. Floating wheel drives with a rim gear on one arm of the octahedron were selected for the tilting movement. The base frame had four drives with 16 motors and 32 wheels for the azimuth drive, with a load of around 100 tonnes per wheel. A concrete ring was to bear the azimuth track with a diameter of 64 metres. Spaces for the power supply and workshops were provided in the foundations.