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.

A 130-metre high crane had to be erected before the building of the steel construction could begin. Responsibility for the assembly and manufacturing work on site fell to the company MAN. A machine in a cabin at the edge of the assembly area was used to produce the complicated cuts of the tubes for the reflector. Step by step the reflector structure was welded together, until finally sector-shaped components were created. Larger constructional elements with rectangular cross-section, like the trestle structure of the telescope, were manufactured from steel plate in the Krupp workshops in Rheinhausen and transported to Effelsberg.

The on-site welding for the reflector had to be carried out with high precision. The angular accuracy with which the tubes were aligned and welded together was especially critical. Eventually the reflector sections were stacked up at the assembly area; some of them were already equipped with reflector panels on the ground. The assembly involved alternately mounting the sector components on opposite sides. This meant that the engineers had to ensure that the instrument could be driven in the azimuthal direction even during the early stages of assembling the telescope.

In 1970 the engineers mounted the reflector’s last sector. A gap of only five millimetres showed the astonishing accuracy with which the steel construction had been built. The remaining panels could now be mounted. Finally, the reflector area was calibrated with the theodolite.

The scientists and engineers had equipped the telescope with complete computer control. This was intended to make it possible to align the antenna with an accuracy of better than ten arc seconds. The drive system came from AEG, the encoder for the telescope axes from the company Heidenhain. An ARGUS 500 system from the company Ferranti was selected as the process computer. The astronomical control programs were developed by the Institute, as were the receivers.

On April 23, 1971 the new radio telescope registered the first signals at a wavelength of eleven centimetres, and only a simple dipole was used in the primary focus. During the official opening on May 12, 1971 the antenna was fully operational with measurements of the radio continuum at a wavelength of 73 centimetres. Observations at 408 megahertz were carried out during the inauguration; they eventually led to the famous survey of the complete radio sky.

The Effelsberg 100-metre telescope has served two generations of radio astronomers and led to thousands of scientific publications. The careful maintenance of the instrument by the technical teams has ensured its continuous operation. In the course of the past 40 years, researchers and engineers have also continuously improved the system – particularly with regard to the reflector’s surface accuracy. In 2006 the sub-reflector was finally replaced. The new reflector has an actively adjustable surface that allows further improvements to be made for observations from the secondary focus.

There is a long list of receivers which have been developed for both the primary focus and the secondary focus in order to cover all frequency bands of interest in the centimetre wavelength range. The Effelsberg telescope has even been used successfully at 86 gigahertz (three millimetre wavelength). Despite its age, it still numbers among the first-rate astronomical precision instruments.

Authors: Richard Wielebinski / Bernd H. Grahl

Editor: Helmut Hornung