History of the Observatorium Hoher List (OHL)

Hoher List Observatory was the observatory of the Argelander-Institut für Astronomie of the University of Bonn. Already when Prussia founded the Rheinische Friedrich-Wilhelms-Universität in Bonn in 1818, a chair of astronomy was added to the Faculty of Philosophy. Between 1839 and 1845, Bonn Observatory was built outside the city on Poppelsdorfer Allee, which connects the Bonn City Palace with the pleasure palace Clemensruhe (nowadays Poppelsdorfer Schloss), under the great astronomer Friedrich Wilhelm August Argelander (1799 - 1875).

Pictures: Bonn Observatory on Poppelsdorfer Allee. Engraving around 1850 and photo 1893.

Over the years, the city of Bonn has outgrown the observatory, and today it is practically located in the city center. Due to light pollution and smog in the big city no meaningful astronomical observations were possible anymore. Therefore, in 1950, the decision was made to erect an outdoor station outside of Bonn, on a flat, 1800 feet (549 meters) high hilltop called 'Hoher List', near the village of Schalkenmehren in the district of Daun city.

Under Director Friedrich Becker (1900 - 1985) and his assistant Hans Schmidt (1920 - 2003), later the first director of the observatory, a total of 8 ¼ hectares were purchased from the Schalkenmehren community and private owners, and the observatory was built from 1950 to 1954. The main building nestles in a slight curve against the slope of the hilltop, on its southeast side. Three observation towers were added to the building, and a fourth, smaller tower was added to the hillside slightly above by Hans Schmidt. A residential building completed the ensemble.

The first major instrument acquired was a 20-inch (50 cm) diameter Schmidt type photographic telescope, named after the Hamburg optician Bernhard Schmidt, the first larger telescope of its kind to allow sky imaging with a wide viewing angle. Unfortunately, this telescope was sold by the University of Bonn shortly before the observatory was placed under historical preservation order.

Another dome houses the Bonn Double Refractor. The unique possession of photographic material from around 1900 makes this telescope a unique piece of scientific and historic equipment.

Pictures: On the left the historical 12/14-inch (30/36 cm) Repsold Double Refractor from 1899 and on the right the 13/20-inch (34/50 cm) Askania Schmidt telescope (both images © M. Geffert).

After being the most important telescope in the Rhine-Ruhr region in its early Bonn days (1899 - 1966), this telescope again gained great importance at the end of the last century. In 1967 the telescope was set up at the Hoher List Observatory, almost without modifications, in order to study stellar motions by the exact comparison of old and new photographic plates. About 30 peer-reviewed papers from the 1980s and 1990s demonstrate the international importance of this work based on the old centennial photographic plates.

Picture: Observatorium Hoher List in early summer of 2002. Below left is the dome of the 1-meter telescope with the adjacent electronics laboratory. In the center is the one-story residential building, above it on the left the main building with three domes, above it the small cone-roofed hut of the Schröder Refractor (returned to Bonn in 2012), and to the right of the main building the small residential bungalow for guests. On the far right rises the dome of the Bonn Double Refractor, adjoined on the left by the precision mechanics workshop.

The main building soon proved to be too small, and a modern telescope was more and more urgently desired. Thus, between 1962 and 1965, Observatorium Hoher List was extended by two large towers, the domes of which are 28-feet (8.5 meters) in diameter. Attached to the towers were the precision mechanics workshop and the new electronics laboratory. In addition, a five-room residential bungalow was built to accommodate visiting observers from abroad. In one of the domes, the Bonn Double Refractor, built in 1899 by the Repsold company in Hamburg, was re-installed. This refracting telescope combines two telescopes in one tube, one for visual observations, the other one for photographic observations. For the second new dome, a modern Nasmyth-Cassegrain reflecting telescope was acquired, the '1-meter telescope'. The main mirror with an aperture of 106 cm was manufactured by Askania-Werke in Berlin. The equatorial fork mount and the truss tube of the telescope, i.e. the mechanical structures, were designed by Ing. B.G. Hooghoudt (Leiden, Netherlands), who was also involved in the construction of the 100 m radio telescope at Effelsberg, and built by the Rademakers company (Rotterdam, Netherlands). The entire electrical system (analog control system, etc.) was provided by the Wesemann company (Rotterdam, Netherlands). The optics proved to be of very high quality (λ/7 wavefront, Strehl 95%).

The electro-mechanical control system was upgraded to a modern computer control system at the beginning of the 21st century, so that the telescope can also be operated under remote control. This computer control, in connection with the high-precision drive mechanics, provide such an exact compensation of the earth rotation that guiding control during a photographic run is completely unnecessary!

Picture: Askania's 106-cm reflector telescope in its equatorial fork mount. In 2012 the Argelander Institute of the University of Bonn discontinued scientific observing and student education/training operations at Hoher List. The operation of the telescope was taken over by the AVV.

More pictures of Hoher List Observatory, the instruments and the history of the observatory can be found in the gallery on our forum page. If you want to read the report of Prof. Dr. Hans Schmidt about the astronomical site testing of the Hoher List and the foundation of the observatory, please refer to his original report (in German), which is a really exciting and entertaining read from the pioneering times.