Hamburg Observatory - Overview:
Buildings & Telescopes

The 1m Reflector Telescope at Hamburg Observatory is, by merit of its Zeiss mounting, one of the most unusual constructions among telescopes. Prior to 1920, and again between 1946 and 1960, it was the largest telescope in Germany. A very similar 125cm reflector telescope of Berlin Observatory (Babelsberg) surpassed the 1m Reflector in the years 1920 - 1946, however, that instrument was dismantled, and transported to the former Soviet Union after World War II as part of the reparations (where it is still in use at Crimean Observatory).

When the relocation of Hamburg Observatory to Bergedorf took shape at the beginning of this century, and future instrumentation had to be chosen, the large refractors were at the peak of their development. With the rise of astrophysics, however, reflector telescopes had become increasingly important. In new observing techniques like celestial photography in particular, they were superior to refractors, because of their larger focal ratios and absolute lack of chromatic aberration. Although some remarkably large reflectors had been built in the 19th century (Birr Castle: 1.80m, Malta: 1.20m, Melbourne: 1.22m), little success was assigned to them, due to their heavy, difficult to polish, metal mirrors. Only after L. Foucault had been able to prove the usefulness of silver-coated glas mirrors for astronomical purposes, and by merit of the excellent exposures made by J. E. Keeler at the turn of the centuries with the 91cm Crossley Reflector (Lick Observatory), did reflector telescopes start their triumph.

The Hamburg 1m Reflector was the world's fourth largest reflector at the time it started to work in 1911 (after Mt. Wilson: 1.52m, Paris: 1.20m, Lowell-Obs. Flagstaff: 1.07m), especially since the metal reflector telescopes no longer existed or were - as was A. A. Commons 1.52m reflector with glas mirror - no longer used.

The order to build the optical and mechanical components of the telescope, as well as the 10m dome, went to Carl Zeiss in Jena. The astronomical section of that firm had heen founded only a few years before in 1897. Prior to the Hamburg instrument, Zeiss had delivered only two medium sized reflector telescopes to Heidelberg (72cm) and Innsbruck (40cm). Thus the 1m Reflector was the first large Zeiss telescope. It was the first telescope to be equipped with a Zeiss-mounting after Fr. Meyer. In such a construction with its characteristic bars and counterweights, the declination and right ascension axes are performed as hollow-axes. Inside, strong iron rods carry the load of telescope and counterweights, while the weight on the bearings of the axes themselves is reduced. A very exact and frictionless motion was thus achieved. The telescope, despite its weight of 26 tons, is indeed easily movable with one hand. Another advantage is that it can be moved across the meridian without - as with a German Mounting - turning the telescope to the other side.

Apart from the Hamburg 1m Reflector, only two further large reflector telescopes were built with this kind of mounting: the above mentioned 1.25m telescope of Babelsberg Observatory, and a twin of the Hamburg instrument which was delivered to the royal Belgian observatory in Uccle (Brussels). Tube and optics of the Belgian telescope were lost in world war II; the mounting was equipped with a new 84/120cm Schmidt telescope in 1958.

The 1m Reflector was built as a Newton system with an extremely high focal ratio (3m focal length). Access to the Newton focus was provided by a platform in front of the dome opening which could be moved up and down, as well as into the dome. Because of the extremely large focal ratio of 1:3, image distortion was naturally very large. Pointlike stellar images could be obtained only very close to the plate centre, however, fields up to 2 x 2 deg² have been successfully investigated. If necessary, the image quality could be improved by reducing the aperture by means of an adjustable iris diaphragm in front of the primary mirror. Tracking corrections could be made with the aid of an off axis guiding eyepiece in the Newton focus, or with the guiding refractor (aperture 20cm, focal length 3.40m). A 10cm finder was also available.

Although the new building was completed in 1907, and the dome in 1909, the telescope could not start operating before the end of 1911. Performance tests proved unsatisfactory, and Zeiss had to make a new mirror cell because of irregular bending of the 17cm thick prime mirror. These works took nearly a year so that regular observations could start only in early 1913. During its first years, the telescope was used mainly by the director Richard Schorr and the Danish astronomer Thiele. Until 1920, these two observers took more than 1700 photografic plates which were used primarily for the search and position determination of comets and planetoids. 30 new asteroids and one new comet (1918III Schorr) were found with the 1m Reflector in these years, and two periodical comets were rediscovered.

In April 1920, young Walter Baade took over the telescope. Had the research program been hitherto mainly concerned with classical astronomy, so now astrophysics became more and more dominant. Baade took countless photographs of stellar clusters, gaseous nebulae, and galaxies. Downright epoch-making were his studies of variable stars in, and close to, globular clusters, in particular M53. Baade could prove - for the first time - the existence of isolated stars in the halo, i.e. far outside the galactic plane. He compared occurence and types of variable stars in fields with different galactic latitudes, which later led to his famous discovery of the two different stellar populations. Further work of Baade with the 1m Reflector included, among others, the Orion nebula, and the discovery of two clusters of galaxies in Ursa Maior. It is little known that Baade managed as early as 1921 (and thus three years earlier than Hubble at Mt. Wilson) to identify three variable stars in the nearby spiral galaxy M33 with the 1m Reflector. Because these were not Cepheids, they could not prove the extragalactic nature of M33. Baade never published this discovery. Besides that, Baade regularly took up Schorr's favourite interest and searched for comets and asteroids. His search resulted in the discovery of comet 1922II Baade, and the rediscovery of three periodic comets. In addition, several new asteroids were found, among them the unusual object 944 Hidalgo which orbits the sun far outside the asteroid belt, between Jupiter ad Saturn.

In 1931, Walter Baade left Hamburg Observatory to enter a new position at Mt. Wilson Observatory (California). The twelve years at his hands were the most productive phase in the history of the 1m Reflector. Back in 1927, after he returned from his first visit to Anmerica, Baade had suggested to move the telescope to a more southern region with better climate. This idea was never realised, nor was Baades suggestion to convert the 1m Reflector into a Schmidt telescope.

During the following years, Richard Schorr took over the telescope again, using the instrument once more for comet and asteroid observations. An exception was the detailed investigation of the emission line spectrum of the Orion nebula by Rudolf Minkowski, F. Goos, and P. Koch in the years 1931 to 1934. For that purpose, a prism spectrograph with Fabry-Perot plate had been mounted in the telescope's prime focus. Further observations, until 1939, were made by Brüggemann, Larink, Dieckvoß and Sandig.

Observational work rested during World War II, yet plans ripened to use the instrument for stellar spectroscopy in the future. An already existing prism spectrograph, originally bought for the Great Refractor, was intended to be used. However, it could not be mounted in the prime or Newton focus. Moreover, a larger focal length was required - usually realised in the Cassegrain or Coudé focus. In the middle of the war, two new auxiliary mirrors were ordered from Zeiss: a hyperbolic convex mirror to increase the focal length to 15m, and a flat mirror to direct the light beam out of the tube towards the Nasmyth focus. Nobody intended to run the risk of drilling a hole through the primary mirror as is usual for Cassegrain telescopes. In 1944, the primary mirror was taken out, and sent to Jena for proper figuring of the new optics. After the capitulation, its fate was uncertain, since Thuringia was first occupied by America, but afterwards added to the Soviet occupation zone. That notwithstanding, the mirror returned safely to Bergedorf about Christmas 1945, the auxiliary mirrors following a little later.

The Zeiss spectrograph was put into service in autumn 1947. This prism spectrograph has an unusual, very compact design with a folded light path to avoid bending effects as far as possible. By exchange of two prism boxes with one or three prisms in series, respectively, and with camera objectives of different focal lengths, dispersions between 8 and 72 Å were available. The light of an iron arc could be reflected into the light path to obtain reference spectra.

Between 1947 and 1972, thousands of star spectra were observed with this instrument, among them the Zeta Aurigae systems, novae, radial velocity standards, MKK standard stars, spectroscopic binaries, and variables. The most untiring observers in these years were P. Wellmann and H.G. Groth. Other successful observers with the 1m Reflector were, among others, D. Labs, J. Hardorp, T. Herczeg, M. Grewing, I. Yavuz, U. Gehlich, and R. Wehmeyer.

In 1974 and 1975, a new grid spectrograph destined for the new Oskar-Lühning Telescope was tested at the 1m reflector. Between 1976 and 1978, finally, optical period monitoring of the Crab pulsar, begun some years earlier at the Great Refractor, was continued with the fast photometer of the 1m Reflector. The old prism spectrograph was remounted in the eighties, and the instrument was used only for teaching purposes (sun spectrum) afterwards.

The 1m Reflector telescope is presumably the historically most valuable instrument at Hamburg Observatory. On one hand, numerous exciting discoveries were made with the instrument in he hands of one of the most important astronomers of the 20th century. On the other hand , except for the change from Newton focus to Nasmyth focus, the telescope is practically in its original state. Finally, it has a special rank in technical history due to its nearly unique construction. Although the telescope is in full working order, its general state is very poor. To avoid further damages, it needs derusting and preserving. Unfortunately, neither the financial nor the personal situation of Hamburg Observatory permits these works to be started.

German text and images: Matthias Hünsch, historical fotos: Hamburg Observatory;
English translation: Kerstin Molthagen