The first mirror telescope
The State Observatory of the University of Heidelberg began to plan a first mirror telescope around 1900. SCHOTT was commissioned to build a mirror substrate with a diameter of 720 millimeters and delivered it to Carl Zeiss in 1903 for final processing. The finished telescope went into operation on the Königsstuhl mountain near Heidelberg in 1906. It was given the name Waltz reflector because it was financed through a generous donation from the private foundation of Käthe Waltz, a relative of the astronomer Max Wolf. He and his staff succeeded in making many epochal discoveries, for example the rediscovery of Halley’s comet in 1909 and the identification of countless variable stars. After the addition of a spectrograph and modern CCD detectors, the Waltz reflector was still used for astronomical observations until recently.
One-meter mirror telescope
Two-meter reflecting telescopes
The development of large-scale astronomical equipment came to a standstill in Germany until 1948. This was the year the director of the Astrophysical Observatory in Potsdam, Professor Hans Kienle, filed an application for a two-meter telescope from the German Academy of the Sciences: “A two-meter universal reflecting telescope is urgently required by the German astronomists in order to be able to return to the forefront of astronomical research with such a high-performance instrument....The construction of this telescope will at the same time provide effective proof of the efficiency of the ‘Jenaer Werke’ of Carl Zeiss and SCHOTT.“
After receiving the commission in June 1949, SCHOTT used ZK 7 glass to produce a mirror blank with a diameter of 2.15 meters and weighing more than 3,000 kilograms. This type of glass was characterized by a heat expansion that is about half as much as other crown glasses used at the time and was therefore far more suitable as an engineering material for astronomical mirrors. The two-meter reflecting telescope began operations at the Thuringian State Observatory in Tautenburg near Jena in 1960.
Mirror substrates made of “Duran” borosilicate glass
Around 1962 came the conversion to astronomical mirrors made from materials with still lower thermal expansion coefficients than that of ZK 7, for example quartz glass and glass ceramic. Using a continuous melting process, SCHOTT GLAS in Mainz succeeded in casting mirror substrates with diameters of between one and two meters from “Duran” borosilicate glass. For example, the largest Egyptian telescope in Heluan was equipped with a 1.95-meter mirror made from “Duran.” The European Southern Observatory (ESO) still uses a reflector with a “Duran” mirror in Chile. It has a diameter of 1.6 meters and continues to deliver excellent images.
The largest “Duran” disk with a diameter of 2.7 meters was cast in 1963. The casting process lasted several hours and the six-month cooling process took place in an annealing furnace that was specially built for this purpose.
Mirror substrates made from “ZERODUR®” glass ceramic
Thin mirrors for telescopes with active optic systems
The introduction of active optic systems in the 1980s ushered in a completely new kind of telescope mirror. The mirrors are produced so thinly for these systems that their shape can be selectively adjusted with the help of electronically controlled mechanisms. Variations from the ideal shape can thus be constantly corrected. This concept led to a clear reduction in costs because far less material and time are required for production. In addition, much less mass has to be moved in adjusting the telescope.
The prototype of this new telescope generation was the New Technology Telescope (NTT) of the European Southern Observatory (ESO). In the case of the primary meniscal mirror, the ratio of diameter to thickness was 15:1 (diameter 3.6 meters, thickness 0.24 meters). It was delivered in 1986.
Astronomical mirrors for telescopes of the 8- to 10-meter class
This revolution in large-scale telescope construction paved the way for still larger telescopes equipped with mirrors with diameters of eight to ten meters. For the construction of Very Large Telescopes (VLT), the ESO commissioned SCHOTT in 1988 to produce four “ZERODUR®” mirror substrates, each with a diameter of 8.2 meters. For this project, SCHOTT developed a novel centrifugal casting process in which the casting mold has a curved bottom and constantly rotates. The first 8.2-meter mirror substrate was delivered in 1993. Three further substrates were supplied each year for the following three years. In the meantime, all four of the large telescopes transmit a wealth of observational data from outer space.
New techniques were introduced for astronomical mirrors with still larger dimensions – such as for the Keck I and Keck II telescopes with diameters of 10 meters located in Hawaii or for the 10.4-meter Grantecan telescope currently under construction on La Palma. These mirrors are no longer produced in one piece, but instead are composed of numerous hexagonal mirror segments. Particularly high demands are made on optical finishing and assembling: all segments are constantly repositioned so that they work together to produce the best possible image quality.
The 100th anniversary
In April 2003 SCHOTT delivered a 4.1-meter mirror substrate made from “ZERODUR®” for the world’s largest wide field survey telescope called VISTA (Visible and Infrared Survey Telescope for Astronomy). It was commissioned by a consortium of 18 British universities as an “entry” to the European Southern Observatory. It will be VISTA’s task to search the skies for interesting objects that will then be investigated in greater detail by the Very Large Telescope.