A Flying Start for a Super Telescope: Schott Successfully Casts an 8-m Mirror Blank
6 February 1991
A test run for the manufacture of mirror blanks in the 8-metre class, for use in the world's largest optical telescope, the ESO 16-m equivalent Very Large Telescope (VLT), has been successfully performed at Schott in Mainz, Germany . The test blank had a diameter of 8.6 metres and a surface area of more than 55 metres squared. This is the first time that has been possible to cast such a large glass-ceramic blank in one piece. To accomplish this impressive feat, Schott has developed a number of new technological procedures.
The ESO VLT will consist of four individual 8-metre telescopes which together will perform as a 16-metre telescope with a mirror surface area in excess of 200 metres. It will be placed on Cerro Paranal , an isolated mountain top in the Chilean Atacama desert at 2664 metres altitude.
During the next years, Schott will produce the four mirror blanks needed for the VLT. Each of them will have a final diameter of 8.2 metres and be unusually thin, only 177 millimetres, in order to be so flexible that their surface form can be easily controlled and maintained in optimal shape by means of an active optics system. This technique has already been successfully installed in the ESO 3.5-metre New Technology Technology for which the mirror blank was also produced by Schott.
For the manufacture of the very large VLT mirror blanks, Schott uses the spin-casting technique. In the 2400 metres squared production hall on the bank of river Rhine which was specially constructed for the VLT Project, 45 tonnes of molten glass is poured into a rotating mould with a curved bottom; it makes about six revolutions per minute. In this way the blank is given the desired, curved shape which is retained when the glass cools and solidifies.
This prototype blank will now spend about three months in an oven while it is slowly cooled to room temperature. Then follows a mechanical correction of the shape and thereafter a renewed thermal treatment, the so-called ceramization process, by which the material achieves its zero-expansion properties, making it insensitive to temperature changes and suited for use in astronomical telescopes.
ESO has congratulated Schott on the successful casting of the first mirror blank of this size. Smaller blanks from Schott of the same material are used in other advanced astronomical instruments like the Keck telescope, ROSAT, Galileo and AXAF.
 This Press Release is published simultaneously by Schott Glaswerke (Mainz, Germany) and the European Southern Observatory. See also eso8808 about the signing of the contract between Schott and ESO. A B/W photo showing the casting at Schott is available at request from: Schott Glaswerke, Presse Abteilung, Hattenbergstrasse 10, D-6500 Mainz 1, Germany; Phone: (06131)837328 -Mrs. Fuhr; Fax: (06131)662003.
 See also eso9011.
ESO EPR Dept