Casting the world's first 8-m mirror (1991)

The first test run for the manufacture of mirror blanks in the 8-metre class was successfully performed at the Schott Glaswerke (Mainz, Germany) in early 1991. Completely new manufacturing techniques had to be developed in order to produce blanks of this size, a process never attempted before.

With a temperature of 1,400 ^oC, 45 tonnes of molten glass were poured into the specially designed mold. Immediately thereafter, the mold was moved to a rotating platform. During the first cooling, the mold was spun on the rotating platform. With the centrifugal forces created by the spin - in combination with the curved bottom of the mold - the glass gradually assumed the rough meniscus shape required for the VLT mirror blank.

During the subsequent manufacturing stages, the glass blank was slimmed down to its final weight of 23.5 tonnes through precision grinding. The glass was then transformed into Zerodur glass ceramic (a material that undergoes no dimensional changes with changing temperature and is therefore ideal for astronomical purposes), making it the world's largest Zerodur blank.