VLT primary mirror: still liquid!

Production of one of the VLT primary mirrors at the Schott company in Germany, in February 1993. The liquid yellow glowing glass is cooled down over several weeks under controlled conditions. With a diameter of 8.2 metres, the VLT primary mirrors (M1s) are among the largest one-piece astronomical mirrors in the world. The VLT M1s are only 17 cm thick and they are made of Zerodur, a special glass-ceramic with a thermal expansion coefficient close to zero. Zerodur is a very stable material that can be polished with the extremely high accuracy required by astronomical mirrors.