The first segments of the main mirror of an extremely large telescope are successfully cast.

In Minza, a casting of the first six hexagonal segments of the main mirror of an extremely large telescope (ELT) was carried out, according to a press release from the European Southern Observatory (ESO). The 39-meter main ELT mirror will consist of 798 such segments. An extremely large telescope will become the largest optical telescope of the world; The first observations on it are scheduled for 2024.

ELT mirror cannot be made from one piece of glass; It will consist of 798 individual hexagonal segments with a diameter of 1.4 meters and about 5 centimeters thick. In the collection, all these segments will work as a single giant mirror, which will collect tens of millions of times more than the human eye.

The manufacture of the mirror is entrusted to the German optical company Schott. The segments of the main ELT mirror, as well as the harvesting of the secondary mirror of the telescope, are made of the Zerodur ceramic material developed by Schott, which has an extremely low coefficient of thermal expansion. Even in the case of significant changes in temperature, it is practically not deformed. The material is resistant to chemicals and can be polished with very high accuracy. An exceptionally thin reflecting layer of aluminum or silver is usually sprayed to the surface shortly before the telescope puts into operation and then periodically updates. Mirrors made of Zerodur have been working reliably for decades on many well -known telescopes, including on a very large ESO telescope in Chile.

Photo: Schott/ESO

The casting of the first segments is especially important, since it allows engineers of Schott to comprehensively evaluate and optimize the process of manufacturing mirrors used in it tools and methods.

The casting of the first six segments has become an important milestone, but there is still a long way ahead: it is necessary to cast and polish even more than 900 segments (798 directly for the main mirror plus a set of 133 spare ones). When the production process is debugged, one segment will be made daily.

After casting, the workpieces will have to undergo the stages of slow cooling and heat treatment, and then they will be polished and polished with an accuracy of 15 nanometers along the entire optical surface of the mirror. Grinding, polishing and additional testing will be performed by a French company.