Schott chooses Siemens automation technology to produce mirrors for the world’s largest telescope

Cerro Armazones, Chile. The European Extremely Large Telescope (EELT), currently the world’s largest telescope, is at present under construction on top of the Chilean mountain of Cerro Armazones. The glass specialist Schott is one of the companies manufacturing the enormous mirrors for the telescope. The manufacturing process is based on a special ceramic glass so that the mirror can withstand the extreme local conditions. This is why Schott has chosen Siemens technology for the manufacturing process.

The process control system Simatic PCS 7 ensures the temperature is measured and controlled extremely accurately during production. In addition to the main mirror with a diameter of almost 40 meters, the European Extremely Large Telescope also has four other mirrors which act as reflectors. Three of these large mirrors, which measure up to four meters, are produced by Schott. The Mainz-based company uses a special ceramic glass (Zerodur) in the manufacturing process as the mirror substrate. The benefit of this ceramic glass is that it has a very low coefficient of thermal expansion. This means that the material does not expand even if temperatures vary considerably. This is an important property if the EELT is to supply sharp images.

Perfect temperature control for perfect mirrors

In order to avoid stress cracks, the special glass is repeatedly heated and cooled over long periods of time during production. Temperature control at this point must be accurate to three decimal places which is why Schott has chosen the Simatic PCS 7 process control system and the Simatic ET 200SP distributed I/O system from its long term partner Siemens for this project. Initially Schott employees and experts from Siemens tested the fundamental feasibility in the laboratory and, after successful tests, Siemens then programmed a new module specifically for this application. The solution includes many types of measurement such as stress, voltage and resistance, as well as resistance thermometers and thermal elements.

Undiscovered worlds

This extremely accurate measurement of temperature allows Schott to produce mirrors which can not only withstand the adverse conditions on top of this Chilean mountain, but can also contribute to taking a look at hitherto undiscovered worlds.

Foto: Schott, ESO