It will be the largest "eye of mankind": the new gigantic optical telescope - Extremely Large Telescope (ELT) - in the Chilean Atacama Desert. To look deep into space, 798 hexagonal mirror segments of the 39-meter main mirror must be arranged with an accuracy of two nanometers. This is possible with the Zerodur glass-ceramic from Schott in Mainz.

Sharp images from space
In 1903, Schott supplied the first telescope mirror carrier with larger dimensions. Since then, the company's astronomical mirrors have been contributing to the exploration of the universe. To be able to look deeper and deeper into space, telescopes need to capture as much light as possible. Schott's glass-ceramic mirror substrates ensure sharp images. With a construction period of ten years, the ELT is the most spectacular project of the European Southern Observatory ESO to date. It is due to go into operation in 2024.

The laterally offset cab with the raised driver's seat ensures a clear view of the mold through the lifting frame as well as past it. © Still/Schott

Schott has invested in new melting capacities and post-processing facilities at its competence center for glass-ceramics in Mainz. The new glass melting furnace is designed for mirror substrates with a diameter of up to 4.25 meters. It is also used to cast the round disks for the primary mirror and the two Zerodur mirror substrates for the secondary and tertiary mirrors of the ELT. The demand for Zerodur glass-ceramic is also very high for other applications. Schott has therefore already invested in a second melting tank.

In order to produce a glass ceramic with the required properties from the raw glass, it must be heated again. During ceramization, added substances form tiny crystals as the temperature rises, which contract when heated and counteract the thermal expansion of pure glass. By precisely coordinating the crystal and glass phases, an expansion of almost zero is even achieved for certain temperature ranges. This process can take up to several months.

The truck has good driving stability even when negotiating tricky bends with the heavy mold. © Still/Schott

Casting and ceramizing the ELT mirror segments therefore takes just under four months in each case. At peak times, one round disk is completed per day.

Vat handling with eight-ton electric forklift truck
The RX 60-80 electric forklift from Still with precise hydraulic control is used to handle the casting mold with the molten glass. The electric forklift is maneuverable and enables aisle widths of less than five meters. The laterally offset cab with the raised driver's seat ensures a clear view of the castings through and past the mast. Large viewing windows, low-lying tilt cylinders and the outer contour of the vehicle provide excellent all-round visibility. Such an overview increases work safety and operational readiness and reduces the risk of injury and damage.