The SCHOTT melting team has started casting the first mirror segments that will make up the 39-meter primary mirror (M-1) of the European Large Telescope (ELT). To make the segments, liquid glass heated to over 1400 degrees Celsius will be poured directly into molds, and transferred into a cooling furnace and subjected to a ceramicization process lasting several weeks.

The result is ZERODUR glass-ceramic, a material with a thermal expansion of near zero that makes it especially suited to astronomy applications. The SCHOTT production facility in Mainz is expected to complete delivery of the segments by 2024.

SCHOTT, the international technology group, will produce up to 949 identical 1.52-meter hexagonal segments for the ELT. The giant mirror will be composed of a total of 798 segments, made of ZERODUR glass-ceramic, with the balance used as replacement segments.

"Casting the mirror substrates for the M1 is a milestone in the European Southern Observatory's (ESO) project plan. The main mirror of the ELT will be humanity's largest eye on the sky, enabling us to reach unprecedented depths of space," said Marc Cayrel from ESO's ELT project team. SCHOTT has developed special casting molds for the production of the M1 segments.

"The glass consumption factor will be as low as possible, so we can work very efficiently," explains Dr. Thomas Westerhoff, Director of Strategic Marketing for ZERODUR at SCHOTT.

After ceramization, each of the round ZERODUR blocks will be cut into five slices of approximately 60 - 70 mm in thickness. Further processing at SCHOTT will take place on state-of-the-art computer-controlled 5-axis CNC machines. SAFRAN Reosc, a company based in France, will polish the M1 segments.

ZERODUR is a proven material that has been selected for four of the five high-precision mirror elements of the ELT. In addition to the segmented 39-meter giant mirror, these will be a convex mirror 4.2 meters in diameter (M2), the M3 concave mirror (3.8 meters in diameter) and a 2.4-meter diameter adaptive mirror (M4).

"Thanks to significant expansion of production capacity at the Mainz site, SCHOTT is very well prepared for the production of ZERODUR glass-ceramic, both for the ELT project and for the currently very gratifying high demand from the high-tech industry," said Dr. Thomas Westerhoff, director of strategic marketing, ZERODUR at SCHOTT Advanced Optics.

Two glass melting tanks are in parallel operation due to the high demand. Furthermore, new jobs were created for around 50 employees, and additional capacity-expanding investments are planned for the coming year.

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