The first six hexagonal segments for the main mirror of the European Southern Observatory’s (ESO’s) Extremely Large Telescope (ELT) have been successfully cast by glass manufacturer Schott AG at its facility in Mainz.

The segments will form parts of the ELT’s 39-m main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024. The primary mirror of the telescope will be the largest ever made for an optical IR telescope. The segments will work together as a single huge mirror to collect tens of millions of times as much light as the human eye.

“It was a wonderful feeling to see the first segments being successfully cast,” said Marc Cayrel, head of ELT’s optomechanics at ESO. “This is a major milestone for the ELT.”

As with the telescope’s secondary mirror blank, the ELT main mirror segments are made from Zerodur, Schott’s low-expansion ceramic material. ESO has awarded the company with contracts to manufacture the blanks of the first four ELT mirrors. The first segment castings allow the engineers at Schott to validate and optimize the manufacturing process and the associated tools and procedures.

More than 900 segments will need to be cast and polished. When fully up to speed, the production rate will be about one segment per day. After casting, the mirror segment blanks will go through a slow cooling and heat treatment sequence, then will be ground to the proper shape and polished to 15 nm across the entire optical surface.

ESO is a European intergovernmental astronomy organization and a ground-based astronomical observatory supported by the 16 countries of Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the U.K., along with the host state of Chile and strategic partner Australia. Schott is a developer of special glass, glass-ceramic and glass innovations.