Schott Casts First Mirror Segments for ESO’s Extremely Large Telescope

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.

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. ESO has awarded Schott with contracts to manufacture the blanks of the first four ELT mirrors.

The 39-m diameter primary mirror of the ELT will be the largest ever made for an optical-IR telescope, consisting of 798 individual hexagonal segments, each measuring 1.4 m across and about 5 cm thick. 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 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 the low-expansion ceramic material Zerodur from Schott. 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, with 798 for the main mirror itself and a spare set of 133. 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 and will then be ground to the right shape and polished to a precision of 15 nm across the entire optical surface. The shaping and polishing will be performed by French company Safran Reosc.

ESO is an intergovernmental astronomy organization and ground-based astronomical observatory supported by the 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. Schott is a technology-based group of glass manufacturers and developers.