First segments cast for world's largest telescope's main mirror

The world's largest optical telescope is well on its way to seeing first light.

The German company SCHOTT has finished casting the first six hexagonal segments for the main mirror of the Extremely Large Telescope (ELT) of the European Southern Observatory (ESO).

When assembled, the mirror will measure 39 metres across and sport a total of 798 segments. The ELT should be completed by 2024.

The huge mirror can't be made from a single piece of glass – it is too large for that. Each of the 798 individual hexagonal segments will measure 1.4m across and will be about 5cm thick. 

“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!”

The first segment castings allow the engineers at SCHOTT to validate and optimise the manufacturing process, so they are extremely important. After casting, the next step is to cool the segments slowly, heat treat them, and then grind them to the right shape. Finally, each segment will be polished to a precision of 15nm across the entire optical surface.