Extremely Large Telescope’s mirror gets closer to completion

The final segment of the Extremely Large Telescope (ELT)’s primary mirror has been cast, signalling another milestone in the super-telescope’s development.

At nearly 40 metres wide, the telescope’s mirror will be the world’s largest once it’s assembled and operating in the Chilean Atacama Desert.

The ELT, which is part of the European Southern Observatory, is expected to be up and running by the end of the decade.

The telescope’s primary mirror will be made of 798 hexagonal segments, all of which are 1.5m across and just 5cm thick.

But German company SCHOTT has just finished casting the 949th segment. An extra 133 segments were made to help with maintenance once the ELT is working, plus another 18 spares.

The mirrors are made from a ceramic called ZERODUR©. Made from oxides of lithium, aluminium, and silicon, the material has very few imperfections and extremely low thermal expansion: that is, it doesn’t change much in size as the temperature changes.

The final segments are now headed on the same journey as their earlier counterparts. First up, they’ll go to France, where they are cut into hexagons and polished so smooth that the largest irregularity will be just 10 nanometres in size. This means that, were a human skin cell to escape PPE and fall on the mirror, it would be 3,000 times the size of the largest impurity.

Then, they’ll be shipped to Chile, where they’ll be coated with silver and added into the telescope.

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