A throne fit for the world’s largest telescope mirror

Did you know that ESO’s Extremely Large Telescope (ELT) will have a primary mirror (M1) that weighs 200 tonnes, about as much as the largest blue whales? This colossal mirror will rest on the structure that is shown under construction in this Picture of the Week, taken with a drone on July 2024.

M1 will be the largest segmented mirror ever built for a telescope, but will need to move constantly and very smoothly during observations while keeping its optical shape, and therefore needs to be held by a cell that is both lightweight and sturdy. Enter the throne that will eventually seat this segmented mirror: the M1 cell seen here. It will hold 798 hexagonal segments that will work together as a single 39-m mirror, staying aligned as the telescope moves, even under varying gravity loads, wind conditions, vibrations or changes in temperature. German company SCHOTT has cast and delivered all these segment blanks, which are now being carefully polished by Safran Reosc in France. More than 70 segments have completed their trip to Chile, where they will be coated with a reflective layer of protected silver. 

The M1 cell has openings and walkable areas underneath the mirror supports that provide access to each segment for maintenance. It is just the lowest portion of the impressive altitude structure that will eventually host all five of the ELT’s mirrors. The hole in the centre of the lattice, visible in the image, will one day house the central tower that holds the M3, M4, and M5 mirrors.

The ELT is expected to see first light later this decade, when this hard work will yield a telescope capable of tackling the biggest scientific challenges of our time.

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• Top-down view of the M1 cell
• Pieces of the M1 cell before being lifted into the dome