It is not possible to make a mirror this big from a single piece of glass. That is why the main mirror of the ELT consists of 798 individual segments, working together as one, to observe the cosmos in depth and detail.
On arrival at the ELT Technical Facility the segments are inspected and coated by our engineers and technicians, giving them the highly reflective finish that will collect starlight.
📷 ESO/A. Russell
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However the coating does not last forever:
➡️ Once the telescope is operating, two segments a day will need to be exchanged to receive a fresh reflective layer, work that our team at Paranal will do throughout the entire ELT operation lifetime.
➡️ To make this possible without ever leaving a hole in the mirror, 133 additional segments have been produced (that is, a total of 931), so that a segment can be taken out for recoating and be immediately replaced.
📷 Safran Reosc/ESO
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While more than half the segments with their supports have now arrived in Chile, most other segments are either en-route or packed and ready to be shipped, with a remaining few in production. This means that, in the coming months, all segments will be in Chile.
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📷 ESO/A. Russell
@esoastronomy how much do the segments weigh? Is there a whole series of customized handlers and lifting gantries/carts perhaps for moving them around?
@ottaross Each segment weighs 250 kg, including the supports. And yes, we have a segment manipulator specially designed to handle this. It will be mounted on a crane that can position it directly above the desired segment. This segment will be then pushed above the rest and grabbed by the manipulator. We're currently testing this device at our headquarters in Germany, stay tuned... ;-)
@esoastronomy nice! Thank you for the background info!
@esoastronomy Bonjour, quand est-ce que le ELT pourra fonctionner ? est ce qu'il pourrait faire de l’interférométrie avec le VLT ?
@DessiniFab Hi! The first observations with a test camera are planned for 2029, and the first observations with the actual scientific instruments for late 2030.
The ELT and the VLT won't work together interferometrically, but some ELT instruments will (independently) complement the capabilities of the VLT Interferometer.
@esoastronomy Merci ^^
@esoastronomy you make it sound so simple.
@Bart_amsterdam We hope that means that we explained it in an accessible way :-) But it's indeed a very complex process. If you want to learn more about the journey these segments go through, here's a useful explainer video: https://www.youtube.com/watch?v=3-s_UZePP_Q
Oct 14, 2026, 12:30 PM@esoastronomy how many different segment shapes are there? Or are they all interchangeable?
@tamme Good question! You can divide the main mirror into 6 radial sectors or petals. Each sector has 133 segments, which are slightly different from each other, but identical to the 133 segments of all the other sectors. That's why in addition to the 798 segments that make up the full mirror there are 133 extra ones – the "exchange sector" you see in the image in post 3 of this thread. Every day we'll remove two segments from the mirror and replace them with their counterparts from this set.
Oct 14, 2026, 1:47 PM@esoastronomy oh cool, now I also understand the shape of the change set. Thank you!