Half of ESO’s ELT main mirror segments are now in Chile

Nearly 500 segments of the main mirror for ESO’s Extremely Large Telescope (ELT) have arrived at the ELT Technical Facility at Paranal Observatory in Chile — roughly half of the pieces destined for the 39-metre primary, ESO announced on 14 October 2026. At the same time, the last segment has entered production at Safran Reosc in France.

Why 798 — and 931 — pieces

A single slab of glass that large is impossible, so M1 is built from 798 hexagonal segments that act as one mirror. Another 133 spares bring the manufacturing total to 931, so operators can pull a segment for recoating without leaving a hole in the array. Once the telescope is running, ESO expects to recoat about two segments a day at Paranal for the whole life of the observatory.

An international supply chain

Blanks start in Germany at SCHOTT, cast from Zerodur glass-ceramic that barely changes shape with temperature. They travel to Safran Reosc near Poitiers for figuring, polishing and mounting on supports designed by Dutch TNO and built by VDL ETG Projects. Position sensors come from the FAMES consortium (Fogale, France; MicroEpsilon, Germany); actuators that keep neighbours aligned are from Physik Instrumente in Germany. Danish logistics firm DSV hauls finished assemblies on a roughly 10,000 km road-and-ship trip to the Atacama.

At the Technical Facility, ESO staff inspect and coat each segment with the reflective layer that will collect starlight. The first segments reached Chile in early 2024; Safran Reosc now turns out about one segment per day. Most of the remainder are already packed, shipping or nearly finished — ESO expects all M1 segments in Chile within coming months.

What comes next

From Cerro Armazones near Paranal, the ELT is meant to study rocky exoplanet atmospheres, the first galaxies after the Big Bang, and whether the constants of physics shift over cosmic time. ESO says the overall project is more than 75% complete, with first science observations planned for late 2030.