More than half of the ELT’s giant mirror is now in Chile

Piece by piece, the largest telescope mirror ever built is making its way across the Atlantic. Nearly 500 segments of the main mirror of ESO’s Extremely Large Telescope (ELT) have now arrived at the ELT Technical Facility at ESO’s Paranal Observatory in Chile, around half of all the segments being made for what will become a 39-metre eye on the sky. Furthermore, the last segment has entered production at Safran Reosc in France. 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 (M1) consists of 798 individual segments, working together as one, to observe the cosmos in depth and detail.

The production of M1 hexagonal segments is a complex process that showcases the international collaboration that makes the ELT possible. The segments’ journey began in Germany, where SCHOTT created the segment blanks from Zerodur©, a glass-ceramic material that barely changes shape as temperatures shift. The blanks then travelled to Safran Reosc near Poitiers, France, to be figured, polished and mounted on their supports, which were designed by TNO and produced by VDL ETG Projects BV, two Dutch companies. The FAMES consortium (Fogale, France; MicroEpsilon, Germany) designed and manufactured the sensors that monitor the relative position of each segment and German company Physik Instrumente manufactured the actuators that maintain each segment perfectly aligned with its neighbours. The final leg of the journey of the M1 segments and their supports, which most assemblies have now completed, is a 10.000-kilometre trip by road and by ship to the Atacama Desert, a task assigned to Danish company DSV.

On arrival at the ELT Technical Facility the segments are inspected and coated by ESO engineers and technicians, giving them the highly reflective finish that will collect starlight. 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 the ESO 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. With the arrival of the latest batch, more than 50% of the 931 segments are now in Chile.

The very first segments arrived in Chile in early 2024, around seven years after the first industrial contracts for their production were signed. Now, however, Safran Reosc has reached a production rate of a segment per day. 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 M1 segments will be in Chile.

When it starts observing the Universe in a few years, from Cerro Armazones near Paranal, the ELT will be the world’s biggest eye on the sky. Its giant mirror, sophisticated systems and the dark skies it will operate under will enable it to study the atmospheres of rocky planets around other stars, observe the first galaxies and stars that formed after the Big Bang, and test whether the constants of physics have changed over cosmic time. The ELT project is now more than 75% complete, and the first observations with scientific instruments are planned for late 2030.

Courtesy of European Southern Observatory