ESO’s biggest eye on the sky getting ready

First segments of the world’s largest telescope mirror shipped to Chile

The construction of the European Southern Observatory’s Extremely Large Telescope (ESO’s ELT) has reached an important milestone with the delivery to ESO and shipment to Chile of the first 18 segments of the telescope’s main mirror (M1). Once they arrive in Chile, the segments will be transported to the ELT Technical Facility, at ESO’s Paranal Observatory in the country’s Atacama Desert, where they will be coated in preparation for their future installation on the telescope main structure. Unable to be physically made in one piece, M1 will consist of 798 individual segments arranged in a large hexagonal pattern, with an additional 133 being produced to facilitate the recoating of segments.

Primary mirror, M1, segments of ESO’s Extremely Large Telescope (ELT) awaiting finishing at Safran Reosc facilities in France. 798 hexagonal segments will work together on the ELT, acting as one giant mirror 39-metres across. To meet the challenge of delivering such a large number of polished segments along with 133 spares within seven years, Safran Reosc has to build up to a peak production rate of one mirror a day.

With a diameter of more than 39 metres, it will be the largest telescope mirror in the world.

The final stage in the production process of M1 segments — polishing — was carried out by world-leading optical systems manufacturer Safran Reosc near Poitiers, central France, at a building completely refurbished to work on this delicate task. As part of the process, Safran Reosc developed new automation workflows and measurement techniques to ensure that the polishing met the high standards required for ESO’s ELT. The surface irregularities of the mirror are less than 10 nanometres (less than one thousandth of the width of a human hair). To reach this level of performance, Safran Reosc used a technique called ion-beam figuring, in which a beam of ions sweeps the mirror surface and removes irregularities atom by atom.

While only 18 segments have been shipped thus far, many more will soon be delivered by Safran Reosc to ESO. On 1 November 2023, the 100th segment went out of the production line and entered into the extensive inspection phase that takes place before final delivery. Furthermore, Safran Reosc has achieved a production rate in excess of four segments per week, with a target of five a week expected soon, a remarkable achievement for the series production of incredibly high-accuracy optics.

Once segments of the Extremely Large Telescope’s (ELT’s) primary mirror, M1, have been polished, they’re mounted on a support system just like this one. Final inspection of the support is happening here, at the Safran Reosc facilities in France, prior to delivery to Chile, where the telescope is under construction in the Atacama Desert. Together, each mirror segment on ESO’s soon-to-be biggest eye on the sky and their supports will weigh 250 kilograms.

The construction of ESO’s ELT has required the close involvement of multiple companies in Europe and Chile with ESO’s teams, highlighting how the telescope is a true international endeavour. The mirror segments were cast by the German company SCHOTT at their facility in Mainz, Germany, before being delivered to Safran Reosc in France for polishing. Other companies involved in the work done on the segment assemblies include: Dutch company VDL ETG Projects BV who produced the delicate segment supports, German-French FAMES consortium who developed and manufactured the 4500 nanometric-accuracy sensors monitoring the relative position of each segment, and German company Physik Instrumente who designed and manufactured the 2500 actuators able to position the segment to nanometric precision. The delicate task of transporting the segments was assigned to Danish company DSV.

At the Safran Reosc factory in Poitiers, France, segments of ESO’s Extremely Large Telescope main mirror, together with their support systems, were packaged for delivery in December 2023, starting their 10 000 km long journey to Chile. Once there, they will be coated with a thin layer of reflective silver. Each segment is close to 1.5 metres across and 5 centimetres thick, and their surfaces have been polished to an accuracy of tens of nanometres — 10 000 times thinner than a human hair. The ELT’s 39-m mirror will comprise 798 of these segments, plus 133 additional segments to facilitate recoating.

Having left France last week, the 18 polished mirror segments are now on their journey of over 10 000 km to the ELT’s construction site in the Atacama Desert. From there, ESO’s ELT will tackle the biggest astronomical challenges of our time and make yet unimaginable discoveries once it starts operating later this decade. (ESO)