The final segment of the world’s largest telescope mirror has been successfully manufactured

The European Southern Observatory’s (ESO) Extremely Large Telescope (ELT), which is being built in the Chilean Atacama Desert, is one step closer to completion. The German company SCHOTT has given the go-ahead to manufacture the last of the 949 segments ordered for the telescope’s primary mirror (M1). With a diameter of over 39 metres, M1 will be the largest mirror ever built for the telescope.

Because it is too large to be a single piece of glass, M1’s primary mirror will be made up of 798 hexagonal segments, each about five centimetres thick and 1.5 metres wide, which together will collect tens of millions of times more light than the human eye. An additional 133 segments have been manufactured to facilitate servicing and coating of the segments once the telescope is operational. ESO has also purchased 18 spare segments, bringing the total to 949.

The M1 blanks—the pieces of material that are shaped and then polished into mirror segments—are made from ZERODUR (registered trademark), a low-expansion glass-ceramic material developed by SCHOTT and optimized for the extreme temperature ranges encountered at the ELT’s installation site, in the Chilean Atacama Desert. The company also manufactured the blanks for the other three ELT mirrors (M2, M3, and M4) at its facility in Mainz, Germany.

“What ESO has ordered from SCHOTT is more than just ZERODUR,” says Mark Carell, Head of ELT Optomechanics at ESO. “In close cooperation with ESO, SCHOTT has fine-tuned every step of the production process, adapting the product to meet and often exceed ELT’s stringent requirements. The excellent quality of the blanks is maintained throughout the entire series production of more than 230 tons of this high-performance material. ESO is therefore very grateful for the professionalism of the qualified team at SCHOTT, our trusted partner.”

Thomas Werner, ELT Project Manager at SCHOTT, says: “Our entire team is delighted to have secured the largest order for ZERODUR in our company’s history. For this project, we successfully completed the series production of hundreds of ZERODUR mirror substrates, whereas we usually run a single operation. It was an honour for all of us to play a role in shaping the future of astronomy.”

CRT mirror segment block number 949, manufactured and ready for molding. (Photo: SCHOTT)

Once manufactured, all segments undergo a multi-stage international journey. After slow cooling and heat treatment, the surface of each blank is formed by ultra-precision grinding at SCHOTT. The blanks are then transported to the French company Safran Reosc, where each of them is cut into a hexagonal shape and polished with an accuracy of up to 10 nanometers over the entire optical surface, i.e. the surface irregularities of the mirror will be less than one thousandth of the thickness of a human hair. The following also took part in the work on the M1 segment units: the Dutch company VDL ETG Projects BV, which produces the segment supports; the Franco-German consortium FAMES, which has developed and is completing the production of precision sensors measuring 4,500 nanometers, which track the relative position of each segment; the German company Physik Instrumente, which has developed and is producing 2,500 actuators capable of positioning the segment with nanometric accuracy; and the Danish company DSV, which is responsible for transporting the segments to Chile.

Once polished and assembled, each M1 segment is sent across the ocean to reach the ELT facilities at ESO’s Paranal Observatory in the Atacama Desert. It’s a 10,000-kilometre journey, during which more than 70 M1 segments have already been built. In Paranal, a few kilometres from the ELT construction site, each segment is coated with a layer of silver to make it reflective, after which it will be carefully stored until the main telescope structure is ready to receive them.

When it becomes operational later this decade, ESO’s ELT will be the world’s largest eye on the sky, tackling some of the greatest astronomical challenges of our time and making discoveries that have never been imagined before. (Credit: ESO. CC BY 4.0)