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Last segment blank of the world’s largest telescope mirror successfully delivered


The European Southern Observatory’s Extremely Large Telescope (ESO’s ELT), under construction in the Chilean Atacama Desert, is one step closer to completion. German company SCHOTT has successfully delivered the blank for the last of the 949 segments commissioned for the telescope’s primary mirror (M1). With a diameter of more than 39 metres, M1 will be by far the largest mirror ever made for a telescope.

Too large to be made from a single piece of glass, M1 will consist of 798 hexagonal segments, each about five centimetres thick and 1.5 metres across, working together to collect tens of millions of times as much light as the human eye. An additional 133 segments have been produced to facilitate the maintenance and recoating of the segments once the telescope is operational. ESO has also procured 18 spare segments, bringing the total number to 949. 

The M1 blanks, shaped pieces of material that are later polished to become the mirror segments, are made from ZERODUR®, a low-expansion glass-ceramic material developed by SCHOTT and optimised for the extreme temperature ranges at the ELT’s site in the Atacama Desert. This company has also manufactured the blanks of three other ELT mirrors — M2, M3, and M4 — at their facilities in Mainz, Germany. 

What ESO ordered from SCHOTT is more than just ZERODUR®,” says Marc Cayrel, Head of ELT Optomechanics at ESO. “In close collaboration with ESO, SCHOTT fine-tuned every single production step, tailoring the product to meet and often exceed the ELT’s very demanding requirements. The outstanding quality of the blanks was maintained throughout the mass production of more than 230 tonnes of this super-performing material. ESO is thus very thankful for the professionalism of the skilled teams at SCHOTT, our trusted partner.”

Thomas Werner, ELT Project Lead at SCHOTT, says: “Our entire team is thrilled to conclude what has been the largest single order of ZERODUR® in the history of our company. For this project, we successfully concluded the serial production of hundreds of ZERODUR® mirror substrates, when we usually have a single-piece operation. It’s been an honour for all of us to play a part in shaping the future of astronomy.”

Once cast, all segments follow a multi-step, international journey. After a slow cooling and heat treatment sequence, the surface of each blank is shaped by ultra-precision grinding at SCHOTT. The blanks are then transported to French company Safran Reosc, where each of them is cut into an hexagon shape and polished to a precision of 10 nanometres across the entire optical surface — meaning the surface irregularities of the mirror will be less than one thousandth of the width of a human hair. Also involved in the work done on the M1 segment assemblies are: Dutch company VDL ETG Projects BV, which is producing the segment supports; the German-French FAMES consortium, which has developed and is finalising manufacturing for the 4500 nanometric-accuracy sensors monitoring the relative position of each segment; German company Physik Instrumente, which designed and is manufacturing the 2500 actuators able to position the segment to nanometric precision; and Danish company DSV, which is in charge of transporting the segments to Chile.

Once polished and assembled, each M1 segment is shipped across the ocean to reach the ELT Technical Facility at ESO’s Paranal Observatory in the Atacama Desert — a 10 000-kilometre journey that over 70 M1 segments have already completed. In Paranal, only a few kilometres away from the construction site of the ELT, each segment is coated with a silver layer to become reflective, after which it will be carefully stored until the telescope’s main structure is ready to receive them.

When it starts operating later this decade, ESO’s ELT will be the world’s largest eye on the sky. It will tackle the biggest astronomical challenges of our time and make as-yet unimaginable discoveries.

More information

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society. 

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