Extremely Large Telescope: World’s Largest Telescope Mirror Will Bring the Stars Closer to Earth

This artist’s rendering shows a night view of the Extremely Large Telescope in operation on Cerro Armazones in northern Chile. The telescope is shown using lasers to create artificial stars high in the atmosphere. Credit: ESO/L. Calçada

Last Segment of the World’s Largest Telescope Mirror Successfully Cast

Set for completion this decade, the Extremely Large Telescope in Chile will be the largest telescope globally, with a main mirror spanning 39 meters and made from 798 precision-engineered segments. It represents a significant international effort in astronomy.

Currently under construction in the Chilean Atacama Desert, the European Southern Observatory’s Extremely Large Telescope (ESO’s ELT) 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 meters, M1 will be by far the largest mirror ever made for a telescope.

Innovations in Telescope Mirror Design

Too large to be made from a single piece of glass, M1 will consist of 798 hexagonal segments, each about five centimeters thick and 1.5 meters 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 primary mirror of ESO’s Extremely Large Telescope (ELT), known as M1, will be by far the largest mirror ever made for a telescope. With a diameter of more than 39 meters, M1 is too large to be made from a single piece of glass and will instead consist of 798 hexagonal segments, each about five centimeters thick and 1.5 meters 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. Now, German company SCHOTT has successfully cast the blank for the last of the 949 segments, seen in this photo. 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 optimized for the extreme temperature ranges at the ELT’s site in the Atacama Desert. The 949th segment is seen in this image before being cut into its hexagonal shape and polished — steps that will be performed by French company Safran Reosc. Credit: SCHOTT

Advanced Materials and Production Techniques

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 optimized 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.

Collaboration and Precision Engineering

“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.”

Computer rendering showing the main mirror (M1) of the ESO Extremely Large Telescope. Credit: ESO

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 honor for all of us to play a part in shaping the future of astronomy.”

International Logistics and Assembly

Once cast (see video below), 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 nanometers 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-kilometer journey that over 70 M1 segments have already completed. In Paranal, only a few kilometers 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.

Final Stages and Future Impact

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.