This artist’s rendering shows a night-time 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 world’s largest telescope mirror successfully cast
The Extremely Large Telescope in Chile, due for completion this decade, will be the world’s largest telescope, with a 39-metre main mirror and made of 798 precision-engineered segments. It represents a significant international effort in astronomy.
The European Southern Observatory’s Extremely Large Telescope (ELT) is currently under construction in Chile’s Atacama Desert.ESO‘s ELT) is one step closer to completion. German company SCHOTT has successfully delivered the blank for the last of the 949 segments that have been commissioned for the telescope’s primary mirror (M1). With a diameter of over 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 is composed of 798 hexagonal segments, each about five centimetres thick and 1.5 metres wide, which work together to collect tens of millions of times more light than the human eye. An additional 133 segments have been manufactured to allow for easier maintenance and recoating of the segments once the telescope is operational. ESO has also purchased 18 spare segments, bringing the total 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. At over 39 metres in diameter, M1 is too large to be made from a single piece of glass and will instead be made up of 798 hexagonal segments, each about five centimetres thick and 1.5 metres wide, which work together to collect tens of millions of times as much light as the human eye. An additional 133 segments have been produced to allow for easier maintenance and recoating of the segments once the telescope is operational. ESO has also purchased 18 spare segments, bringing the total to 949. Now, the German company SCHOTT has successfully cast the blank for the last of the 949 segments, seen in this image. The M1 blanks, shaped pieces of material that will later be polished to become mirror segments, are made of ZERODUR©, a low-expansion glass-ceramic material developed by SCHOTT and optimized for the extreme temperature ranges at the ELT site in the Atacama Desert. The 949th segment is seen in this image before it is 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, formed pieces of material that are later polished to become mirror segments, are made of ZERODUR®, a low-expansion glass-ceramic material developed by SCHOTT and optimized for the extreme temperature ranges at the ELT site in the Atacama Desert. The company also manufactured the blanks for three other ELT mirrors — M2, M3 and M4 — at its 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 refined each individual production step and adapted the product to meet and often exceed the very demanding requirements of the ELT. The excellent quality of the blanks was maintained during the mass production of more than 230 tons of this super-performance material. ESO is therefore very grateful for the professionalism of the skilled teams at SCHOTT, our trusted partner.”
Computer rendering of the main mirror (M1) of the ESO Extremely Large Telescope. Credit: ESO
Thomas Werner, ELT Project Lead at SCHOTT, says: “Our entire team is excited to complete the largest single order of ZERODUR® in our company’s history. For this project, we successfully completed series production of hundreds of ZERODUR® mirror substrates, while normally we would have a one-off operation. It is an honor for all of us to play a role in shaping the future of astronomy.”
International logistics and assembly
Once cast (see video below), each segment follows a multi-step, international journey. After slow cooling and heat treatment, the surface of each blank is shaped by ultra-precise grinding at SCHOTT. The blanks are then transported to French company Safran Reosc, where each one is cut into a hexagonal shape and polished to a precision of 10 nanometers across the entire optical surface — meaning the mirror’s surface irregularities will be less than one-thousandth the width of a human hair.
Also involved in the work on the M1 segment assemblies are: the Dutch company VDL ETG Projects BV, which produces the segment supports; the German-French FAMES consortium, which produces the 4500 nanometeraccuracy sensors that monitor the relative position of each segment; the German company Physik Instrumente, which designed and manufactures the 2,500 actuators that can position the segment with nanometer precision; and the Danish company DSV, which is responsible for transporting the segments to Chile.
Once polished and assembled, each M1 segment will be 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 has already been completed by more than 70 M1 segments. At Paranal, just a few kilometres from the ELT construction site, each segment will be coated with a layer of silver to make it reflective, and then carefully stored until the main telescope structure is ready to receive them.
Final stages and future impact
When it becomes operational later this decade, ESO’s ELT will be the world’s biggest eye on the sky, tackling the greatest astronomical challenges of our time and making discoveries hitherto unimagined.