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The Very Large Telescope: The largest telescope mirror in the world will bring the stars closer to Earth

The main mirror of the European Southern Observatory's (ESO) Extremely Large Telescope (ELT), known as M1, will be the largest mirror ever built for a telescope. More than 39 meters in diameter, and will consist of 798 hexagonal sections, each about five centimeters thick and 1.5 meters wide. The last of which was recently installed

Subjects: Astronomy, Southern European Observatory

Imaging the Very Large Telescope (ELT) in action. This artist's rendering shows the Extra Large Telescope at night, in action atop Cerro Armazones in northern Chile. The telescope uses lasers to create artificial stars high in the atmosphere. Credit: ESO/L. Calçada
Imaging the Very Large Telescope (ELT) in action. This artist's rendering shows the Extra Large Telescope at night, in action atop Cerro Armazones in northern Chile. The telescope uses lasers to create artificial stars high in the atmosphere. Credit: ESO/L. Calçada

The assembly of the last segment of the largest telescope mirror in the world has been successfully completed.

The Very Large Telescope, whose construction is scheduled to be completed this decade in Chile, will be the largest telescope in the world, with a 39-meter-diameter main mirror made up of 798 segments engineered with high precision. This is a significant international endeavor in the field of astronomy.

In Chile's Atacama Desert, the European Southern Observatory's Extremely Large Telescope (ESO's ELT) is taking another step toward completion. The German company SCHOTT has successfully supplied the raw material for the last of the 949 segments ordered for the telescope's main mirror (M1). At more than 39 meters in diameter, M1 will be the largest mirror ever built for a telescope.

Innovations in the design of the telescope mirror

The 949 segment is seen in this photo before being cut into a hexagonal shape and polished - steps that will be carried out by the French company Safran Reosc. Credit: SCHOTT
The 949 segment is seen in this photo before being cut into a hexagonal shape and polished - steps that will be carried out by the French company Safran Reosc. Credit: SCHOTT

Because M1 is too large to be built from a single piece of glass, it will consist of 798 hexagonal segments, each about five centimeters thick and 1.5 meters wide, which will work together to collect an amount of light tens of millions of times larger than the human eye. In addition, 133 additional segments were fabricated to facilitate maintenance and recoating of the segments when the telescope becomes operational. ESO also acquired 18 additional segments, bringing the total to 949 segments.

The M1's raw material, shaped pieces of material that are then polished to become mirror segments, is made of ZERODUR, a low-expansion glass-ceramic material developed by SCHOTT and adapted to the extreme temperature ranges of the ELT site in the Atacama Desert. This company also produced the raw materials for three additional ELT mirrors – M2, M3, and M4 – at its factories in Mainz, Germany.

Collaboration and precision engineering

"What ESO has ordered from SCHOTT is more than ZERODUR," says Mark Kierl, Head of ELT Optomechanics at ESO. "In close cooperation with ESO, SCHOTT planned every stage of production, adapting the product to meet and sometimes even exceed the highest requirements of ELT. The excellent quality of the raw materials has been maintained throughout the mass production of more than 230 tons of this super-performance material. ESO is very grateful for the professionalism of the skilled teams at SCHOTT, our trusted partners.”

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

Thomas Werner, ELT Project Manager at SCHOTT, says: “Our entire team is excited to finalize what was the largest ZERODUR order in our company's history. In this project we successfully completed the serial production of hundreds of raw materials for ZERODUR mirrors, when normally we perform a one-time process. It was an honor for all of us to be a part of shaping the future of astronomy.”

International logistics and assembly

At the end of production (see video below), all segments undergo a multi-stage international journey. After a slow cooling process and heat treatment, the surface of each raw material is shaped by ultra-precise grinding at SCHOTT. The raw materials are then delivered to France's Safran Reosc, where each 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 a thousandth the width of a human hair.

Other companies are also involved in the work on the segments of M1: the Dutch company VDL ETG Projects BV, which produces the supports for the segments; The German-French consortium FAMES, which developed and is finalizing production of 4500 nanometer precision sensors that monitor the relative position of each segment; the German company Physik Instrumente, which designed and manufactures 2500 motors that can position the segment with nanometer precision; and the Danish company DSV, which is responsible for transporting the segments to Chile.

After polishing and assembly, each M1 segment is transported across the ocean to reach the ELT technical facility at ESO's Paranal Observatory in the Atacama Desert – a journey of 10,000 kilometers already completed by more than 70 M1 segments. At Paranal, just a few kilometers from the ELT construction site, each segment is coated with a layer of silver to become reflective, then carefully stored until the main telescope structure is ready and they can be installed.

Final stages and future impact

When it becomes operational later this decade, ESO's ELT will be the world's largest eye on the sky. He will tackle the greatest questions of astronomy of our time and make discoveries hitherto unimaginable.   

to the ELT telescope site      

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