ESO’s Extremely Large Telescope Is One Step Closer To Completion

4th Jul 2024
ESO’s Extremely Large Telescope Is One Step Closer To Completion

The last hexagonal segment of the primary mirror for the Extremely Large Telescope (ELT) is ready. With a diameter of more than 39 meters, it will be the largest mirror ever made for a telescope. It is part of the European Southern Observatory (ESO) and will be positioned on top of Cerro Armazones in the Atacama Desert of Chile, 3000 meters above sea level.

The World’s Largest Eye On The Sky

The Extremely Large Telescope is an astronomical observatory under construction. When completed, it will be the world’s largest telescope. The design consists of a reflecting telescope with a 39.3-meter-diameter segmented primary mirror and a 4.2-meter-diameter secondary mirror. The telescope will be supported by adaptive optics, six laser guide star units, and multiple large science instruments. 

Some of the tasks of the ELT are:

  • To enable detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes;
  • To explore the nature of the Universe’s dark sector;
  • To detect water and organic molecules in protoplanetary disks around other stars.
5-mirror optical system of ESO's Extremely Large Telescope (ELT)
The diagram shows the 5-mirror optical system of ESO’s Extremely Large Telescope (ELT). Before reaching the science instruments, the light is first reflected from the telescope’s 39-meter segmented primary mirror (M1); it then bounces off two further 4-meter-class mirrors, one convex (M2) and one concave (M3). The final two mirrors (M4 and M5) form a built-in adaptive optics system to allow extremely sharp images to be formed at the final focal plane. Credit: ESO

Making The Giant Mirror For Extremely Large Telescope

The primary mirror (M1) for ELT is one of the most impressive and challenging features of the entire project. Initially, the M1 will consist of 798 segments, each about five centimeters thick, measuring around 1.5 meters in diameter and weighing 250 kg. However, an additional 133 segments have been produced to facilitate maintenance and recoating. ESO has also purchased 18 spare segments, so the total number is 949. 

Since all the segments have to work together as a single mirror, they require specific construction, and all the details need to fit perfectly. The mirror’s surface irregularities will be 10,000 times thinner than a human hair.

The German company SCHOTT successfully produced the first six segment blanks at its facility in Mainz in January 2018. The last segment blank was delivered in June 2024.

The artist’s rendering of the ELT primary mirror
The artist’s rendering of the ELT primary mirror is based on the detailed construction design for the telescope. Credit: ESO

A Journey Of Mirror Segments

The process of making the segments is complicated. After a slow cooling and heat treatment, 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 a hexagon shape and polished. Once polished and assembled, each M1 segment is shipped to the Atacama Desert. In Paranal, not far 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.

German company SCHOTT
German company SCHOTT has successfully delivered the blank for the last of the 949 segments for the telescope’s primary mirror (M1). Credit: ESO

What Is It Made Of?

The mirror is being cast from a specific material, Zerodur—a lithium-aluminosilicate glass ceramic produced by SCHOTT. Zerodur has been used for several very large telescope mirrors and some smaller telescopes. It is suitable for producing mirrors that maintain acceptable figures in extremely cold environments, such as deep space. 

ESO’s Extremely Large Telescope is expected to be ready in 2028. It is a great step forward for cosmos exploration. Like a giant eye in the sky, the telescope will observe space and help our scientists make new astrophysical discoveries.

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