The ELT (Extremely Large Telescope) observatory on the Cerro Armazones mountain in Chile

Extremely Large Telescope

SCHOTT is proud to play a leading role in the development of the European Southern Observatory’s Extremely Large Telescope (ELT). Made with SCHOTT ZERODUR® glass-ceramic, the telescope’s segmented primary mirror is 39 meters in diameter and is widely expected to break new ground in space exploration.


In 2006, the European Southern Observatory (ESO) started the ELT program with detailed studies for the telescope. Conceived as a ‘window into distant galaxies’, the gigantic telescope depends on a series of five mirrors producing exceptionally accurate high-resolution images from millions of light-years out in space. The ESO looked to SCHOTT’s unrivalled expertise in glass-ceramics, and in 2017 commissioned the company to begin manufacturing of the mirror substrates required to achieve its unprecedented telescopic range.

The ELT (Extremely Large Telescope) observatory of the European Southern Observatory (ESO)
ELT of the European Southern Observatory (ESO) with SCHOTT ZERODUR® glass-ceramic as the mirror substrate. Source: ESO


The ELT project is one of the largest SCHOTT ZERODUR® has ever been involved with, producing the substrate for a total of four glass-ceramic mirrors. Two have a diameter of approx. 4 m, with the secondary mirror (M2) as the largest convex monolithic glass components. The production of the secondary mirror, with a diameter of 4.25 m, is a huge challenge, requiring the grinding of raw material down to just 10 cm or 3.94 inches in thickness. For the primary mirror, SCHOTT has the task of manufacturing 949 mirror segment blanks 1.5 m in diameter at the rate of one per day, each with exactly the same dimensions and properties.


SCHOTT is ideally prepared for this project with its state-of-the-art equipment: A five-axis CNC competence center enables the manufacturing of ZERODUR® glass-ceramic mirrors up to a size of 4.25 meters in diameter with a precision never achieved before . A second melting tank, installed for ZERODUR®, intends to meet the long-term high demand for the material and to contribute to customers’ success.

The optical system of the ELT and the role of ZERODUR®

This diagram (artist's rendering) shows the advanced five-mirror optical system of ESO's Extremely Large Telescope (ELT) and the location of the ZERODUR® mirror substrates. It is based on a giant main mirror that combines 798 hexagons (M1) and four smaller mirrors (M2 to M5) as reflectors. The main mirror collects light from the night sky and reflects it onto the M2, at four meters in diameter. This in turn reflects the light to a reflector (M3). The task of this third mirror is to irradiate the light onto the adaptive optics mirror (M4), which changes its surface a thousand times a second to correct the starlight distorted by the earth atmosphere in the telescope build-in adaptive optic systems.  Finally, the light reaches the fifth mirror (M5), which stabilizes the extremely sharp images at the final focal plane compensating lower frequency vibrations of the telescope structure. 
Illustration showing the five different mirror stages of the European Southern Observatory’s Extremely Large Telescope (ELT)
Construction of the primary mirror for the ELT (Extremely Large Telescope) observatory

A gigantic 39-meter primary mirror

The main mirror will have 798 hexagonal ZERODUR® segments in total – SCHOTT produces 949 segment blanks, each 1.5 m in diameter and 50 mm in thickness. They will work together as a single mirror able to collect tens of millions times more light than the human eye.

Construction of the secondary mirror for the ELT (Extremely Large Telescope) observatory

A technological masterpiece

The four-meter class secondary mirror (M2) for the ELT is the largest convex mirror ever built. It exhibits an extreme curvature, a diameter of 4.25 meters with a thickness of only 100 mm.

Construction of the tertiary mirror for the ELT (Extremely Large Telescope) observatory

Sweat-inducing work

After casting, the tertiary mirror substrate (M3) was ground down to its final weight of 3.2 tons by performing several machining steps. The final mirror measures four meters in diameter, is 100 mm in thickness, and is located in the telescope optics below the huge primary mirror.

Consistent image quality in extreme environments

At the heart of the ELT are the outstanding thermal properties of SCHOTT ZERODUR® glass-ceramic. With an extremely low coefficient of thermal expansion, it can be relied upon to perform consistently and reflect light in a highly precise manner in temperature varying environments. SCHOTT’s ability to manufacture it in a wide range of shapes and sizes has also been crucial to the ELT project.
Everything you need to know about SCHOTT and the ELT in six minutes
Markus Kissler-Patig, Senior Astronomer at the European Southern Observatory
A project like the ELT would not be possible if a material such as ZERODUR® did not exist.

Visualizing the vastness of the universe

The power and scale of the Extremely Large Telescope is simply astonishing. With almost crucial elements of the project being done for the first time, it aims to gather 100 million times more light than the human eye, with images 16 times sharper than those achieved by the Hubble telescope. It will also enable more detailed studies of planets surrounding other stars, along with distant galaxies and supermassive black holes, expanding our knowledge and understanding of the universe.
Roberto Tamai, ELT Program Manager, European Southern Observatory
Roberto Tamai, ELT Program Manager
Nothing like this has been done before. The ELT is the first of its kind. It’s a big machine with very stringent requirements in terms of positioning and tracking. We’re putting a giant 3,000-tonne piece of steel in a set position and need to move it with extreme precision. Yes, we have all the simulations and computational analyses to show that we can achieve it, but there’s a big step between simulations and reality. Taking this leap is one of the biggest challenges.

Used materials & similar products 

This unique glass-ceramic was developed by SCHOTT to achieve an extremely low CTE (coefficient of thermal expansion), making it ideal for demanding environments that require highly precise positioning. Its potential to be manufactured in large formats is also vital to the ELT project.

An unprecedented challenge that demands true teamwork

The planning and building of the ELT is a massive undertaking and one that requires close cooperation between hundreds of technical experts based all over the world. Simply travelling to the ELT site in the middle of Chile’s Atacama Desert is a logistical challenge, so processing and transporting the massive mirrors and components has pushed SCHOTT and its partners to new levels of dedication and innovation.

Glass made by

Polishing by
Safran-Reosc, France

Segment support system units for the primary mirror
CESA (Spain) VDL (Netherlands)

Support cells for M2 and M3 mirrors
SENER group (Spain)

Quaternary mirror
AdOptica, INAF (Italy)

Dome construction
ACe consortium (Italy)

ELT project owner
ESO, the European Southern Observatory

Want to know more? Let’s talk

Whether you need more information, samples, a quote, or advice for a project, we would be delighted to talk to you.

Get in touch
Man in glasses in business office on phone while working on laptop_605x350.jpg