Image: The first hexagonal segments for the main mirror of ESO’s Extremely Large Telescope (ELT) are shown being successfully cast by the German company SCHOTT at their facility in Mainz. These segments will form parts of the ELT’s 39-metre main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024. Credit: SCHOTT/ESO
The first six hexagonal segments for the main mirror of ESO’s Extremely Large Telescope (ELT) have been successfully cast by the German company SCHOTT at their facility in Mainz. These segments will form parts of the ELT’s 39-metre main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024.
SulutPos.com, Garching bei München, Germany – The 39-metre-diameter primary mirror of ESO’s Extremely Large Telescope will be by far the largest ever made for an optical-infrared telescope. Such a giant is much too large to be made from a single piece of glass, so it will consist of 798 individual hexagonal segments, each measuring 1.4 metres across and about 5 centimetres thick. The segments will work together as a single huge mirror to collect tens of millions of times as much light as the human eye.
Marc Cayrel, head of ELT optomechanics at ESO, was present at the first castings: “It was a wonderful feeling to see the first segments being successfully cast. This is a major milestone for the ELT!”
As with the telescope’s secondary mirror blank, the ELT main mirror segments are made from the low-expansion ceramic material Zerodur©  from SCHOTT. ESO has awarded this German company with contracts to manufacture the blanks of the first four ELT mirrors (known as M1 to M4, with M1 being the primary mirror) (eso1704).
The first segment castings are important as they allow the engineers at SCHOTT to validate and optimise the manufacturing process and the associated tools and procedures.
The casting of the first six segments is a major milestone, but the road ahead is long — in total more than 900 segments will need to be cast and polished (798 for the main mirror itself, plus a spare set of 133). When fully up to speed, the production rate will be about one segment per day.
After casting, the mirror segment blanks will go through a slow cooling and heat treatment sequence and will then be ground to the right shape and polished to a precision of 15 nanometres across the entire optical surface. The shaping and polishing will be performed by the French company Safran Reosc, which will also be responsible for additional testing (eso1717).
ESOcast 145 Light: First ELT Main Mirror Segments Successfully Cast (4K UHD)
 Zerodur© was originally developed for astronomical telescopes in the late 1960s. It has an extremely low coefficient of thermal expansion, meaning that even in the case of large temperature fluctuations, the material does not expand. Chemically, Zerodur© is very resistant and can be polished to a high standard of finish. The actual reflective layer, made of aluminium or silver, is usually vaporised onto the extremely smooth surface shortly before a telescope is put into operation and at regular intervals afterwards. Many well-known telescopes with Zerodur© mirrors have been operating reliably for decades, including ESO’s Very Large Telescope in Chile.
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and by Australia as a strategic partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research.
ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.
ESO Organisation Release