German company Schott started casting the first hexagonal segments of the main mirror of ESO's Extremely Large Telescope (ELT) early this year. The main mirror will eventually be 39 metres in diameter, making it the largest ever made for an optical/infrared telescope.
The casting of the first segments marks an important milestone in the creation of the ELT. The segments will form part of the ELT's 39-metre main mirror, which will eventually consist of 798 segments. When it catches its first light in 2024, the ELT will be the largest optical telescope in the world.
With a 39-metre diameter, the main mirror of ESO's Extremely Large Telescope will be by far the largest ever made for an optical/infrared telescope. Such a behemoth is far too large to be made from a single piece of glass. Therefore, it will consist of 798 separate hexagonal segments, each 1.4 metres wide and about five centimetres thick. These segments will work together as one enormous mirror capable of collecting tens of millions of times more light than the human eye.
Ceramic glass
To make the segments, liquid glass heated to more than 1,400 degrees is poured directly into moulds and transferred to a cooling furnace and subjected to a ceramisation process that takes several weeks.
The result is ZERODUR® ceramic glass, a material with near-zero thermal expansion, making it particularly suitable for astronomy applications. ESO has awarded this German company the contracts to manufacture the first four ELT mirrors (known as M1 to M4, M1 being the main mirror), and the SCHOTT production facility in Mainz is expected to complete delivery of the segments by 2024.
Marc Cayrel, head of ELT optomechanics at ESO, attended the first castings: 'It was a great feeling to see the first segments successfully cast. This is an important milestone for the ELT!'
Long road
The first segments now cast are significant because they allow SCHOTT's engineers to validate and optimise the production process and associated tools and procedures.
Casting the first six segments is a major milestone, but there is still a long way to go. In total, more than 900 segments need to be cast and polished (798 for the main mirror itself, plus a spare set of 133). Once the factory is running at full capacity, about one segment per day will be produced.
After casting, the mirror segments will undergo a slow cooling and heat treatment procedure, be ground into the correct shape and then polished to 15 nanometres accuracy. The grinding and polishing will be done by French company Safran Reosc, which will also be responsible for additional testing.
This video shows part of the extraordinary process.
More about Zerodur®
Zerodur® was developed in the late 1960s for use in astronomical telescopes. It exhibits almost no thermal expansion even when exposed to large temperature fluctuations. In addition, the material is very chemically resistant, allowing it to be polished very precisely. The actual reflection layer, consisting of aluminium or silver, is dammed onto the extremely smooth mirror surface shortly before a telescope is put into operation - a process that has to be repeated at regular intervals afterwards. Many renowned telescopes with Zerodur® mirrors, such as ESO's Very Large Telescope in Chile, have been in operation for decades.
About ESO
ESO is the leading intergovernmental astronomical organisation in Europe and by far the most prolific observatory in the world. It is supported by 16 member states: Austria, Belgium, Brazil, Czech Republic, Denmark, Finland, France, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, as well as host country Chile. ESO runs an ambitious programme aimed at designing, building and managing large observatories that enable astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising astronomical collaboration. ESO manages three world-class observatories in Chile: La Silla, Paranal and Chajnantor. Paranal is home to ESO's Very Large Telescope (VLT), the world's most advanced optical observatory, and two survey telescopes. VISTA operates in the infrared and is the largest survey telescope in the world and the VLT Survey Telescope is the largest telescope specifically designed to map the sky in visible light. ESO also plays an important partner role in ALMA, the largest astronomical project today. And on Cerro Armazones, near Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will be "the world's largest eye on the sky".
By: Kelly Bakker
Source: Schott/ESO
