Secondary Mirror of ELT Successfully Cast
Largest convex mirror blank ever created
22 May 2017
The casting of the secondary mirror blank for ESO’s Extremely Large Telescope (ELT) has been completed by SCHOTT at Mainz, Germany. The completed mirror will be 4.2 metres in diameter and weigh 3.5 tonnes. It will be the largest secondary mirror ever employed on an optical telescope and also the largest convex mirror ever produced.
ESO’s 39-metre Extremely Large Telescope (ELT) will be the largest telescope of its kind ever built when it achieves first light in 2024. A new milestone has now been reached with the casting of the telescope’s secondary mirror (M2), which is larger than the primary mirror of many of today’s research telescopes.
The mirror blank is the cast block of material — in this case Zerodur® glass-ceramic  — that will then be ground and polished to produce the finished mirror. In January 2017, ESO awarded SCHOTT the contract to manufacture the M2 mirror blank (eso1704). ESO has enjoyed a fruitful collaboration with SCHOTT, who also produced the 8.2-metre meniscus main mirrors for the Very Large Telescope at ESO’s Paranal Observatory (ann12015). A manufacturer of exceptional astronomical products to a very high standard, SCHOTT has already delivered the blanks of the deformable thin shell mirrors that will make up the ELT’s quaternary mirror, M4 (ann15055), and will also provide the blank of the tertiary M3 mirror.
The blank of the secondary mirror now has to go through a slow cool-down, machining and heat treatment sequence over the next year. It will then be ready to be ground to precisely the right shape and polished. The French company Safran Reosc will carry this out, along with additional testing (ann16045). The blank will be shaped and polished to a precision of 15 nanometres (15 millionths of a millimetre) across the entire optical surface.
When completed and installed, the M2 mirror will hang upside down above the telescope’s huge primary mirror and forms the second element of the ELT’s novel five-mirror optical system. The mirror is strongly curved and aspheric and is a major challenge to make and test.
 Originally developed for astronomical telescopes in the late 1960s, Zerodur® has almost no thermal expansion even when subjected to large temperature fluctuations, is highly chemically resistant, and can be polished to a high standard of finish. Many 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, Czechia, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. 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, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in 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”.
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