ESO’s Extremely Large Telescope (ELT), scheduled to see first light in 2024, is at the cutting edge of telescope technology. Its optical system will consist of no fewer than five separate mirrors, each of them a significant engineering challenge.
ESO has now awarded the contract to polish the third mirror in the light path, known as M3, to the French company Reosc , a subsidiary of Safran Electronics & Defense. They will receive the blank from SCHOTT, design the mirror and its mounting interfaces, polish the surface, and complete all necessary optical tests before delivery . Reosc were also awarded the contracts to design, polish and test the telescope’s secondary mirror in July 2016, and to manufacture the deformable shell mirrors that will comprise the ELT’s fourth mirror (M4).
M3 will be a giant 3.8-metre concave mirror — as big as the primary mirror of many of today’s world-class telescopes. It will be an unusual feature, as most current large telescopes such as ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope use only two curved mirrors, sometimes using a flat tertiary mirror to redirect light to a convenient focus. The curved surface of M3 will work together with the primary and secondary mirrors to deliver a better image quality over a large field of view.
The structural element of the mirror, before the reflective coating is applied, will be made of a sophisticated material called Zerodur™ from SCHOTT . It will then need to be shaped and polished to a precision of 15 nanometres.
 Reosc, a subsidiary of Sagem, a Safran high-technology company, is a world leader in the design, production and integration of high-performance optics, including for astronomy, space, high-energy lasers and the semiconductor industry. Reosc develops and produces high-performance optics for satellites, large telescopes and high-energy lasers, as well as the semiconductor industry. The company also built the single-piece 8-metre mirrors for ESO’s Very Large Telescope and the Gemini international telescopes, the 11-metre mirror for the Gran Telescopio de Canarias, mirrors for Europe’s Nirspec instrument on NASA’s James Webb Space Telescope, and mirrors for ESA’s GAIA astronomy satellite.
 The contract to cast the M2 mirror blank was awarded on 18 January 2017.
 Zerodur™ was originally developed for astronomical telescopes in the late 1960s. It has almost no thermal expansion even in the case of large temperature fluctuations, is highly chemically resistant, and can be polished to a high standard of finish. The actual reflective layer, made of aluminum or silver, is usually vapourised onto the extremely smooth surface shortly before the telescope is put into operation. Many well-known telescopes with Zerodur mirrors have been operating reliably for decades. They include, for example, ESO’s Very Large Telescope in Chile.