ESO Signs Contract to Polish the E-ELT Secondary Mirror
French company Reosc will polish the largest secondary mirror ever built
6 July 2016
ESO has signed a contract with the French company Reosc , a subsidiary of Safran Electronics & Defense, to polish the secondary mirror of ESO’s forthcoming European Extremely Large Telescope (E-ELT) from a pre-formed glass-ceramic blank. Reosc will design the mirror and its mounting interfaces, polish the surface, and complete all necessary optical tests before delivery, which is expected in 2022.
The contract was signed by ESO Director General Tim de Zeeuw and M. Thierry Batut, President of Reosc, in the presence of M. Jean-Claude Brunet, the French Consul General in Munich, other senior representatives of Reosc and key ESO staff members.
The secondary mirror is, as the name suggests, the second of five mirrors that guide light from distant objects through the telescope, eventually to emerge at the telescope’s Nasmyth focus where a suite of scientific instruments will be sited. Hanging upside-down at the top of the telescope structure, high above the 39-metre primary mirror, this will be the largest secondary mirror ever employed on a telescope and the largest convex mirror ever produced. At over 4 metres in diameter, it is as large as the primary mirror of many top-class professional telescopes. The mirror will be shaped and polished to a precision of 15 nanometres (15 millionths of a millimetre) over the optical surface . As a highly convex, aspherical mirror, its fabrication is a considerable challenge and the result will be a truly remarkable example of precision optical engineering — as with many elements of the E-ELT it will be a genuine first in this area of technology.
 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 telescope, 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.
 If the mirror was 700 km in diameter (the distance between ESO’s Headquarters in Garching, Germany, and Reosc’s plant in Saint-Pierre du Perray, in France) the precision would correspond to just 3 mm.
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