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Precise processing of telescope mirrors for the world’s biggest eye on the sky

The European Southern Observatory (ESO) is constructing the world's largest ground-based telescope, the Extremely Large Telescope (ELT), at Cerro Armazones (3046 m) in Chile. This giant telescope shall help to broaden astrophysical knowledge and provide new insights, e.g. about black holes and the history of the universe. The ELT has a novel five-mirror-design, with a primary mirror of 39 m diameter. This main mirror consists of 798 hexagonal segments, each is 1.4 m wide, 5 cm thick and weighs 245 kg. With its huge size the ELT gathers more light than all the existing large research telescopes on the earth together. It will also provide 15 times sharper images than the NASA/ESA Hubble Space Telescope.

For an optimum view into space each of the 798 primary mirror segments requires an individual surface profile with nanometer precision. Safran Reosc, a subsidiary of Safran Electronics & Defense, has been awarded by ESO for the production of all five mirrors for the ELT. Safran Electronics & Defense even built its own new 5000 m2 production site in Poitiers, France, to handle the complex production of the main mirror segments.

scia Systems is pleased to supply two scia Finish 1500 to Safran Reosc. The systems are used for a final polishing error correction of the primary mirror elements. By applying ion beam polishing (IBF) the scia Finish 1500 enables to remove material with a thickness of about 600 nm. An individual layer thickness profile can be realized on each hexagonal segment. Such a precision is needed to avoid imaging defects and to optimize the brightness of the images.

Safran Reosc, a subsidiary of Safran Electronics & Defense, is a world leader in the design, production and integration of high-performance optics for astronomy, space, high-energy lasers and the semiconductor industry. Since its foundation 1937 the company has developed unrivaled expertise in optics for ground-based astronomy and offers a complete range of high-performance optics and high-precision opto-mechanical equipment. More information:

European Southern Observatory (ESO) is an intergovernmental science and technology organization. Its program includes the design, construction and operation of powerful ground-based astronomical telescopes, in order to enable important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. More information: