The European Southern Observatory (ESO) awarded Safran Reosc with the contracts for the manufacturing of the main optics for the Extremely Large Telescope (ELT) : in 2015 for the twelve ELT M4 Zerodur shells, in 2016 for the ELT M2 4.25-m diameter Zerodur convex mirror, in 2017 for the ELT M3 4-m diameter Zerodur concave mirror, and in 2019 for the ELT M5 2.7-m diameter Silicon Carbide flat mirror. We present for each of these optics their key requirements and challenges. The ELT M4 shells are thin glass sheets in Zerodur of 1.95 mm thickness with strong requirements on thickness uniformity lower than 15 μm and on surface deformation lower than 14 nm RMS. The ELT M2 and ELT M3 mirrors are large Zerodur mirrors of class 4-meter diameter with strong requirements on bonding and surface deformation, requiring dedicated WFE test benches. We present the main parameters to achieve and make an overview of their test benches developed in Safran Reosc facility. The ELT M5 mirror is a large piece of Silicon Carbide of dimension 2.7m x 2.2m that presents high challenges for its substrate manufacturing and surface deformation. We detail its main requirements and the test bench specially developed for this mirror. Finally, we summarize for each of these optics the main steps and results achieved, and the current progress status.
In the past 20 years, a new generation of telescopes with large primary mirrors has appeared, based on the segmented mirrors design. The Thirty meter Telescope and the European Southern Observatory (ESO) Extremely Large telescope (ELT) are now under construction. They have, respectively, a 30m aperture and a 39m aperture. To cope with these apertures, they both have a large monolithic secondary mirror. These two convex aspherics are the TMT secondary mirror, a 3.1 m convex hyperboloid, and the ELT secondary mirror, a 4.25 m diameter aspheric with an asphericity of 2 mm. The surface measurement of these mirrors is challenging and auxiliary optics, like Test Plates (TP) and Computer Generated Holograms , are needed. In this paper, we present the Interferometry Test Bench, designed at Safran Reosc to polish and validate the ELT secondary mirror (ELT M2). The test bench is composed of three main components: a temporal phase shift interferometer, a Test Plate and the ELT M2 mirror on its polishing support. We perform interferometry between the concave face of the test plate and the mirror surface, forming an analogue to a Fizeau interferometer. The interferometer, named IRIDE, was designed and manufactured by Safran Reosc. It is a temporal phase shift interferometer, where environmental vibrations are addressed by increasing the interferograms acquisition rate. The interferometer has a low coherence light source, to reduce parasitic fringes. The Test Plate is a ZERODUR® meniscus lens fabricated at Safran Reosc. . Its concave surface is an off-axis isophase portion of the ELT M2 mirror and it is the Fizeau cavity’s reference surface. The ELT M2 mirror blank is mounted on its support for metrology and polishing (SMP). The SMP supports the mirror blank and interfaces it with the turning table. A measurement of the entire ELT M2 clear aperture is composed of 12 sub pupils measurements.
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