The interferometric test bench for the ELT M2 polishing and validation at Safran Reosc

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.