The interferometric test bench for ELT M2: the largest convex precision mirror ever made

Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO in 2016. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. As of spring 2024, this mirror is in its final phase of polishing. It is regularly controlled on a dedicated interferometric test bench (ITB) specifically designed to achieve the best accuracy on such a large mirror. The test bench was thoroughly validated in September 2023 and all its requirements were demonstrated. This bench includes a convex-concave test plate together with a unique custom interferometer system, both specifically designed and manufactured for this test bench. In fact, the specific requirements of this very large mirror forbid the use of off-the-shelf component or interferometer and it was necessary to design a specific interferometer to cope with all the requirements. Furthermore, it was necessary to manufacture an almost perfect 2-m reference plate to control the mirror. It took about 3 years to manufacture this reference plate with the requested accuracy. <br/> <br clear="all"/> The test bench makes a measurement on 24 sub-pupils which are stitched together to reconstruct the final surface error. The full aperture measurement and the map stitching lasts around 2 hours. Specific sensors are used to monitor the stitching process and the stitching algorithm had to be adapted in order to fulfil the stringent performance requirements. <br/> <br clear="all"/> In this paper, we present the main features of the bench, the alignment strategy, the test results obtained on the bench and the stitching algorithm developed to reconstruct the surface error.