ELT secondary mirror manufacturing progress at Safran Reosc

Safran Reosc has been awarded by ESO the manufacturing of the Secondary Mirror of the Extremely Large Telescope in 2016 [4]. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. We present the main ELT M2 manufacturing steps, namely adhesive qualification, pads bonding on the back and lateral sides, convex surface grinding and polishing. The necessary metrology instruments using interferometry and coordinate measurements are presented. The requirements driving the project with extraordinary means are presented, namely the schedule, the mirror volume, the convex surface figuring errors, the mirror lifetime of 30 years and the pad positioning accuracy (down to 0,125mm). We eventually present how these challenges are overcome: how the bonding lifetime was qualified, how Safran Reosc facility was re-configured to accommodate the polishing and control of 4-meter diameter mirrors. The polishing method (to accelerate the process and meet the requirements) are presented and discussed, including the presentation of the innovative double-head robot polishing station. We present the metrology devices adapted for the 4m mirrors and implemented in our factory. A focus on the 20-meter long interferometric bench designed and built by Safran Reosc is presented, to allow measuring the M2 convex surface and overcoming critical issues like birefringence and vibrations. This bench includes a convex-concave test plate together with a unique custom interferometer system, which are both presented.