Surface error correction of ELT primary mirror hexagonal segments by ion figuring

Ion Beam Figuring is a key technology after polishing, hexagonal cutting and integration to obtain ELT-M1 hexagonal segments without edge effects. This process used at REOSC over 30 years offer an efficient solution for finishing these large freeform optical components. This paper presents a dedicated figuring process strategy to maximize throughput and process convergence as well as to limit temperature increase during processing. We analysed the results of optical surface improvement over 100 ELT-M1 segments. These results highlight the high accuracy of IBF technology to correct hexagonal surfaces all the way to the edge. The final surface figure error was obtained while keeping low surface roughness. This is obtained after less than two figuring iterations leading to a throughput over one segment per day.