Edge-cutting analysis for segmented optics for large telescopes such as the Thirty Meter Telescope

Abstract

For the light-collected optical elements used in astronomical or space applications, there is a requirement to increase the effective area as much as possible relative to the physical area. For example, segmented mirrors such as the primary mirror of the Thirty Meter Telescope (TMT) or that of the ELT, it is necessary to reduce the gap between adjacent mirrors, thereby improving the light collection efficiency and preventing the generation of unnecessary diffraction patterns. In these cases, it is popular to cut the edges with multiple machines after polishing in roundel shape, but there are risks of damage to the polished surfaces during the handling operation. In addition, use of multiple machines increase the overall machining time which itself is a problem. As a solution to these issues, we studied a method in which a mirror segment is machined along a pattern of cutting, and then is easily cut by a cleaving mechanism on the same machine. We estimated the stress intensity factor and the lifetime based on fracture theory. FEM analysis was used to examine suitable processing patterns which should be applied to the optical element to achieve edge-cutting. We carried out an experimental cleaving using a sample glass, and the difference between theoretical predictions and experimental results matched well. These results are used as basic data to determine the processing procedure of the upcoming segmented mirrors for the TMT.