Designing a hydraulic support system for large monolithic mirror’s precise in-situ testing-polishing iteration

In order to improve the fabrication efficiency and testing accuracy of meter-scale, large monolithic mirrors, hydraulic support units (HSUs) are commonly used. However, the challenges to reduce the disparity of the HSUs’ stiffness and keep the stability of the mirrors’ altitude are hard to resolve, especially for large-scale mirrors. In this paper, we found the air ratio of the working fluid for HSUs is a key factor for designing the HSUs to resolve the challenges from the analytical solution that we derived for supporting large mirrors. Here we designed, tested and fabricated dozens of HSUs and used a four-meter SiC mirror, the world’s largest monolithic SiC reported in public, as a study case. It shows that the stiffness values of grouped HSUs vary within ± 3%, and the mirror’s reference surface PV is less than 20 μm in 10 days, producing a mirror tip/tilt angle less than 1.5″. The surface error of the supported mirror is about 20 nm, which is very close to the ideal case where uniform stiffness exists, for randomly distributed stiffness values. The repeatability of the in-situ interferometric test with 0.019 λ RMS of the mirror surface demonstrates the supporting system in a high precision. With such a supporting system, the fabrication process of this mirror was estimated to be sped up by 47% compared to the typical fabricating iteration. With minor modifications and easy extensions, such a novel supporting system could be used widely for many in-situ, high-quality fabricating-testing processes.