Xiaoqin Yan, Zibo Jiang, Xingtao Liu, Lijun Wu, Xiaojie Jiao
At present, the secondary mirrors of large ground-based telescopes are increasingly adopting adaptive configurations, with most deformable mirrors fabricated from glass-ceramic materials. However, such materials suffer from several limitations, including low thermal conductivity, susceptibility to thermal deformation induced by voice coil motor heating, and a tendency to fracture during fabrication, alignment, and operation. An innovative approach is proposed in which titanium alloy is employed as the material for adaptive secondary mirrors. Owing to its high thermal conductivity, low coefficient of thermal expansion, and high fracture toughness, titanium alloy has the potential to overcome the inherent drawbacks of glass-ceramic materials. Through experimental investigation, the evolution of surface roughness of titanium alloy mirrors under different grinding abrasive sizes was characterized. In addition, the surface roughness, reflectivity, and surface microstructure of titanium alloy mirrors processed with different polishing abrasives were systematically analyzed. An optimized process chain consisting of boron carbide (B4C) fine grinding, cerium oxide (CeO2) prepolishing, and silicon dioxide (SiO2) final polishing was established. Using this process, a Φ150 mm titanium alloy reflective mirror was fabricated. Measurements show that the surface roughness (Sq) reached 7.680 nm, the average reflectivity in the visible wavelength range (350 to 750 nm) was 57% (which can exceed 90% after coating), and the surface figure accuracy achieved an RMS value better than 15.82 nm. These results provide a technical foundation for the development of titanium alloy adaptive deformable mirrors.