Long focal length aspherical mirror testing with CGH and auxiliary lenses (invited)

At present, the radius of curvature of the primary mirror in some large-aperture optical telescopes has reached the order of tens of meters. If the surface of the mirror is tested simply by CGH, the length of the testing optical path is not lower than the length of its radius of curvature. Due to factors such as site size and ambient airflow disturbance, it is difficult to achieve high-precision measurement of the mirror surface under these conditions. In order to solve the problem of high-precision surface testing of large-aperture long focal length optical mirror, a hybrid compensation method is proposed. This hybrid compensation method combines CGH and auxiliary lenses to effectively shorten the length of the testing optical path, and can realize null testing of aspherical mirrors. In the optical path design, it is necessary to effectively optimize the optical design parameters of the hybrid compensation optical path and the separation of the CGH diffraction order. At the same time, the optical path length should be less than the radius of curvature of the aspherical mirror to achieve the purpose of shortening the length of the testing optical path. By testing the EELT main mirror, the simulation test shows that the testing path length of the method can be shortened to less than 1/8 of the length of the radius of curvature of the surface. Designing testing accuracy is better than that of RMS λ/100 (λ=632.8 nm). The above simulation results show that this method can not only realize the shortening the length of the testing optical path, but also achieve the high-precision surface testing of the aspherical mirror.