Alignment of the Secondary Mirror of a Space Telescope Based on Holographic Modal Wavefront Sensor

On-orbit wavefront sensing is essential for a space telescope to align the mirror. For a space telescope with an active second mirror (SM), we propose an automated alignment method based on the holographic modal wavefront sensor (HMWFS). Compared with the traditional alignment methods that require either an intricate instrument or a lot of iterations to optimize a metric function, only a single-shot image is captured for rapid measurement of the misalignment of the SM in five freedom of degree. The HMWFS is realized by a computer-generated hologram (CGH) into which all aberrations modes caused by the misalignment are encoded. Furthermore, the improved HMWFS applies to both point and extended objects. A kinoform CGH is adopted and the optimization is analyzed. Numerical simulation has been taken to demonstrate the feasibility of the proposed method.