Alignment for active secondary mirror of space telescope using model-based wavefront sensorless adaptive optics

The precise alignment of the space telescope with an active secondary mirror (ASM) is a key factor to ensure the imaging quality. The traditional alignment methods, like sensitivity table method and SPGD, are not suitable for on-orbit instant alignment of space telescope. In this paper, a model-based wavefront sensorless adaptive optics (WSLAO) method is proposed for the alignment of ASM of space telescope. This method does not require an additional wavefront sensor (like Shack-Hartmann) or a lot of iterations (like phase retrieval). The low spatial frequency content of images is used as the metric function. The misalignment errors can be estimated from the quadratic relationship between the metric function and the coefficients of gradient orthogonal modes (GOM). Here GOM is derived from the response functions of five degrees of freedom of the ASM. A three-mirror anastigmatic telescope with an ASM is modeled for the performance tests. Simulation results show high corrective accuracy and rapid convergence of our method. The RMS of the residual aberration can be reduced to less than 0.02 rad after three correction cycles.