Electrohydraulic high‑accuracy position and orientation control system of the primary mirror for a large-aperture high resolution telescope

Generally，owing to variations in gravity，temperature，and external disturbances under differing conditions，the positions and orientations of primary mirrors of high-resolution large-aperture telescopes often change significantly in the free state；in this scenario，subsequent optical axes cannot be aligned with the primary mirrors，causing optical misalignment errors and degraded adaptive high-resolution imaging qualities，sometimes even leading to image fly off from the field of view. To eliminate these imaging errors resulting from variations in the positions and orientations of primary mirrors（POPMs），this paper proposes a novel high-accuracy electrohydraulic control system for the POPM of a large telescope. For this，a mathematical model of the POPM is established for design and analysis for active control. First，a POPM resolving control model of an entire telescope is constructed，and the variation principle of the POPM is analyzed. Second，a five part muti-motor electrohydraulic control system is adopted to realize active control of the POPM. To guarantee control accuracy，we construct the electrohydraulic control system model of each part and use a multivariate linear fitting feed forward controller based on the position error resulting from a change in the telescope elevation；meanwhile，a linear active disturbance rejection controller is adopted for POPM control. Finally，experiments on large telescopes are performed. When the elevation of a 4 m telescope moves at a constant speed，the Z shift can be reduced from 91. 5 μm to 0. 5 μm，and the deflection shift can be controlled under 0. 05 arcsec from 3 arcsec. Next，when the elevation of a 1. 2 m telescope moves at a variable speed，the Z shift can be reduced from 5. 04 μm to 0. 2 μm，and the deflection shift can be controlled under 0. 65 arcsec from 0. 05 arcsec. Further，when multi-point force actuators are added to the primary mirror，the Z shift can be reduced from 12. 2 μm to 2 μm，and the deflection shift can be controlled under 0. 03 arcsec from 1 arcsec. This can effectively realize the optical axis stability of the primary mirror while guaranteeing the alignment of subsequent optical axes and high-resolution self-adaption image quality. © 2023 Chinese Academy of Sciences. All rights reserved.