High-accuracy electrohydraulic control system for the position and orientation of the primary mirror for a large telescope

Due to variations in gravity, temperature, and external disturbances, the optical axes of a telescope’s primary mirrors can shift and, consequently, become misaligned with the light path. Such misalignment significantly reduces image quality; thus, the position and orientation of primary mirror (POPM) control systems must maintain the optical axis in an ideal position. Therefore, in this paper, we proposes a novel high-accuracy electrohydraulic control system for the position and orientation of the primary mirror (POPM) of a large telescope. To this end, a POPM control system with five electrohydraulic partitions is adopted, and a mathematical model of the POPM is derived. In addition, a mathematical model of each partition of the electrohydraulic system is derived for the telescope controller design. A linear active disturbance rejection controller (LADRC) and a sliding mode controller (SMC) are adopted in each electro-hydraulic partition to ensure positioning accuracy. Experiments are carried out on 4 and 1.2 m large telescopes. The corresponding results show that by both keeping constant and varying the elevation of the large telescopes, the position error of the primary mirror can be limited to less than 1 μm, and the orientation of the primary mirror can be maintained with an error of less than 0.05 arcsec, even in the presence of external disturbances. This control accuracy can guarantee both the inalterability of the optical axis of the primary mirror and the possibility to adjust the light according to the requirement in order to obtain high-resolution images.