Active optics technology application on Four-Meter Telescope

Active optics technology is pivotal in the development of next-generation large-aperture telescopes. The Four-Meter Telescope independently developed by the Institute of Optics and Electronics, under the Chinese Academy of Sciences, was completed and put into operation by the end of 2021. To maintain excellent imaging quality, it includes an active primary mirror with 110 active actuators. During active optics control, there are some innovation characteristics. First, the active optics system is operated using a combination of the look-up-table method and the real-time correction method. Second, the active optics and adaptive optics are united to eliminate the distortion caused by structural deformation and atmospheric turbulence. Third, our proposed control strategy based on the compensation plane method and constrained least-squares algorithm are applied in the real-time correction. We described the compositions and function principles of the active optics system for the Four-Meter Telescope. In addition, current star observation results are reported. The observation results show that the average FWHM results of the star images for different elevation angles are 0.6 to 0.8 arcsec after active optics correction, which indicates that the convergence results of active optics systems closely match the seeing conditions of the local observation site. The FWHM for the long-exposure image can reach 0.44 and 0.55 arcsec in x and y directions, respectively.