Research On Active Controlling Technology For Large Optical Telescope

Active optics based on light-weighted primary mirror is one of the key technologies for building modern large telescopes.Active optics technology dynamically controls the shape of the primary mirror,which solves the problem of low stiffness of the lightweighted mirror.During the operation of the telescope,the active control technology can not only correct the mirror surface,but also correct the low-frequency and loworder aberrations caused by other parts of the optical system and atmospheric disturbance,so as to improve the imaging quality of the telescope system.Based on 4-m active thin primary mirror,the research work of active control technology is carried out in this paper,mainly including the following contents:Firstly,the supporting system of 4-m thin primary mirror is introduced,including axial support arrangement,radial support arrangement and.The core problem of 4-m active support control technology is the solution of active correction force.Then,the deformation of 4-m thin primary mirror under gravity and temperature loads and the active correction ability of 4-m thin primary mirror are analyzed by finite element method.The results show that the positioning method based on hard points reduces the fitting ability of the primary mirror for the low-order Zernike aberration and destroys the rotational symmetry of its free-vibration modes.Secondly,the existing active correction force algorithms are analyzed,including least-squares algorithm,damped least-squares algorithm,bending algorithm and so on.For the active support system with hard points,a constrained least-squares algorithm is proposed.The local force smoothing constraint is introduced into the algorithm for the first time,which effectively reduces the internal stress of the primary mirror in the process of surface correction.Based on 4-m thin primary mirror,the fitting results of each algorithm are simulated and analyzed,and the existing algorithms are compared from the aspects of force amplitude,fitting residual and local force smooth.Thirdly,in the process of surface detection and surface fitting,a method to establish the compensation plane is proposed,which calculates the compensation plane based on the detected deformation and actual deformation at the hard points.After compensation,the relative movement between the primary mirror and the reference plane is effectively avoided.Therefore the high-order dent at the hard points is eliminated and the aberration fitting ability of the primary mirrors is improved.Fourthly,a distributed active support control system based on Ether CAT bus is designed for 4-m thin mirror.The control system satisfies the synchronous control requirements of multi-channel support points of 4-m thin mirror.Then,a series of surface correction experiments of 4-m thin mirror are completed under the interferometer tower,including transfer functions calibration,standard Zernike aberrations fitting,gravity deformation correction and so on,which verified the studies in previous chapters.Finally,the installation and outfield experiment of 4-m thin primary mirror are completed,the active correction ability of 4-m thin primary mirror is further verified on 4-m telescope.The far-field images of star are obtained.Before and after active correction,the diameter of the circle containing 80% of the geometrical energy is reduced from 4 arcsec down to 1.6 arcsec.The research results of this paper have been successfully applied to Chinese large optical telescope,which verified the technology roadmap of active control technology.They can also provide practical experiences for the development of larger optical telescopes in China.