Research On Thermal Control System Of 4m SiC Lightweight Primary Mirror

Thermal issues are concerned in all aspects of the design of the large ground-based telescope. The effect of temperature on the telescope’s performance and stability has become increasingly evident with the increase of the telescope aperture. For a 4m telescope or above, the primary mirror thermal control system has become an essential part. In this paper, based on the 4m Si C lightweight primary mirror system of the telescope, the necessity, the target, the program and the structure design of mirror thermal control system is studied.Firstly, the composition of 4m telescope is introduced. The thermal stability of the primary mirror made of Si C or Zerodur is analyzed with the method of the finite element, when the telescope is running on the whole day. The advantages of Si C primary mirror and the need for thermal control is explained, then the need for thermal control system is further proved by the simulation and experiment of 2m Si C lightweight primary mirror. A mathematical model of temperature distribution in the primary mirror is built with the method of Zernike polynomial fitting. The primary mirror thermal deformation and the aberration is computed under different temperatures distributions, the thermal gradient indexs of primary mirror thermal control system is raised based on the correcting deformation ability of active optics, the temperature difference of between the upper and lower mirror surface should be controlled in less than 0.2K, the horizontal and vertical temperature difference in less than 0.5K. The impact of the turbulence on the mirror seeing is analyzed from the perspective of aero-optical. The thermal control indicator of temperature difference between primary mirror and ambient, which the temperature difference should be controlled between-1K and 0.5K, is determined. According to thermal control indicators, the primary mirror thermal control system is designed, which is composed of mirror back ventilation system and mirror surface ventilation system, the thermal control forms and the structure of the thermal control system are optimized designed, and a detailed work and control plan are proposed. Finally, according to the 4m Si C primary thermal control system, a mirror back thermal control experiment based on 1.23 m Si C mirror is designed. By analyzing the experimental results, the feasibility of thermal control system is proved.In this paper, a primary mirror thermal control system for the 4m telescope is detailed studied and designed. The relevant engineering experience achieved from the experiment is preliminary explored and accumulated for the future implementation of mirror thermal control system engineering, which has a very high reference value for the final determination and implementation of the future 4m telescope primary mirror thermal control scheme.