Research On The Lightweight Design And Support Of The2m-SiC Primary Mirror For Ground-based Telescope

With the enlargement of aperture of ground-based telescope, the collecting andresolution power were getting higher and higher. But in furthest improving thesurface figure of mirror and reducing the weight of equipment were also animportant target for engineers. In particular, with the development of SiC materialand lightweight technology, SiC lightweight mirror as a new option was provided forlarge mirror support.As a new optical material, the merits of SiC that high stiffness ratio and thermalconduction and excellent polishing performance were obvious. But the highcoefficient of thermal expansion compared to classical glass materials (for instanceZerodur and ULE) increased the difficulty of the mirror support. In addition, veryhigh requests were also put forward to the supporting structure by mirror opticalmanufacture and alignment.As the subject investigated, taking mirror lightweight structure, opticalmanufacture, mechanical alignment and thermal influence into account, thelightweight design for2040mm SiC mirror and supporting structure were finishedapplying the theory calculation and FEM from two aspects that simulation analysisand mechanical embodiment.Firstly, according to a great deal of investigation, the basic principle and general methods for large-aperture mirror support were summarized. Meanwhile, theimportance of passive support was emphasized and the force actuator as the bridgebetween passive and active support was provided. Then, the overall plan for thisstudy including mirror lightweight structure was determined according to theconsideration in various design factors.Secondly, lightweight structure for the primary mirror was optimized byparametric modeling, and the lightweight ratio was77.6%. According to thecharacters of load in different work conditions, the axial support was divided intoequal-force floating support and unequal-force floating support. Meanwhile, thehydraulic Whiffletree support for optical manufacture and the force actuator supportfor observation were also provided. In the premise that the low-order residual errorwas0.025λ (λ=632.8nm)，the surface error was4.32nm RMS (about0.007λ) atlast, according to the optimization of supporting forces by FEM. Based on thetangential shearing support principle, the equal-angle-force plan which was suitablefor lightweight mirror improved the mirror surface error to be1.8nm RMS (about0.003λ) for radial support.Thirdly, based on the principle of mechanism kinematics, the conception of“hardpoint” which was used for mirror positioning and alignment was introduced.The features of hardpoints for manufacture support system and operation supportsystem were introduced. In order to meet the request of mirror support stiffness inoperation condition, the simulation for axial and radial hardpoints was finished.Then, the thermal deformation and temperature influence for large-aperturemirror under the different thermal loads were discussed by FEM. In order tocompensate the distortion, the axial flexible rod and radial lever support wereadopted and the diameter-length ratio of flexible rod was also optimized. Theoptimization results showed that the rational length of rod (diameter3mm) was120mm. Then, for overcoming the surface severe distortion under axial temperaturegradient, as a thermal control measure, air jet array which was suitable forlightweight mirror was provided. At last, on the basis of previous calculation and analysis, the Engineeringstructure design and model building of hydraulic Whiffletree support for opticalmanufacture and force actuator support for observation were finished. Using theFEM, analyse on static, dynamic and thermodynamic were accomplished, the resultsshow that the max deformation of mirror was3.24nm (elevation angle from0°～90°), the first order resonance frequency was47.9Hz, and the mirror thermaldeformation (-30℃～30℃) satisfied the request of free heat expansion.Fabrication, Optical manufacture, mechanical alignment and observation for2mSiC mirror were the objects which thesis focused on. A new support system wasfinally completed, and the perfect lightweight ratio and surface figure came true. Theapplication of force actuator in the operation condition provided the interface unit foractive support at last. The author hopes that this thesis could have some referencevalue to the development of larger-aperture lightweight SiC mirror.