Research On Uniform Removal Method In Fabrication Of Large Aspherical Mirror Based On NC Machining

In modern large optical systems, the use of the high steepness aspheric reflecting primary mirror can simplify the structure of system; reduce the size and weight of the system, thereby improving the performance of the optical system. For development of space optics and astronomical observation, the manufacturing of large-diameter aspherical reflecting primary mirror has an important significance. With the development of these applications, the higher demands in manufacturing of large-diameter aspherical reflecting primary mirror at machining precision and efficiency have been put forward.Computer Controlled Optical Surfacing as a optical processing technology is widely used. In the processing, there are still have some further questions need to be solved. The first is hard to repair the grinding wheel at molding and grinding process. The second is the size of lap mismatch. And the last is lap motion parameters are not chosen appropriately. Those reasons can make the existence of higher and lower points at annuls or part in the surface. The small spatial frequency error of surface can make ‘leak point’ and ‘missing link’ at interferogram in the whole mirror aperture. It makes hard to obtain accurate surface shape data. Knife-edge method is often being used to measure surface qualitatively. Then the surface error which is being measured is repair by hands. It would greatly affect the processing efficiency of the workpiece and produce local mid-and-high frequency errors. Based on combination active lap and NC manipulator technology, the processing of grinding the mirror surface uniform removal is proposed. ’Uniform removal’ means the process of equal removal at the whole surface in grinding and rough polishing stage. It aims to remove the subsurface damage layer at forming and grinding stage. It also smooths the surface for reducing or eliminating mid-and-high-frequency error on the mirror. It can achieve full-caliber interferometer measurement docking with the early measurement method. Through the establishment of mathematical model and experimental method, lap dwell time, pressure and feed rate and other parameters are studied in the paper. It can make the mirror removed uniformly. The main contents include the following:(1) The problems of current large-diameter aspherical machining process are analyzed. The necessity of uniform removal process is put forward from the lackof the abrasive grinding characteristics of the optical brittle materials and the phenomenon of the missing data of the interferometer in the rough polishing processing.(2) Based on the characteristics of NC tools and active lap in different spatial frequency and similar to the numerical control processing method, a unified theory of ring removal function model is being established. This method for analysis of active lap and gadgets are large and small spatial frequency error and eliminate the edge effect of joint. And based on this, a new algorithm of dwell time for matrix operation is proposed.(3) The finite element pressure model of active lap edge effect is put forward. Through calculation, the pressure of the lap on the workpiece contact area had been found nonlinear change in the position of the center hole and the edge of the workpiece. The edge of the ring removal function is obtained through the finite element calculation. The uniform removal algorithm of lap on the edge of the workpiece was modified and the experimental study is carried out. The experimental results accord with theoretical model.(4) Through the study of the ring removal function in machining with NC manipulator, the correction ability of zonal error in processing of large aspheric mirror is analyzed. The analysis results have been verified by experiment.(5) For matching of the spatial frequency error model, a combined processing method of NC manipulator and active lap combination based on uniform removal has been put forward. The uniform removal effect is obtained for large aperture mirror for using this method.In this paper, the research result has been applied to the 1.25 m f/1.5 primary mirror grinding and polishing. The processing method is based on the active lap, combined with NC tools. The surface accuracy of grinding processing is controlled under 2~3um for using uniform removal processing. And the full aperture interferogram of He-Na light can be gained after the processing of rough polishing. The final surface error converges to PV 141 nm, RMS 19.6nm. The combined processing method is achieved for high precision and high efficiency processing in large aspheric mirror.