Kev Technique Compyter Controlled Active Lap In Fabrication Of Large Aspherics

Modern large optical systems are used widely in astro-observation, space observation and other fields. Large aspheric primary mirror, as one of the key elements in large optical system, is needed more and more. The diameter of primary mirror determines the aperture of optical system. Light collect ability and resolution of system are higher when aperture is larger. So the primary mirror is bigger and bigger. The largest diameter of monolithic mirror is up to8.4m, and of mosaic mirror is up to39m. Also both higher figure accuracy and higher fabricating efficiency are demanded with the development of the optical systems. Computer Controlled Active Lap (CCAL) is a high efficiency fabricating method for large aspheric primary mirror, and has great application prospects. This paper presents deep research results on several key technologies in large aspheric fabricating based on the CCAL. Breakthrough progress has been made in follow aspects:(1) The algorithm for active lap real-time deformation calculation in fabrication has been deduced based on homogeneous coordinate transformation theory. The relationship between maximum deforming speed and lap position, workpiece rotating speed and lap rotating speed has been analyzed.(2) The Finite Element Analysis (FEA) models for active lap and workpiece contact analysis have been built in ANSYS Workbench. The pressure distribution on contact region when active lap inside the mirror and overhang the mirror out edge has been analyzed. A thin film type pressure measuring system from TekScan has been used to measure the pressure distribution. The measured results are coinciding with the FEA results. This solves the difficult problem of describing the pressure distribution between lap and workpiece correctly.(3) The mathematical model for CCAL ring Tool Influence Function (TIF) has been built based on the lap-workpiece motion rules and the pressure analysis results. The influences to TIF from pressure distribution, gaps on grinding layer and lap-workpiece rotate speed are analyzed. The result shows that the shape of TIF could be changed by adjust the pressure distribution and rotate speed. And high frequency errors can be introduced by using the circular gaps. On the other side, smooth TIF can be got by using the grid gaps. The active lap TIF experiments are carried out, and the results show that the theoretic TIF model is correct.(4) Two methods are proposed to model the edge TIF for CCAL. One is adopting the pressure distribution from FEA. The other is building up an equivalent parametric pressure model to fit the experimental data directly. Experiments are carried out to get the active lap edge TIFs and fit the parametric model. Comparison between experimental TIFs and modeled TIFs proves that the edge effect models are valid. Model fit errors of these two models are all below16%, which is better than skin model (30%).(5) Methods for CCAL to achieve un-axisymmetric fabrication are studied. Two ways are presented:lap pressure controlling based way and lap dwelltime controlling based way. Simulations and experiments are carried out. The simulation results indicate that both two ways are effective for removing the un-axisymmetric figure errors. The dwelltime controlling based method has been used in1.8m primary mirror polishing, and shorten the polishing time.The findings in this paper have been successfully applied in the grinding and polishing of a1.8mf/1.5hyperboloid lightweight primary mirror. The axisymmetric fabricating model is used the grinding stage. The figure error is reduced from PV90.5μm, RMS23.3μm to PV11.7μm, RMS2.1μm after116hours grinding. In the polishing stage, both axisymmetric and un-axisymmetric fabricating model are used. And after about375hours polishing, the figure error is converges to PV125nm, RMS13.5nm. The research results and experience can be used in the future fabrication of4mf/1.5parabolic primary mirror.