Study On The Key Techniques Of Magnetorheological Finishing For Off-axis Aspheric Optical Elements

With the rapid development of modern science and technology, lots of reflectingoptical systems based on off-axis aspheric optical elements are employed widely inmany leading edge fileds which includes space-based observation, ground-basedobservation, and extreme-ultraviolet lithography (EUVL) etc. Compared to theseon-axis optical elements, not only the convergence of surface form error and thereachability of figuring accuracy, but also the control of characteristic parameters, themetrology of surface form error and the high-precision and high-efficiency processingtechnics are all present in the process due to the complicated manufacturing features ofthe off-axis aspheric optical elements. Therefore, the fabrication of off-axis asphericoptical elements is always the emphasis and difficulty in the field of opticalmanufacturing.Nowadays, the requirement of off-axis aspheric optical elements increases a lot inmodern optical system, so the conventional machining methods can not satisfy therequirements of practical application. As one of these new optical machining methods,Magnetorheological Finishing (MRF) owns many unique advantages includinginvariable and controllable influence function, deterministic and controllable figuringprocess, accurate and forecast machining result, little edge effect, good surface quality,low sub-surface damage, as well as excellent flexibility of technics, which is proposedto be potentially applied in the fabrication of off-axis aspheric optical elements. Thisthesis focuses on the high-precision/high-efficiency fabrication of off-axis asphericoptical elements and the key theory and technology in MRF process, aims at thedouble-control of surface form accuracy and characteristic parameters, as well as theestablishment of technological chain, which should improve the national manufacturingability of the off-axis aspheric optical elements.This thesis’s outline is as follows:1、The complex manufacturing features of the off-axis aspheric optical elementsincluding the variant curvature, nonlinear forming, large sagitta, large off-axis distanceand large aperture etc produce lots of theoretical problems which are referred tocurvature-variant influence function model, nonlinear surfacing process model, and highdynamic dwell time model and algorithm. The study of these theoretical problemsprovides the theoretical basis and technology reference for MRF figuring of off-axisaspheric optical elements.2、Based on the MRF figuring theory and its application, the MRF figuringtechnics of off-axis aspheric optical elements are optimized. By means of optimizing themachining position-attitude model, the machining difficulty can be reduced; moreoverthe machining ability and accuracy should be improved. The influence function characteristic of off-axis aspheric optical elements is estimated in order to establish twocompensating figuring processes which refer to influence function linear-compensatedfiguring technology and influence function nonlinear-compensated figuring technology.The study of figuring technology provides the technical approach for MRF figuring ofoff-axis aspheric optical elements.3、Surface accuracy and characteristic parameters evaluate the quality of off-axisaspherical mirrors from two aspects, so we need to effectively control the characteristicparameters as well as the surface accuracy. Theoretical model describing the keycharacteristic parameters (off-axis distance, conic constant and vertex radius ofcurvature) of off-axis aspheric optical elements are established to achieve themonitorition and qualification of characteristic parameters. The nonlinear distortioneffect and the adjustment-induced aberration are two key problems presenting in thenull test of the off-axis aspheric optical elements. To realize the accuracy positioningand effective evaluation of surface form error, the nonlinear distortion control modeland the aberration-separated model of constrained characteristic parameters’ toleranceare both established. The control of characteristic parameters provides the machiningprecondition and constraint criteria for MRF of off-axis aspheric optical elements.4、Based on the analysis of conventional machining technological chain of theoff-axis aspheric optical elements, the novel technological chain with MRF is proposed.The theory and technology in this thesis are both applied in experimental validation, andthe surface form accuracy and tolerance of characteristic parameters are qualified finally,which provides an effective support for the study achievement of this thesis.