Study On Ion Beam Polishing Technology Of Off-axis Parabolic Mirror

With the advancement of modern processing and manufacturing technology,the processing precision of aspheric optical elements in non-contact processing has been greatly developed.Different optical curved surfaces free up optical design and enables the optical system to achieve higher imaging quality.Off-axis parabolic mirrors are commonly used optical devices in optical design and are widely used in off-axis reflection systems.However,the manufacturing accuracy of large diameter and large off-axis is the biggest factor limiting its use.This paper uses ion beam processing technology to study the processing of off-axis parabolic mirrors.The main research includes the following contents:(1)Solve for the minimum material removal.Use the axisymmetric method and the coordinate rotation method to obtain the smallest asphericity respectively;Then design and write the closest spherical search algorithm program through MATLAB software,and design the GUI interface to search for the closest spherical surface of two different off-axis parabolic mirrors;Finally,determine its closest spherical radius and minimum asphericity to determine the minimum material removal.(2)Research material properties.For K9 optical glass,first study its physical and chemical properties,and then use the spot method and Faraday cup method to determine its removal function.Three factors affecting the stability of the removal function are analyzed:thermal effect,stepper motor stability,and ion beam output stability.Finally,based on the above analysis,methods of compensation and improvement are proposed.(3)Research dwell time theory and simulation.The dwell time is solved by linear equations,and the initial surface shape of a planar optical element with an aperture of 220 mm is detected by a Zygo interferometer.Through the obtained removal function,the dwell time is simulated and verified,and a simulation result with a primary convergence ratio of 4.75 is obtained.(4)Polish off-axis parabolic mirror with ion beam.According to the minimum material removal amount obtained by programming the off-axis parabolic mirror closest to the spherical surface,the off-axis parabolic mirror with an aperture of 110mm and an off-axis amount of 85mm is completed.In the processing,a sequential processing method is used that combines the original aperture processing,small beam diameter processing,and small beam diameter local high point processing.This sequential processing method replaces the repeated iterative processing method,and finally the high-precision surface shape value is obtained,where the PV value is 0.36 λ and the RMS value is 0.029 λ.At the same time,the roughness detection is added during the processing to analyze the evolution of the roughness during the ion beam practice.In this paper,a sequential processing method is adopted,which provides a method for ion beam processing to reach the convergence limit faster.