Research On Optimization Strategy And Design Method Of Off-axis Reflective Free-form Surface Optical System

In order to guide the design of off-axis reflective freeform optical system,this thesis focuses on the optimization strategy and design method of off-axis reflective freeform optical system.First,several commonly used characterizations of freeform surfaces and vector aberration theory with tilted components are analyzed,then establish a connection between surface and aberration.Compare the design ideas of the direct and indirect design method,and lay the foundation for the design method of off-axis reflective freeform system.For the multi-parameter optimization method,the surface optimization strategy and the field of view optimization strategy are proposed.In the surface optimization strategy,the Zernike standard polynomial is used in order to characterize the optical system wavefront aberration.Based on the coefficients square of Zernike standard polynomials,sequence of the aberration is determined.Then the corresponding XY polynomial term is selected as the optimization variables.In the field of view optimization strategy,starting from a small field of view,and expanding to a large field of view gradually,and the optimization steps are repeated.Through the calculated sum of squares of the coefficients of the Zernike standard polynomial,the RMS value of the wavefront corresponding to each field of view is obtained,then set the optimization weights of each field of view.According to a known off-axis three-mirror system with a field of view of 30°×1°,F-number of 12 and focal length of 1200 mm,a similar system was designed by combining the design method of the surface optimization strategy and field of view optimization strategy.The freeform surface only used 15 XY polynomial coefficients to characterize.Compared to an optical system using the same structure type,freeform surface type,XY polynomial terms,F-number and focal length,the field of view is expanded three times larger than before,the system image quality is close to the diffraction limit.Except for evaluating the optical property and imaging quality of the design results with the conventional evaluation criteria,two different calculation methods for the focal length of the off-axis reflective freeform optical system are proposed and analyzed due to its off-axis characteristics and freeform surfaces.A detailed calculation method and calculated example are given for the push-pull distortion and the ellipticity of the optical system.