Dwell time calculation and curvature effect compensation method of magnetorheological finishing for large-aperture aspheric optics

Aspheric optics are widely utilized in high-performance optical systems due to superior properties. The fabrication of projection objectives for extreme ultraviolet lithography (EUVL) imposes extremely stringent requirements on surface accuracy, yet the ultraprecision machining of aspheric optics remains a fundamental challenge in optical manufacturing. Magnetorheological finishing (MRF), characterized by high material removal efficiency, minimal subsurface damage, and excellent surface error convergence ability, has become a key technology of optical manufacturing. However, large-aperture aspheric optics significantly increase the number of trajectory points, and the polishing paradigm based on a constant tool influence function (TIF) fails to accommodate the complex curvature distribution of aspheric optics. This instability in the TIF leads to large-scale and ill-conditioned dwell time computation, which severely constrain manufacturing accuracy and consistency. Based on the characteristics of MRF process and the principles of fluid mechanics, this paper analyzes the TIF model for aspheric surfaces in MRF, and compensates the curvature effect of aspheric surface based on point-by-point adaptive compensation method. Through the theoretical model, the curvature of the dwell point is solved point by point and the global spherical mapping is realized, which avoids the local fitting error and significantly improves the compensation efficiency and accuracy. Based on the solution method of linear equations of dwell time, a splicing removal efficiency matrix construction model is established and converted into a C language package, which significantly expands the computational scale and improves solution efficiency. Finally, the simulation experiment is conducted on an aspheric mirror with an aperture of 300mm×300mm and a conic constant of -15.6. The results demonstrate that the root mean square (RMS) of the surface error is reduced from 98.53nm to 0.96nm, thereby verifying the applicability of the proposed method in high-precision aspheric polishing, and provides an effective technical path for the ultra-precision manufacturing of aspheric lenses in the EUVL system.