Due to the different curvature everywhere, the aspheric surface is hard to achieve high-precision accuracy by the traditional polishing process. Controlling of the mid-spatial frequency errors (MSFR), in particular, is almost unapproachable. In this paper, the combined fabrication process based on the smoothing polishing (SP) and magnetorheological finishing (MRF) is proposed. The pressure distribution of the rigid polishing lap and semi-flexible polishing lap is calculated. The shape preserving capacity and smoothing effect are compared. The feasibility of smoothing aspheric surface with the semi-flexible polishing lap is verified, and the key technologies in the SP process are discussed. Then, A K4 parabolic surface with the diameter of 500mm is fabricated based on the combined fabrication process. A Φ150 mm semi-flexible lap is used in the SP process to control the MSFR, and the deterministic MRF process is applied to figure the surface error. The root mean square (RMS) error of the aspheric surface converges from 0.083λ (λ=632.8 nm) to 0.008λ. The power spectral density (PSD) result shows that the MSFR are well restrained while the surface error has a great convergence.
The high-precision aspheric surface is hard to be achieved due to the mid-spatial frequency error in the finishing step.
The influence of mid-spatial frequency error is studied through the simulations and experiments. In this paper, a new
polishing process based on magnetorheological finishing (MRF), smooth polishing (SP) and ion beam figuring (IBF) is
proposed. A 400mm aperture parabolic surface is polished with this new process. The smooth polishing (SP) is applied
after rough machining to control the MSF error. In the middle finishing step, most of low-spatial frequency error is
removed by MRF rapidly, then the mid-spatial frequency error is restricted by SP, finally ion beam figuring is used to
finish the surface. The surface accuracy is improved from the initial 37.691nm (rms, 95% aperture) to the final 4.195nm.
The results show that the new polishing process is effective to manufacture large-aperture and high-precision aspheric
Combination technology of MRF and sub-aperture smoothing in off-axis asphere manufacture was researched. The
asphere polishing with computer control polishing is not very deterministic , as removal rate of computer control
polishing is variation over time in asphere polishing due to the tool misfit, pad wear, or slurry variation.
Magnetorheological Finishing is deterministic, subaperture finishing technology in asphere manufacture, but it is limited
to smooth mid-spatial-frequencies and high-spatial-frequencies. An example was given in a 290mm circle aperture offaxis
aspherical optic polishing process with combination technology of MRF and sub-aperture smoothing. The final
figure error was λ/50 rms from the initial 0.8λ rms. The result shows that the combination technology is practical and
have high convergence efficiency.