Computation of the Removal Function for Ion Beam Figuring Curved Surface

Ion beam figuring (IBF) is a deterministic method for final figuring process of optical surface. The removal functions of IBF should be measured and calibrated strictly to guarantee the accuracy of the figuring results. The incident parameters of ion, surface profile and ion density distribution on the surface are the main factors which affect the material removal rate within the removal function. For the curved surfaces these factors vary from point to point so that the removal functions keeps changing when figuring curved surface especially for steep surface. Unlike the first two factors, the ion density distribution can’t be described by mathematic expression directly. Moreover, the ion is not in Gaussian distribution on the curved surface any more, which will result in variation of removal functions. Faraday cup is used to get the ion space distribution. So that the ion density on an arbitrary point can be obtained exactly. Based on the material removal features, revision method is proposed to fix the difference of the removal functions between the curved surface and the flat surface. Through experiments and theoretical analysis, the removal functions changes distinctly and not in Gaussian distribution. The computation method can fit the variety of the removal function effectively.