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The transmitted wavefront mid-spatial-frequency(MSF) errors of spherical lens has a great influence on the quality of the transmitted beam. Aiming at the problems of poor convergence precision and low convergence efficiency in mid-spatial frequency errors polishing of large square spherical lenses, this paper proposes a mid-spatial-frequency errors correction technology by using full-aperture rigid polishing combined with numerically controlled sub-aperture smooth polishing. In the full-aperture polishing stage, the surface shape distribution that is conducive to subsequent sub-aperture polishing is obtained through radius compensation technology to reduce the sudden change of surface shape in the corner area of the component. The more rigid polishing pad is used to smooth the whole surface then, so that the component has better MSF errors condition before the sub-aperture high-precision surface correction. In the stage of small tool CNC polishing, the transmission structure and mass distribution of the polishing disc are optimized, and the ideal transmission characteristics and size parameters of the polishing disc are obtained through mechanical simulation analysis to reduce the overturning moment of the polishing disc when the direction changes suddenly. This optimization also improves the pressure distribution of the polishing interface. Stability, a flexible polishing disc combined with a high dispersion polishing slurry is used to correct the surface errors. The concentration of the polishing slurry is optimized and the supply method is well changed. So when the surface shape errors convergence process is more efficient and controllable, it will not cause the deterioration of the MSF errors. The smooth tool is applied in the last stage with reducing the temperature change of the polishing interface. All these measures are aimed to increase the stability of smooth polishing and to achieve high-efficiency, high-precision and stable convergence of MSF errors. The experimental verification was carried out on four square spherical lenses with a size of 440mm×440mm. The final PSD1:RMS values have all reached within 1.8nm. Additionally the overall processing time has been greatly shortened.
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