Multi-tool sub-aperture polishing high-slope asphere with low mid-spatial frequency errors

Abstract

The reasons and methods for generating and suppressing mid-spatial frequency (MSF) errors were deeply studied, and a new method of combining multiple small tools was used to smooth out MSF errors. First, an elastic tool was employed to pre-polish the aspheric surface after grinding, aiming at fast removing sub-surface damage as well as correcting surface figure. Then a second tool-self-designed smoothing tool -was mainly used to smooth the surface, which was followed by an air bonnet tool to continue figure correction until Zernike residual was not convergent. The effective combination of the three small tools was not only able to avoid the high slope fabrication difficulty of the aspheric surface, but also to suppress the MSF errors. The surface was tested by CGH, which can ensure the accuracy of the surface. This method was successfully used to polish a 150mm asphere with a maximum departure of 0.26mm. After five iterations, the surface accuracy converged to 4.05nm RMS. The result shows that this method can realize the valid polishing of high-accuracy aspheric surface.