Improvement of the surface shape error of the pitch lap to a deterministic continuous polishing process

Continuous polishing using pitch lap is a key process in finishing large flat optical elements. The figure of the optics is primarily dependent on the surface shape of the pitch lap. In this study, a novel method is proposed to determine the lap shape error by rotating the lap while the measurement point is moved radially using a precision slide mechanism. The systematic measuring error including the slide straightness error and slide perpendicularity error with respect to the lap rotary axis is divided into 1) the straightness and absolute horizontality error of the slide and 2) the absolute verticality error of the lap rotary axis. It is proposed that the former can be determined using a displacement sensor to measure water surface while moving along the slide, while the latter can be determined by an electric gradienter placed on the rotating lap. Furthermore, a small heating tool is proposed for correcting the local surface shape error of the pitch lap considering its viscoelastic property, and thus obtaining a favorable shape on the entire surface by controlling the dwell time of the small tool. Our experiments validated that the surface shape error of the pitch lap can be corrected by the method significantly, and that the optic figure is significantly promoted.