Studies on Super-Smooth Polishing for Aspherical Optics (1st Report)

Contour Measuring Machine on Large Aspherical Surfaces

Abstract

For the purpose of fabricating free-form optical elements to an accuracy of 80 nmPV, the CSSP (Canon Super-Smooth Polisher) has been developed. This device finishes workpieces by alternately repeating contour measuring and corrective polishing. In such a system, contour measuring is important because it limits the final accuracy of the workpiece. This paper focuses on the CSSP's on-machine contour measuring method. A contact probe is employed to ensure adaptability to free-form contours with maximum tilt angles of 35 degrees. A unique probe structure is proposed, by which both inclination and motion errors of the probe are simultaneously compensated. The flaw problem is discussed from an experimental point of view. Using a ceramic air slide and a linear motor, the contact force was controlled to a constant of 2 mN, which is much smaller than the force of 260 mN that causes a yield stress on workpieces made of calcium fluoride. The probe's scanning speed of 4 mm/s was achieved by speeding up this force controller. The margin of force error during contour measurement was under 0.2 mN. A coordinate measuring method using fourteen axes interferometers was also proposed for compensation of the major mechanical motion errors of the probe and the tables. The measurement results showed good repeatabilities of 3 nmRMS for a 540 mm line measurement, and 9 nm RMS for a φ 500 mm aspherical surface measurement.