Dragging three-point method for measurement of telescope optics

Abstract

We developed the improved three-point method as measuring machine that is applicable for measurements of various telescope optics, including concave, convex, and off-axis mirrors of high aspheric. The method is small and robust against temperature change and vibration and can extend a conventional machining tool to a measurement tool. We measured the cross section of a flat mirror of ϕ300 mm with this method on an old milling machine. The result was consistent with that obtained with an interferometer with a discrepancy of 5.5 nm (RMS). For the next stage, we executed two-dimensional measurements of a concave spherical mirror of ϕ794 mm. The result showed good agreement with that obtained with an interferometer and the method successfully detected an amplitude of 30 nm of small-scale structure in the lateral direction.