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1 November 1990 Robust, angstrom level circularity profilometry
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The measurement of circularity of cylindrical grazing incidence optics is a formidable task, because required fabrication accuracies are typically on the order of several micro-inches, with resulting measurement accuracies on the order of a micro-inch or less. Such demanding accuracy requirements have evolved as the need for high resolution extreme ultraviolet and X-ray systems has increased. Current measurement approaches involve rotating the element (or one or more measurement probes) about the element's axis, and sensing the surface runout. Sensing can be mechanical or interferometric, but in either case the approach is clearly sensitive to runout in the rotation itself. Even when multiple probes are used to eliminate repeatable runout errors, nonrepeatable runout errors or drifts in probe position can severely degrade accuracy. In this paper we discuss a new, noncontacting approach (patent pending) that involves measuring the local circumferential curvature of the test piece by simultaneously measuring its circumferential slope at two slightly displaced locations. As the pair of sensing beams is scanned along the circumference, a profile of curvature is built, from which the circularity profile is deduced. The sensing of curvature not only makes the approach insensitive to all types ofvibration and drift (both in surface height and in surface slope), but also makes it insensitive to runout errors in the relative rotation. Thus, one can achieve absolute accuracies that are orders of magnitude smaller than typical drifts and runouts. We summarize the special qualities of this approach which make it well suited to measuring cylindrical optics, and which make it able to accommodate radii as small as twenty millimeters, working on either the inside (concave) or the outside (convex) surface. Finally, we discuss some preliminary experimental results and compare them with typical accuracy requirements.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Paul E. Glenn "Robust, angstrom level circularity profilometry", Proc. SPIE 1333, Advanced Optical Manufacturing and Testing, (1 November 1990);


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