In the past five chapters we have discussed measurements on imaging systems, interferometric measuring systems, wavefront sensors, and various beam parameter measurements. In the final chapter we concentrate mainly on component level metrology.
In Chapter 3 we showed that an interferometer can measure the surface figure (or âdeparture from sphereâ) of a concave mirror. But you cannot determine a mirror's radius of curvature directly from an interferogram. To do this you need two null interferograms and a means of accurately measuring axial distance from the interferometer. Consider the test arrangement in Appendix 3.1 (page 71, column 2, row 2). Suppose we have adjusted the axial separation so that a null interferogram is displayed. This means the focus of the reference transmission sphere is coincident with the radius of curvature of the mirror.
We now shift the test mirror axially away from the interferometer until another null interferogram is obtained. This occurs when the light from the transmission sphere is focused directly on the mirror surface. By measuring the axial separation between these two null interferograms we obtain the radius of curvature of the mirror.
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