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Optical metrology for the segmented optics on the Constellation-X soft x-ray telescope

We present the metrology requirements and metrology implementation necessary to prove out the mirror technology for the Constellation-X (C-X) soft x-ray telescope (SXT). This segmented, 1.6m diameter highly nested Wolter-1 telescope presents many metrology and alignment challenges. A variety of contact and non-contact optical shape measurement, profiling and interferometric methods are combined to test the forming mandrels, some of the replication mandrels, the formed glass substrates before replication and the replicated mirror segments. The mirror segments are tested both stand-alone and in-situ in mirror assemblies. Some of these methods have not been used on prior x-ray telescopes and some are feasible only because of the segmented approach used on the SXT. Methods to be discussed include axial interferometric profiling, azimuthal circularity profiling, midfrequency error profiling, and axial roughness profiling. The most critical measurement is axial profiling, and we compare the method in use to previous methods such as the long trace profilometer (LTP). A companion paper discusses the method of non-contact 3D profiling using a laser sensor and distance measuring interferometers.