Stitching interferometric metrology for steeply curved x‐ray mirrors

We have developed a new type of relative angle determinable stitching interferometry (RADSI) for hard x‐ray nanofocusing mirrors. The system consists of a microscope Michelson interferometer, and a large‐area Fizeau interferometer. The surface figure profile of each local area is measured by the Michelson interferometer and, at the same time, the stitching angles between neighboring shots are determined by using the large‐area Fizeau interferometer. Tests using a flat mirror were carried out for calibration. The repeatability was found to be 0.8 nm root mean square (RMS). To demonstrate the accuracy of the developed system, we measured an elliptically figured mirror used for hard x‐ray focusing. The obtained results were compared with a profile measured by another method. Both profiles were in good agreement at the subnanometer level.