A Fizeau interferometer system with double-pass and stitching for characterizing the figure error of large (>1m) synchrotron optics

A Fizeau interferometer based system has been developed to measure the figure error of large synchrotron optics using single-pass, double-pass, and stitching geometries. The system, which uses a λ/100 reference flat, is designed to measure optics up to 1.5m in length, and is capable of nanometer level repeatability. Fizeau measurements, in single pass geometry, are conventionally limited to the diameter of the laser beam, typically 150mm or 300mm. Stitching adjacent fields of view together or using a double-pass geometry, allows much larger optics to be characterized. Results for the single-pass, double-pass, and stitching geometries are shown to give consistent figure error values. Data is also in good agreement with an autocollimator-based slope profiler. The Fizeau method is also advantageous since data can be acquired in less than 1 minute, particularly useful for characterizing the many degrees of freedom of active or adaptive optics. To obtain results consistent with alternative techniques, the importance of an a priori knowledge of the surface topography of the reference optics is also demonstrated.