Self-Calibrated Stitching Test of X-Ray Flat Mirrors for Achieving Single-Nanometer and Nanoradian Long-Term Reproductivity

X-ray flat surfaces utilized in most advanced light sources have posed challenges to achieve form accuracy of PV 2 nm, RMS 0.2 nm and RMS 50 nrad. Measurement of the form error with high long-term reproductivity, i.e., repeatability over time of at least one day and with test surface re-mounted, is the key to direct deterministic figuring process. For this end, the models of two main error sources – long-term environment disturbance and retrace error have been established. The different error periods components of the long-term environment disturbance and their influence on the full aperture test accuracy has been investigated. The method of determining the adequate sub-aperture size, overlapping ratio, and the tilt misalignment threshold for decreasing the influence of the long-term environment disturbance and the retrace error is reported. The PV value standard uncertainty is evaluated to be 0.092 nm. Experiments on an X-ray flat with clear aperture of 250 mm × 30 mm was tested with the optimized test parameters and other sets of parameters for comparisons. Self-consistency experiments have been conducted to verify the long-term reproductivity can reach PV 0.908 nm, RMS 0.076 nm and RMS 32 nrad which meet the typical test accuracy posed by the 4th generation synchrotron radiation and free electron lasers.