LTP-corrected stitching interferometer achieving sub-nanometer accuracy for x-ray mirror metrology

This paper presents a sub-nanometer accuracy stitching interferometry system for metrology of large curved x-ray mirrors, incorporating Long Trace Profiler (LTP) low-frequency correction. Utilizing a Fizeau interferometer with a reference transmission flat to measure sub-apertures of flat or curved optics, the system employs a mixed stitching method to achieve high-precision metrology of large x-ray mirrors. Crucially, the instrument overcomes cumulative angular errors in sub-aperture stitching through LTP-guided correction, achieving 0.18 nm RMS measurement repeatability validated on high-precision optics. After calibration of the transmission reference flat and zoom factor, the system demonstrates exceptional consistency with direct LTP measurements (<0.2 nm RMS), surpassing software-only techniques, while successfully verifying curved-mirror capability via <1% radius error measurement on spherical mirror with a radius of curvature of 24.6 m. For challenging elliptical synchrotron x-ray mirrors, our measurements of a specimen with 50 nrad RMS slope error and 0.5 nm RMS height error demonstrate excellent agreement with both manufacturer's RADSI and LTP reference data. By eliminating angular drift propagation during the stitching process, this metrology system achieves sub-nanometer measurement accuracy for strongly curved x-ray optics.