Development of relative angle determinable stitching interferometry for high accuracy X-ray focusing mirrors

X-ray focusing mirrors with elliptical shape are widely used in synchrotron facilities for micro-, nano-scale focusing experiments. Surface interferometry plays an important role in the x-ray mirrors figuring with subnanometer accuracy. To avoid the second order error in stitching interferometry, relative angle determinable stitching interferometry (RADSI) is under development. This method was first developed by Yamauchi et al from Osaka University, which uses a planar mirror to correct the relative stitching angle between the neighboring subapertures. Here, we use RADSI to measure the x-ray spherical and elliptical mirrors with 300mm aperture Fizeau interferometer. The interferometer is combined with 4 accurate rotation and tilt stages for the stitching measurement. To ensure the stitching accuracy, we first studied the measurement accuracy within every single subaperture. Multiple measurement is used to decease the random error of single subaperture. The subaperture positioning is also carefully corrected to ensure the pixels of the adjacent subapertures in overlapping areas can be matched well. A first stitching measurement result of a spherical mirror with 30 meters radius is shown.