“…Among them, specklebased methods (Wang et al, 2015;Berujon et al, 2014Berujon et al, , 2020aZhou, Wang, Fox et al, 2018;Wang, Kashyap & Sawhney, 2015a) have become popular due to their relative simplicity in terms of experimental setup, relatively short data acquisition time and less stringent requirement for transverse coherence. Speckle-based at-wavelength metrology has been used for the measurement of X-ray reflecting mirrors Xue et al, 2019); although, in these cases, either the influence of any wavefront error derived from the incident beam was ignored or a reliable 2D map of the wavefront slope error was not produced (Xue et al, 2019). The wavefront error resulting from imperfections of upstream optics on a beamline can introduce significant systematic error for measurements of super-polished X-ray mirrors with slope errors <100 nrad.…”