Calibration of SPS. (a) Schematic diagram of experimental arrangement, with SPS being 100-μm wide and 200-μm high; (b) diffraction pattern recorded by CCD at 800 eV; (c) intensity profiles across η axis in Fig. 5(b), where red plot is in Cartesian coordinates, and black plot is log-scaled, and red plot is normalized to the peak intensity of zero-order diffraction.