Top (a–c): the three ronchigrams used to calculate the phase of the X-ray beam, with a magnified close-up of the fringes below. The images are taken with gratings with the different spatial periods (225 nm for (a,c), and 275 nm for (b)), at the same position with respect to the focus, but with an angle of the grating with respect to the vertical of −37.9° for (a), −15.4° for (b) and 29.6° for (c). The red rectangle is the analysis mask of the zeroth order, the green and blue rectangles are the position of the −1 and +1 orders respectively. There is no interference and hence no phase information in the white shade area in (a). Bottom (d–f): False-color image of the difference (i.e. errors) between the phase derived from the Ronchigrams, and the phase derived after re-shearing the recovered wavefront of the beam. RMS error of the images is λ/55 for (a) and λ/40 for (b,c).