Protocol to study wavefront preservation capabilities of reflective X-ray optics with coherent synchrotron light

Wavefront preservation of reflective X-ray optics, i.e., homogeneity and coherence properties of the reflected beam, are of crucial importance for their application in combination with high-brilliance synchrotron light sources. In order to compare the performance of optical elements in a quantitative manner, a protocol has been established using the Talbot effect to access the coherence properties of the reflective beam as well as long propagation distance imaging to study its homogeneity. The basic idea is to operate in a single-bounce geometry: a high-resolution imaging detector translated at short propagation distances along the beam is used to measure the visibility of a diffraction grating in transmission geometry placed close to the mirror under study. The change of the fringe visibility as a function of distance between the grating and the detector gives access to the angular source size. A second high-resolution imaging detector at longer propagation distances of up to several meters allows one to measure the homogeneity of the beam. This article outlines the concept as realized at beamline ID19 of the European Synchrotron Radiation Facility, gives insight into some of the technical details to be considered for implementation at other facilities and ends with an example application: the study of a W/B4CW/B4C multilayer mirror.