In a coherent scattering experiment, one of the important experimental parameters is the speckle visibility, which can be changed by adjusting the size of the X-ray beam on the sample. Focussing optics provide therefore a convenient tool to increase the speckle visibility, while simultaneously increasing the flux density and thus the dose on the sample. At the Coherence Applications Beamline P10 different focussing options are used to optimize this interplay of dose and speckle visibility for a given experiment. Using compound refractive lenses, three different realizations are presented using a lens-to-sample distance of 0.45, 1.6 and 10.8 m. The resulting focal spot sizes range from around 1 µm × 1 µm to around 15 µm × 7 µm (horizontal × vertical direction).
Westermeier et al. (Wed,) studied this question.