Ultra small-angle X-ray scattering: conditions, limits and results in speckle experiments

To carry out small-angle scattering experiments with X-rays in the scattering vector range q < 10^-3 Å^-1 with area detectors, background must be carefuly reduced. The principal source of background is slit scattering. Careful polishing of slits allows slit diffraction to be observed, which can be calculated from wave equations. In this case, the background takes the shape of thin streaks perpendicular to the slit edges. This introduces the design of new beamstops, which are cross-shaped. Typical results obtained on the D2AM beamline of the ESRF are shown, especially for the case of coherent scattering.