Schematic showing the optical layout of the beamline and the BNP (not to scale). Sh and Sv are the horizontal and vertical source sizes, respectively, while dzp is the zone plate diameter. La is the distance from the source to the horizontal white-beam slits (with a width dh, which serve as a secondary X-ray source in the horizontal direction). L is the distance from the source to the zone plate. Adequate spatial coherence for nanofocusing is provided when the full source size multiplied by the full acceptance angle phase space area is no larger than the X-ray wavelength λ, or dhdzp/(L − La) < λ in the horizontal and Svdzp/L < λ in the vertical direction. This condition is met by the APS source size in the vertical direction and can be reached in the horizontal direction by adjusting the width of the white-beam slits dh.