Coherence requirements for multislice coherent imaging of large specimens

Coherent X-ray imaging methods such as ptychography and holography can provide 3D images of millimeter-thick specimens at nanoscale resolution when combined with multislice reconstruction methods, which account for wavefield propagation through the full sample depth rather than using a simple projection of optical effects along the illumination direction. Here, we derive the transverse and longitudinal coherence requirements for multislice imaging, based on the illumination wavelength λ, sample thickness t, and the target spatial resolution δ_r. The sample can be larger than either the coherence width or length, and the required coherence lengths can be achieved using either multilayer or crystal monochromators depending on thickness t. When considering our findings in the context of tomography, we find that the number of rotation angles required for full angular sampling depends on resolution δ_r and wavelength λ, but not on sample thickness t. These results confirm the feasibility of providing sufficient coherence for 3D X-ray imaging of samples that are much thicker than the normal depth-of-focus limit.