  Ptychography employs a set of diffraction patterns that capture redundant
information about an illuminated specimen as a localized beam is moved over the
specimen. The robustness of this method comes from the redundancy of the
dataset that in turn depends on the amount of oversampling and the form of the
illumination. Although the role of oversampling in ptychography is fairly well
understood, the same cannot be said of the illumination structure. This paper
provides a vector space model of ptychography that accounts for the
illumination structure in a way that highlights the role of the Wigner
distribution function in iterative ptychography.
