In the field of diffraction microscopy, a coherent illuminating beam of finite extent impinges on a specimen and the resulting diffraction pattern is recorded. The complex transmission function of the specimen is recovered using iterative algorithms that exploit redundancies in the measured data. This is normally oversampling of the diffraction pattern when it is known the object or illumination is of the finite size. In the case of curved illumination, there is no direct relationship between the collection angle and the resolution of the recovered image. The result is a recovered image with varying resolution over the field of view as different parts of the object are illuminated by different wave-vectors. An extension of the Coherent Diffractive Imaging (CDI) technique (employing a single diffraction pattern) is to use multiple diffraction patterns collected from adjacent parts of the object and is called ptychography. In ptychography, translation of the illuminating wave across the specimen introduces translational diversity that leads to faster convergence of iterative phase retrieval as well as extending the field of view. In this paper we investigate the expression of resolution information in the diffraction pattern using curved illumination in order to facilitate specimen recovery with uniformly improved resolution over the entire field of view.
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