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The combination of a Gerchberg-Saxton phase retrieval algorithm with a genetic algorithm is used to retrieve the phase of a TEM or STEM probe and pinpoint its defocus. Results from modelled data and initial experimental work are included.

Ptychographical iterative phase retrieval is a promising new transmission imaging technique which uses a set of measured intensities from the consecutive illumination of overlapping regions of the specimen to form an image of the transmission function in phase and amplitude. Although the technique has been shown to work very effectively in both the light-optical and X-ray domains, electron ptychography poses significant difficulties, not least of which is the uncertainty in probe position due to drift and other instabilities. We demonstrate three methods for deriving the relative positions of the illumination spot on the specimen a-posteriori.

A possible configuration for undertaking diffractive imaging microscopy in the conventional scanning transmission electron microscope (STEM) is to defocus the probe by a large distance in order to minimise the number of diffraction patterns required to reconstruct the ptychographical image. Although the characteristics of STEM probes have been well explored and measured near the beam crossover, they are rarely observed (or calculated) at very large defoci. In this paper we compare probes calculated under a variety of approximations with measured data from a JEOL 2010F in STEM mode. When the illumination aperture is narrow and the defocus is large (near parallel illumination useful for ptychographical imaging) the Ronchigram cannot be used easily to characterize aberration, We develop an alternative method of estimating the effective aperture size and condenser lens spherical aberration.