STEM probe characteristics at large defoci for use in ptychographical imaging

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.