Effect of three-fold astigmatism on high resolution electron micrographs

The effect of 3-fold astigmatism G on high resolution bright field imaging is explored through image simulation. Images of isolated atomic columns and of two practical specimens are computed for different 200 keV high resolution electron microscopes using EMS software modified to include a 3-fold astigmatism term. The visibility of the effect of 3-fold astigmatism on image contrast depends on the ratio G(Cs3/4λ1/4), and is therefore strongest for large G and small Cs. Several images calculated for G = 1 μm and Cs = 0.5 mm depart from the symmetry of the sample in ways that bear a close resemblance to commonly observed image contrast effects. The astigmatism also causes lattice images to be shifted. The shift is different for different crystal orientations. It therefore needs to be taken into account when one is measuring rigid lattice displacements across grain boundaries.