OBSERVATION OF LENS ABERRATIONS FOR VERY HIGH-RESOLUTION ELECTRON-MICROSCOPY .1. THEORY

Least-squares fitting methods are described, currently the most accura te known, for determining the imaging aberrations and other important parameters. The data required are either the image displacement or the image diffractogram shape (or both), as a function of injected beam t ilt; the analysis is largely common to the two cases. Explicit solutio ns are given for some simple cases (e.g. coma-free alignment on the ba sis of three images only) and some promising developments are consider ed; the relevant aberration theory is set out in detail. Experimental work reported in a companion paper has confirmed the presence of three fold astigmatism at significant levels, but has not shown any evidence of other higher order aberrations.