ABSTRACT
Given that electron lenses are already of high optical quality, it is now clear that the corrections being made to the wavefront by a spherical aberration corrector are relatively small and need to be extremely precise. In order to function effectively, an aberration corrector also needs to compensate for the additional "parasitic" aberrations that arise from imprecise machining and assembly, and magnetic inhomogeneities. Practical aberration correction has only now become practical through the development of techniques for measuring and correcting the parasitic aberrations, which has itself required the development of computers fast enough to make such measurements in a practical length of time.
