On a New Test Method for Spherical Aberration of Electron Lenses

The focus of rays from a lens with spherical aberration appears as a spot surrounded by a discrete halo ring if a diaphragm with fine circular aperture is placed across the beam in front of the focus. The diameter of the halo allows an estimate of the magnitude of the aberration involved. The geometry of rays forming the halo is explained here by schematic drawings. The practical application of the halo test is illustrated by some examples. In particular, the negative spherical aberration produced by an electronic space charge in a saddle field lens is demonstrated.