Practical Electron Microscopy and Database

A plane in which the all paraxial rays originating from the center of the object plane join in a single point is called an image plane. From the analysis of electron trajectories in electron lenses, for paraxial rays (small values of r in Figure 4275) the electron speed in the optic axis direction (v_z) is constant.

Figure 4275. Schematic illustration of rotationally symmetric magnetic fields and electron lenses.

Without introducing other multipole elements, the spherical aberration correction only by hexapoles can be done. In this case, the correcting system consists of round lenses and hexapoles only. Therefore, the Gaussian optics is exclusively determined by the round lenses because the hexapole fields do not affect the paraxial region.