This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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A perfect lens ideally takes a plane wave from the electron source or EM specimen, produces a perfect spherical wave and then, focuses it to a point. To illustrate the geometric aberrations, the first-order (or Gaussian) trajectories are normally extracted from the actual aberrated trajectories. It is reasonable to consider that the first-order trajectories are the ray paths that depend only on simple focusing (spherical wave) and lead the positions and magnifications of the image at the imaging planes as the electron rays progress down the EM column. The first-order trajectories determine the ideal focusing, while the high-orders are the deviations with distances from the optic axis (in the case of geometric-aberration). In other words, the aberrations are the deviations from these ideal (first-order) trajectories [1]. This also means that the first-order focusing has a different status and more fundamental effects than the higher order aberrations.