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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See page3749, the ray aberration of a ray leaving an object point with a diffraction vector g is given by the displacement vector δ_r(g) from the ideal Gaussian image point to the point where the ray, after passing through the lens, intersects the image plane.

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.