ABSTRACT
This chapter provides explicit information about the geometric and chromatic aberrations of the main types of optical element. Various methods have been proposed and tested, admittedly with unequal thoroughness, to correct the principal aberrations of round lenses. The chapter also provides formulas for the aberration coefficients of the principal electron optical elements: round lenses, quadrupoles, octopoles, sextupoles, combined focusing and deflecting elements, energy analyzers, and mirrors. Aberrations of position and gradient are both considered, and systems with and without a limiting aperture are likewise both envisaged. The Wien filter is a device that can be used as a monochromator or as an energy analyzer or even as an aberration corrector. The chapter focuses on two important aspects of aberration studies, namely, matrix representation and the effect of system symmetry. There has been a revival of interest in the aberrations with the development of computer alignment, adjustment, and control of the electron microscope.
