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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Coma-free alignment is almost influenced only by the objective lens, while the voltage-center is influenced by the whole lens system in the microscope. In HRTEM measurements, the voltage-centering alignment is applied to reduce the image intensity attenuation due to chromatic aberration of the projector lens (PL) or of the intermediate lens (IL). If coma-free alignment in TEM operations is needed, it is done before the alignment of the voltage centering or current centering.

In many commercial TEMs, both coma-free and voltage-centering alignments use the same beam deflectors above the objective lens for adjusting the direction of incident beam, the coma-free alignment is achievable only at the expense of the voltage-center alignment, and vice versa. However, it was also proposed [1] that the coma-free alignment can be obtained by using beam defectors above the objective lens, while the voltage-centering alignment is adjusted by beam deflectors below the objective lens. The object point corresponding to the viewing-screen center (image plane) is not generally a point along the axis of the objective lens-field due to the leakage-field as shown in Figure 1922. In this design, the deflector is introduced into the location of the image plane of the objective lens.

Figure 1922. Schematic illustration of voltage-centering alignment using deflectors.

[1] T. Yanaka. K. Shirota, A. Yonezawa and Y. Arai, in: Proc. 8th Int. Cong. on Electron Microscopy. Canberra. 1974. Vol. I. p. 128.