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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In advanced STEM-EELS systems, the monochromators induce an energy spectrum of the electron source at a dispersion plane, where a small fraction of the electron distribution is selected by a narrow energy-selecting slit for the measurements as shown in Figure 1716. A drawback of this method is that the electron beam width is increased in the dispersion direction as shown in Figure 1716 (a). In some high-dispersion designs [1], this large dispersion is compensated and the electron beam is reduced to its original size by another monochromator which produces an achromatic image of the electron source as shown in Figure 1716 (b).

Figure 1716. (a) Non-compensated EELS, and (b) Dispersion compensated EELS.

[1] Tsuno, K., 2000. Electron optical considerations of a 1 Å STEM with a monochromator. Microsc. Microanal. 6 (Suppl. 2: Proceedings), 142–143.