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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Sextupole elements can be conveniently used to produce three-fold symmetry of electromagnetic fields. These elements are normally employed in charged-particle optics to compensate for the primary second-order aberrations arising in the systems with a curved optical axis, such as spectrometers or imaging energy filters.