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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Sextupoles cannot be used to correct spherical aberrations if both the signs of the aberrations generated by sextupoles and round lenses are the same. Unfortunately, this is exactly the case if the second-order aberrations are eliminated. However, it is possible to reverse the sign of the spherical aberration by employing a round lens doublet.