It has been known since the early days of electron optics that the rotationally symmetric lenses employed in electron microscopes and similar instruments suffer from severe aberrations that cannot be eliminated by skillful lens design (Scherzer, 1936). Immense effort has been devoted to finding lenses with small aberrations and devising aberration correctors. The original demonstration that the two most important aberrations, spherical and chromatic, cannot be eliminated required that several conditions be satisfied and, by relaxing one or the other of these conditions, correctors can be designed. A nearexhaustive list was published by Scherzer (1947) and reviews charting trends in thinking about aberration correction and progress in implementing correctors are to be found in Septier (1966), Hawkes (1980), and Hawkes and Kasper (1989). These contain very full accounts of earlier attempts to correct aberrations with extensive reference lists and the material presented there is not always reproduced here. In particular, a survey of attempts to build apochromats and aplanatic lenses by H. Rose and colleagues in Darmstadt is to be found in the article by Hawkes (1980). The types of corrector that seem most promising today are examined below but first, we describe the various kinds of aberration and explain why they are important. We then look more closely at the aberration coefficients themselves, which leads naturally to a study of the correctors.
KeywordsSpherical Aberration Chromatic Aberration Aberration Correction Magnetic Lens Electrostatic Lens
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