An electron beam passing through a magnetic deflecting field is, in general, subject to astigmatism. For circular pole-pieces, this takes the form of focusing in the plane of deflexion; for square pole-pieces, focusing perpendicular to the plane of deflexion. Deflexion free from astigmatism can be achieved by means of circular pole-pieces from which semicircular portions have been removed. An application of this to reflexion electron microscopy is described.
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Archard et al. (1958) studied this question.