Abstract
Contrast and resolution are the most crucial parameters in biological electron microscopy. These parameters are sensitive functions of spherical aberration and astigmatism. Axial astigmatism is produced by a rotational asymmetry in the axial magnetic field distribution Bz (r, ) in the objective pole piece, and is adjusted with a stigmator that locally compensates for the rotation asymmetry. Spherical aberration is frequently determined from a measurement of the on-axis field distribution only; it is presupposed that the axial field distribution is rotationally symmetric and that the pole piece axes are coliniar straight lines.Commercial probes used to measure axial magnetic fields in conventional electron microscope pole pieces are so large that off-axis fields cannot be measured, and consequently the degree of rotational symmetry cannot be assessed.
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