Astigmatism occurs when the electrons sense a non consistent magnetic field as they spiral round the optic axis. As shown in the three-dimensional schematic the two planes at right angles to one another passing through the optical axis are the meridian plane and the sagittal plane, the meridian plane being the one containing the off-axis object point. Rays not in the meridian plane, called skew rays, are focused farther away from the lens than those lying in the plane. In either case the rays do not meet in a point focus but as lines perpendicular to each other. Intermediate between these two positions the images are elliptical in shape. This happened because the soft iron polepieces are not ideal cylindrical and ideal symmetrical. The soft iron may also have micro structural heterogeneities which cause local variations in the magnetic field strength. In addition, if the apertures are not exactly centered on the axis, it can also disturb the field. Moreover, if the apertures are not clean, the contamination charges up and deflects the beam. Astigmatism distorts the image by an amount rast, where:
rast=βΔf
- β is the maximum semi angle of the collection of the objective lens aperture.
- Δf is the maximum difference in focus induced by the astigmatism.
Astigmatism is simply corrected using stigmators, which are small octupoles that set up a compensating field to balance the heterogeneities causing the astigmatism. There are stigmators in both the illumination system (condenser lens) and the imaging systems (objective lens).
There are several minor defects, such as barrel and pincushion distortion, which also deform the image. They are irregularly seen at very low magnification where electrons traveling close to the bore of the polepiece appear in the image.
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