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crystal diffracting conditions will appear in the
diffracted discs. As focus is more closely approached,
the magnification of the images in the discs will increase
until, at focus, the magnification in the discs reaches
infinity. By watching the image "blow-up" in the disc,
the exact location of the beam on the sample can be
monitored. This technique is referred to as the shadow
technique (Steeds, 1979). The image in the transmitted
disc is the shadow image. This method is the only sure
way to eliminate diffraction error that is generally
present if the probe is focussed in the imaging mode
rather than in the diffraction mode, as just described.
(Diffraction error is the noncoincidence of the probe
position in the imaging and diffraction modes
respectively.) If the probe is formed by overexciting the
objective lens, the resulting out of focus image is the
shadow image. There is thus no diffraction error if this
latter technique is used to form the convergent probe.
Diffraction error is not a serious problem and can be
easily eliminated. In any event, it does not affect the
information that is present in the pattern, only the area
from which the information is taken. This information is
usually confined to the discs of the diffraction pattern.
There can be considerable detail in both the diffracted
and transmitted discs, depending on the diffracting
conditions under which the pattern is formed.