5 OPERATION
5-26
EM210F-1
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The objective aperture must be properly inserted.
Fig. a shows a hole image when the objective aperture is properly inserted into the
electron beam path. In this figure, the background structure is readily discernible, and the
edge of the hole can be clearly observed all the way round. On the other hand, Fig. b
shows a hole image when the aperture is improperly inserted. In this figure, the image is
indistinct, the background structure exhibits unidirectional blurring, and the edge of the
hole appears as if astigmatism exists.
a: Hole image when objective
aper ture is aligned
b: Hole image when objective
aper ture is misaligned
Fig. 5.26 Effect of objective aperture insertion
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The objective lens astigmatism correction must be complete.
If astigmatism exists in the objective lens, the focusing becomes unidirectional, and it
will be impossible to focus the image correctly in all directions. To illustrate the point,
Fig. a shows a hole image when the lens is free from astigmatism, and Fig. b shows the
same image when lens astigmatism is present.
It will be seen that when the lens is free from astigmatism, the background structure is
clear and the edge of the hole can be readily observed all the way round; and that when
lens astigmatism is present, the image is blurred and the Fresnel fringe is asymmetrical
around the edge of the hole.
Astigmatism correction should be carried out at a magnification higher than that used for
actual photography. However, once astigmatism correction is complete, photography can
be carried out at lower magnifications without the need to repeat astigmatism correction.
If the amount and direction of astigmatism vary as time goes by, a dirty objective
aperture or dirty specimen holder is indicated. In this case, clean the contaminated
part/parts as per the maintenance manual.
Utilizing the background structure in conjunction with the Fresnel fringe (at slightly
over-focus) is extremely effective when correcting objective lens astigmatism at
magnifications of 100,000 times or more. That is to say, after first of all removing the
astigmatism as much as possible using the Fresnel fringe, focus the image and then
remove any remaining astigmatism so as to make the background structure as clear as
possible.
