5 OPERATION
EM210F-1
5-27
a: Hole image when
astigmatism does not exist
b: Hole image when
astigmatism exists
Fig. 5.27 Effect of objective lens astigmatism
■
The specimen must be tightly secured.
If the specimen is not tightly secured, it will result as time passes in a gradual shift of the
specimen (image). This phenomenon is referred to as “image drift”. Fig. a shows a hole
image under drift-free conditions, and Fig. b shows the same image when image drift
exists. In the former, the background structure is distinct, and the edge of the hole can be
clearly discerned all the way round.
In the latter case, the background structure appears to be unidirectional, and the edge of
the hole exhibits one-way blurring.
a: Hole image when there is no drift
b: Hole image when there is drift
Fig. 5.28 Effect of image drift
The chief causes of image drift and the countermeasures to be taken are as follows:
·
Damaged, wrinkled, improperly secured and/or insufficiently strong supporting film
Exercise great care when preparing the supporting film. Use adequate adhesive when
securing the film to the grid, and reinforce the film using the carbon coating method.
·
Bent specimen grid
Discard and replace with new (unbent) grid.
·
Electrically charged specimen and/or supporting film
Enhance conductivity by coating the specimen and/or supporting film with carbon.
