Stereo Scan 440 - DCU

3. The retractable backscattered electron detector occupies a fmite space
beneath the fmallens. All parts of the sample should therefore be just
clear, particularly when the dete~r is to be moved in or out of position.
A safe working distance is 12mm.
,4.- The geometry used for X-ray analysis, at a 350 take-off angle, dictates
that the surface to be analysed should be at a working distance of 25mm.
~ An improvement in depth of focus, the vertical distance over which the
sample appears to be in focus, can usually be made by increasing the
working distance. An increased working distance has the effect of
decreasing the angular aperture of the electron beam. (See Figure 3.3)
BEAM
FIgUre 3.3 Enhancement of Depth of Focus
Item 5 above directly conflicts with item 1. However, improvements in depth of
focus are usually required at relatively low magnifications where the minimal
loss of resolution caused by using a longer working distance would not be
noticed. Depth of focus can also be improved by using a smaller aperture, say
30Jlm or 20Jffi1, or by selecting Optibeam depth of focus mode. This selects the
lens settings which give the greatest depth of focus while maintaining the
selected probe current.
Stereoscan 440 Operator Manual