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Particles can sometimes move as a result of the stage motion (as in Figure 3), so it is not sufficient just to subtract the two images. In at least one case, the substrate itself rotated between images.
Originally posted by Phage
The samples are positioned on a wheel which holds 60 samples. The samples are "stuck" to a sticky silicone coating. The wheel sits on a stage which moves in order to focus the image.
Click the image to see the whole apparatus.
Particles can sometimes move as a result of the stage motion (as in Figure 3), so it is not sufficient just to subtract the two images. In at least one case, the substrate itself rotated between images.
Source
It looks like that's what's happening here. The larger particles, which may not be very well stuck to the sticky substrate, move a bit when wheel and/or stage moved between shots.
[edit on 27-9-2008 by Phage]