We have a Leica rapid movement z-stage which performs the z translation by
rotating about a pivot at one end. This mechanism can cause several artifacts
which are not very well documented (at least- I didn't know about them).
One is that there might be some motion in the y direction due to the rotation-
this is neglible when the specimen is centered at the level of the stage since
the angle is small and the effect goes as the cosine. However, if the specimen
is raised (as in our case by 30 mm) or lowered from the level of the stage
then the effect goes as the sine, leading to a skewing of thick sections.
Another is that the z translation depends on the distance from the pivot.
Again, this is not a problem if the specimen is centered.
Cheers
Alistair