SPM Tip Convolution

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Why is it that tip convolution effects rotate with the scan angle in a Nanoscope-based AFM (and possibly other systems)? The tip certainly doesn't rotate in its holder as the scan angle is changed! I posed this question on the SPM mailing list, and got several answers in return, but I was still puzzled until I put all of them together and thought a while. However, I may not be the only puzzled soul out there, so let me attempt to summarize what I've come to understand:

A good way to think about this is to consider a righttriangle tip approaching a cube on a surface:

If the tip approaches the cube along the plane of the page screen, the left side of the cube will appear sharp in an AFM image and the right side of the cube will have a slant introduced into it due to a tip convolution effect.