In addition to what others have said, the concern about pixel-imperfect star tracking is somewhat moot, as we're talking about two different cameras. The effect

In addition to what others have said, the concern about pixel-imperfect star tracking is somewhat moot, as we're talking about two different cameras. The effect on the imaging camera is very small. There is the star tracker camera (widefield, fast), and the imaging camera (narrowfield, slow). Star trackers use a fast widefield camera because it's easy to get a good signal:noise ratio (there is a lot of contrast between the stars and the background). The imaging camera, on the other hand, generally takes much longer exposures (HST subjects are generally very dim, relative to stars). All those beautiful, nebulous HST photographs you see? Those have exposure times on the order of hours or days. In practice, being off by a pixel momentarily is not a big concern -- the amount of "bad" photons you collect during that time is very small.