How accurate are dithers between observations?
For observations within a single visit of less than 8 orbits, the dither accuracy is about 3 milliarcseconds. For programs exceeding 8 orbits, or for different visits to the same target, position errors up to 500 milliarcseconds and field rotations up to ~0.1 degree can occur, although experience indicates the errors are typically tens of milliarcseconds. Note that large dithers will incur other errors. The camera distortion increases with strength toward the CCD corners, and alters the image scale by about 2% at the corners. Hence a 1.993 arcsecond dither will be 20.0 WFC pixels at the field center, but suffer a 0.4 pixel error at the CCD corners. The individual CCDs are misaligned by up to ~0.5 degrees from their nominal orientations, and again, this implies errors when attempting to dither by certain pixel amounts. A POS TARG = 1.993, 0.000 arcsecond dither in X on WF3 would cause spurious motion in Y of 0.17 pixel on WF4, due to the rotation. Large dithers may also require a differ
