For FOS-1 the image of a spectral line moves by about
0.03 pixels per hour in the dispersion direction as the telescope tracks
through zenith, and the worst flexure in a test tracking 
5 hours
RA is 0.15 pixels per hour.
For FOS-2 the image of a spectral line moves by about
0.02 pixels per hour in the dispersion direction as the telescope tracks
through zenith.
Since the design philosophy of FOS calls
for integrations to be broken up into periods of no more than 2000
seconds
this corresponds to a shift of less than 0.01 pixels in a typical
integration.