For moving targets, the HST
ground system assumes that the position of the target as seen from the center of the Earth can be approximated as a linear function of time. This is done using rates of motion calculated from the object's ephemeris for a time near the start of each orbit. If the target's geocentric ephemeris departs significantly from this linear approximation over the course of an orbit, this can lead to small but noticeable pointing offsets. If you think this may adversely affect your science, please consult with your PC about possible strategies for ameliorating this effect.
The correction for the variable parallax to a Solar System object due to HST
's orbit around the Earth is calculated on-orbit using the best ephemeris for HST
's orbit that was available at the time the detailed weekly schedule was created. However, the real position of HST
along its orbit often differs by up to 1 to 2 seconds (7 to 15 kilometers) from the predicted ephemeris position. This leads to small errors in the phasing of the calculated parallax correction. For example, when observing a Solar System object at a distance of 0.5 AU from the Earth, this can cause typical pointing offsets over the course of an orbit of order 20 to 40 milliarcseconds. As these offsets depend on unpredictable changes in HST
's orbit, such drifts are unavoidable when observing near-Earth objects.