STScI currently uses the best planetary system ephemerides available from JPL. The accuracies of these ephemerides are posted at http://ssd.jpl.nasa.gov/sat_eph.html. Note that we have just acquired the newest ephemerides for the Galilean satellites of Jupiter (JUP166 in the ephemerides web page mentioned above). The RMS accuracies of the positions of these satellites relative to Jupiter have improved substantially since the previously used ephemerides, from 0.04 - 0.17 arcsec to 0.002 arcsec.
In planetary system spectroscopic observations using STIS, it is sometimes necessary to use a satellite as an acquisition object for an offset to a science target on another satellite or on the planet. When planning such an observation with STIS, one should consult the help desk to see whether or not the ephemeris files in use at STScI will be able to provide an offset accurate enough for the chosen slit size.
Observers should be aware that moving targets are tracked by HST with the assumption that the object is moving on a linear track with constant velocity. Depending on the visit structure, the same linear track may continue through several exposures. The scheduling software sets a generally appropriate limit on the duration of a linear track (which depends on the target being tracked, but not on details of the target motion at the time). In some situations (for example, near the inflection point of an orbit), the approximation could result in inadequate tracking when using a narrow slit. If this may be a problem, the observer can consult the Program Coordinator of his/her proposal to find out how long the same track will be used for a series of exposures, and whether it is possible to break the visit into shorter tracks. Alternatively, one can plan to observe during a more linear part of the orbit if possible.