... HST . ... Data . ... HST Overview . ... STIS . ... STIS Data Handbook . ... STScI currently uses the best planetary system ephemerides available from JPL. ... 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. ... 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...