Observers are responsible for the accuracy and appropriateness of the coordinates they supply and any changes made to them. Only you can determine where we should point HST.
Equinox is always a critical quantity when specifying coordinates. All astronomical coordinates change with time because of the precession of the Earth’s rotation axis. Equinox specifies a time to which a coordinate system is tied. Thus J2000 refers to the location of an object in celestial coordinates for the coordinate reference frame of January 1, 2000.
Epoch is generally important only for objects that move. In particular, the epoch of a star’s coordinates refers to a specific time at which the star is at that location. For example, the Hipparcos output catalogue lists coordinates of the brighter stars in the ICRS reference system (which is very nearly the same as J2000), and the coordinates themselves are for epoch 1991.25. The proper motions in the Hipparcos output catalogue are also epoch 1991.25, the midpoint of the mission. If you specify the epoch and equinox correctly, we can easily compute the location of an object at the time it will be observed with HST. (Note: An epoch is purely a time, and one of the form "J1991.25" is nonsensical.)
Proper motions sometimes cause problems. Units are especially crucial. The proper motion in Right Ascension (
RA_PM) can be in sec. of time/yr, arcsec/yr or mas/yr.
DEC_PM can be listed in arcsec/yr or mas/yr.
As we have emphasized above, we urge you to use signs on quantities, even when they are not required. A value of +0.060 for
RA_PM, say, makes it clear that the sign has been considered. Just specifying 0.060 leaves ambiguity because sometimes observers forget a minus.
Targets near the Celestial Poles: Be very careful if your target lies near a celestial pole. Many precession routines break down in this regime, and uncertainties in position can cause problems too. Also, patterns that you may execute with an instrument could cross the pole, leading to confusion in position. All these issues can be resolved, but careful attention is needed.
Targets in Binaries with Large Orbital Motion: If a target’s apparent orbital motion is large, the position and proper motion of a target in a catalogue may not characterize the target’s location to the accuracy required by the target acquisition aperture. For such objects, a custom proper motion calculation is recommended to ensure the target will be in the target acquisition aperture at the time of the observation.
