Measuring Accurate Positions
First, of course, you must have an accurate position for your target. For stationary targets, you must measure the coordinates in the Guide Star reference frame and include the PLATE-ID in the target information section of the Phase 2 Proposal. If you measure positions from WFPC2 or STIS images, you can get the PLATE-ID using files included with those datasets in the archive. If the target-centering procedures described below would not work well for your target because it is extended, complex, or near a brighter object, you will need to measure the coordinates of an offset target and measure the displacement from that object to your science target. For information on specifying the positions of solar system objects, see Chapter 4 of the Phase 2 Proposal Instructions.
Once the telescope acquires the guide stars for your target, your specified target position will be within ~1-2 arcsec of the aperture center. For observations taken through slits which are smaller than 3 arcseconds in either dimension, and for use of the coronographic bars, you will need to specify a target acquisition exposure (ACQ) to center the target in the science aperture. For V < 21 mag, the nominal accurancy of a point source acquisition is 0.01 arcsecond.
If you use slits less than or equal to 0.1 arcsecond wide or the narrow coronographic bars, and you want to center a point or point-like source more accurately (to about 5% of the slitwidth), you should follow the acquisition with a peakup procedure (ACQ/PEAK). You may also want to do a peakup on your object if you have moved to it from an acquired offset star, in case there are uncertainties in the offset. If your observations with narrow slits or coronographic bars span more than 4 or 5 orbits, you should repeat the peakup to compensate for the drift of the telescope.