The ACQ/PEAK Procedure
An ACQ/PEAK moves a small or narrow aperture in a pattern of steps centered on the target location determined by the ACQ. The aperture is stepped by a large fraction of its dimension in a one dimensional pattern (for long slits) or a two dimensional pattern (for small rectangular apertures). At each step in the pattern, the flux detected on a CCD subarray (which limits slit length for long slits) is measured. The lowest flux in the pattern is subtracted from all the fluxes to improve the centroiding on small-scale structure. The flux-weighted centroid of the pattern is chosen as the target location.
You must choose an optical element for the exposures. In most cases, the MIRROR is used to give undispersed light. Gratings can also be used for dispersed-light peakups in a strong emission line or for targets too bright for the minimum exposure time (0.1 sec) with the MIRROR. (Alternatively, in the latter case, the MIRROR can be used with an aperture that restricts the incoming light sufficiently, as discussed below.)
You must choose a long slit or an echelle slit as the aperture. Usually one uses the same slit for the ACQ/PEAK as for the science exposure, but a smaller aperture may be chosen when higher positional or photometric accuracy is required, when coronographic observations are made, or when the target is so bright that saturation in the minimum exposure time (0.1 sec) would occur. In the latter case, echelle slits with neutral density filters can also be used.
For the given optical element and slit, you must now determine an exposure time which gives adequate signal to noise and avoids saturation. Underexposure is the leading cause of poor centering, so care should be taken to meet or exceed the minimum recommended signal level. The exposure time must be at least as long as 0.1 sec, but shorter than a defined maximum time imposed to limit the effect of multiple coincident cosmic rays on the centering accuracy.
The STIS Target-Acquisition Exposure Time Calculator can be used to find the appropriate exposure time for imaging peakups of point sources. Use CLEAR for the filter, and adjust the result for aperture throughput.
The STIS Spectroscopic Exposure Time Calculator can be used to find the appropriate exposure time for dispersed-light peakups of point sources.