The ACQ Procedure
The ACQ procedure begins with a 5 x 5 arcsecond (100 x 100 pixel) STIS CCD subarray image of your point source target, or an appropriately larger image of your extended target. A source with well measured coordinates should be within ~1-2 arcsec of the center of this image. The image is boxcar-summed over a "checkbox": 3 x 3 pixels for a point source, or your selection of N x N pixels for an extended source. The geometric or flux-weighted center of the brightest checkbox, assumed to be the target location, is used to recenter the target, and a second image is taken. The target location is determined again, and a small move is made to center the target in the science aperture (slit). (If a brighter companion object appears in either image, you will acquire it instead - in which case you should use an offset target acquisition strategy.) The acquisition procedure is described in more detail in the STIS Instrument Handbook. Examples of acquisitions are also given there.
Parameters for the ACQ
Considering the structure of the source, you must choose the ACQTYPE (POINT or DIFFUSE), the CHECKBOX sixe (3 is automatic for POINT, 3 to 105 for DIFFUSE), and the algoithm for choosing the center of the brightest CHECKBOX (FLUX-CENTROID is automatic for POINT, DIFFUSE-CENTER = GEOMETRIC-CENTER or FLUX-CENTROID for DIFFUSE).
You must select the aperture. The F28x50LP aperture is generally recommended for sources with V magnitude between 10 and 23, but other apertures can be used for special cases. The minimum exposure time allowed for an acquisition is 0.1 sec, so you may need to select a different aperture if your source is exceptionally bright.
You must choose the exposure time, being careful to get sufficient signal to noise (40 over the checkbox for a point source) while avoiding saturation (144,000 electrons per pixel for the required CCDGAIN=4).
Solar System Acquisitions
Solar system acquisitions are similar to fixed target acquisitions, with a few caveats on the selection of acquisition offset targets and on coordinate specification.
Helpful Tools
The STIS Target-Acquisition Exposure Time Calculator can be used to find the appropriate exposure time for point sources using the allowed apertures (filters).
The STIS Target Acquisition Simulator can be used to check the performance of the acquisition algorithm on a STIS image of the target field, or on a reasonable approximation thereof.