Target Acquisition with STIS
Because all STIS spectroscopic aperatures are smaller than 3 arcseconds, STIS spectroscopic and coronagraphic observations require an acquisition (ACQ) and possibly and acquisition peak up (ACQ/PEAK) to center the slit or coronagraphic bar on the target. The STIS target acquisition uses the CCD camera to image the target and the on-board software allows for locating the position of the target.
Measuring Accurate Positions
Before observing even starts, 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 the Phase II Proposal Instructions.
Acquisition of the Target
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 an ACQ exposure to center the target in the science aperture. For V < 21 mag, the nominal accurancy of a point source acquisition is 0.01 arcsecond.
Resources and Information
Below are helpful resources and information regarding target acquisitions with STIS. For even more information, please see the STIS Instrument Handbook.
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, which also provides examples of target acquisitions.
Considering the structure of the source, you must choose the ACQTYPE (POINT or DIFFUSE), the CHECKBOX size (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 are similar to fixed target acquisitions, with a few caveats on the selection of acquisition offset targets and on coordinate specification.
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.
Peaking Up on the Target
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.
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.
Parameters for the ACQ/PEAK
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.