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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.

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.

LAST UPDATED: 08/27/2021

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