5.2 Target Acquisitions

5.2.1 Target Acquisition without the Ground System

5.2.2 Target Acquisition with Ground System Support

Target acquisition is the procedure used to ensure that the target is in the field of view of the requested aperture to the level of accuracy required by the observer. There are several distinct methods of target acquisition; each method has a different approach and different accuracy and will take different amounts of time and resources to complete. The required level of accuracy depends on the size of the aperture used to obtain the science data and on the nature of the science program.

5.2.1 Target Acquisition without the Ground System

Blind acquisition

Onboard acquisition

Early acquisition

Blind acquisition

For blind acquisition, guide stars are acquired and the FGSs are used for pointing control. The pointing is accurate to the guide star position uncertainty, which is approximately 0.3" RMS, plus the instrument-to-FGS alignment error.

Onboard acquisition

For onboard acquisition, software specific to the scientific instrument centers the fiducial point onto the target. Onboard target acquisitions are needed for COS and STIS spectroscopic observations (except slitless) and all coronagraphic observations with STIS. WFC3 does not have onboard acquisition capabilities, which means that all WFC3 target acquisitions are blind acquisitions. For specific information on methods and expected pointing accuracies, see the HST Instrument Handbooks.

Early acquisition

For early acquisition, an image is taken in an earlier visit and analyzed by the PI. The PI may then update the target coordinates in the Phase II proposal for use with subsequent visits.

5.2.2 Target Acquisition with Ground System Support

Target acquisitions that cannot be accomplished successfully or efficiently via one of the above mentioned methods may still be possible with STScI ground system support. This is accomplished by analyzing certain data, calculating and uplinking appropriate pointing corrections to the telescope.

One example is a technique to avoid repeating multi-stage target acquisitions for an object that will be observed in several visits with the same instrument configuration, using the same guide stars. Multi-stage target acquisitions to center a target in a small aperture (as in STIS) consume a large fraction of viewing time in an orbit. However, if the offsets are determined from engineering and image data obtained from the initial successful target acquisition, those offsets can then be applied to subsequent visits. Request for this support is implicit in the specification of the “Save Offset” and “Use Offset” special requirements in the Phase II proposal.