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 to be used to obtain the science data and on the nature of the science program.
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.
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.
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.
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 and calculating and up-linking appropriate pointing corrections to the telescope. An example is a technique to avoid repeating multi-stage target acquisitions. Instruments that require multi-stage target acquisitions in order to center a target in a small aperture as in STIS, for example, consume a large fraction of the viewing time in an orbit. Target acquisition time may be saved if repeated observations of the same target are planned for multiple visits. In this mechanism, engineering and image data are obtained from an initial successful target acquisition. The telescope offset resulting from the acquisition is retrieved from the data and used to bypass the acquisition sequence in repeat visits as long as the guide stars of the initial visit are also used. Request for this support is implicit in the specification of the “Save Offset” and “Use Offset” special requirements in the
Phase II proposal.
