Hubble Space Telescope Primer for Cycle 17

5.2 Target Acquisitions

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.

5.2.1 Target Acquisition without the Ground System

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 ACS, NICMOS, and STIS. The WFC3 does not have onboard acquisition capabilities. 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 viaþone of the above methods may still be possible withþSTScI groundþsystem support. This is accomplished by analyzing certain data and calculating and up-linking appropriate pointing corrections to the telescope. Two examples are

ACS coronagraphy target acquisitions

STScI staff support target acquisitions forþACSþcoronagraphy to ensure that the target is centered correctly on the coronagraphic spot which is observed to drift up to a few pixels per month. STScI monitors this drift and uplinks the appropriate slew offset to the spacecraft via real-time command. This offset is applied to the spacecraft position between the completion of the onboard target acquisition and the start of the science exposures.þRequest for this support is implicit in the specification of the ``Use Offset'' special requirement in the Phase II proposal.þþ

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.þIn 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. This technique is also referred to as the ``Reuse Target Offset'' technique (see Section 4.1.7 of the Call for Proposals).