5.2 Target Acquisitions

Target acquisition is the method used to insure 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 acquire 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 1" rms, plus the instrument-to-FGS alignment error, which is currently <0.5" for all FGSs for STIS.

Onboard acquisition

For onboard acquisition, software specific to the scientific instrument centers the fiducial point onto the target. Onboard target acquisitions are needed for all STIS spectroscopic observations (except slitless), and also for coronagraphic observations with ACS, NICMOS and STIS. The WFPC2 does not have onboard acquisition capabilities. For specific information on methods and expected pointing accuracies, see the HST Instrument Handbooks (see Section 1.2 ).

Early acquisition

For early acquisition, an image is taken in an earlier visit to provide improved target coordinates for use with subsequent visits.

5.2.2 Target Acquisition with the Ground System

Target acquisitions that cannot be accomplished reliably or efficiently via one of the above methods may still be possible by transmitting the relevant data to STScI, analyzing them to determine the needed pointing corrections, and then providing those corrections to the telescope. This description covers two kinds of activities, the "real-time target acquisition" and the "reuse target offset", both of which are described briefly here. You should contact the STScI Help Desk (see Section 1.3 ) if either of these capabilities is required.

Real-time target acquisition

This method is available for all scientific instruments except the FGS, but generally used only in exceptional circumstances. High data rate TDRSS links are required at the time the data are read out to transmit the data to the ground, and at a subsequent time to re-point the telescope before the science observations, all of which adds a constraint to the scheduling. The PI, or a designated representative, must be present at STScI at the time of the acquisition. The acquisition data, usually an image, are analyzed by STScI support personnel to compute the image coordinates and the centering slew for the target identified by the PI.

Reuse target offset

An offset slew -derived from an onboard acquisition, or an image done on a previous visit-is used to reduce the amount of time required for acquisitions in subsequent visits to the same target. The data from the initial visit are analyzed by STScI support personnel to provide the offset slew to be repeated for subsequent visits. All subsequent visits to the target must use the same guide stars as the initial visit, which limits the time span of all visits to a few weeks.