|COS Instrument Handbook for Cycle 25|
• Coordinate accuracy and target brightness will inform your choice of target-acquisition strategy and optional parameters. Imaging acquisitions are more precise and often faster, but restrictions on the local count rate (Chapter 10) can prevent their use. While the TA modes can be used in any order or even repeated, the recommended strategies are given in Table 8.1. We suggest evaluating these strategies in the following order:
3. The scenarios outlined here are for isolated point sources. See Section 8.10 for additional information regarding crowded or complex fields and offset-target TAs.Table 8.1: Basic COS Target Acquisition Strategies
2 × 2 × 1.767” ACQ/SEARCH
• It is the responsibility of the observer to provide coordinates and proper motions with the required accuracy. Be especially mindful of nearby white dwarfs, which generally have high proper motions, and binary stars, whose motions on the sky are highly non-linear. Observations that fail because of an inaccurate target position will not be repeated. STScI cannot be responsible for target-coordinate or proper-motion errors in published or on-line catalogs, or in the literature. If there is any doubt that the available coordinates meet the required accuracy, then an ACQ/SEARCH should be performed.
• If a target falls near the edge of the aperture at the initial pointing, the TA algorithms may slightly miscalculate its position. Users who require the best possible photometric or absolute wavelength accuracy may wish either to begin with an ACQ/SEARCH to ensure that the target is reasonably well centered before the final stages of the acquisition are performed, or to perform additional acquisition exposures in case the observation at the initial pointing was partially vignetted.
• For ACQ/IMAGE exposures, both the preliminary and confirmation images are downlinked and delivered to the observer. For the other three TA modes, no images or spectra are recorded.