|COS Instrument Handbook for Cycle 25|
8.1 The COS science apertures are 2.5 arcsec in diameter. An observation will yield high-quality data only if the target is properly centered in the science aperture. This chapter discusses the available target-acquisition (TA) methods, demonstrates the dependence of data quality on centering accuracy, and recommends acquisition scenarios for various combinations of target coordinate accuracy and brightness. Recommended parameters for all COS TA modes are presented in Section 8.9.Based on improvements in the COS-to-FGS alignment, together with an analysis of the distribution of positional errors in the GSC2, we estimate that an HST guide-star acquisition will place a target with good coordinates in the COS aperture 98.5% of the time. As a result, we do not require that all COS observations begin with an ACQ/SEARCH sequence if the uncertainty of the target coordinates is ≤ 0.4 arcsec. We do, however, recommend in almost all cases that some sort of target acquisition be performed to center the target in the aperture. The APT (the Astronomer’s Proposal Tool) will issue a warning if an acquisition is omitted. Target acquisition is required only once for a series of observations in contiguous orbits (i.e., once per visit). Moving targets require an acquisition at the beginning of each orbit.The COS detectors are vulnerable to damage or performance degradation if exposed to too much light. Imaging acquisitions present a special risk because they concentrate the light of an object on a small area of the detector. Users of COS must demonstrate that their targets are safe. Information on bright-object protection and screening is provided in Chapter 10.