|COS Instrument Handbook for Cycle 26|
8.10 In some situations an observer may need to obtain an independent ultraviolet image of a region in order to ensure that no objects violate safety limits and that the target to be observed can be acquired by COS successfully. Such an early acquisition should be included in the Phase I proposal, and the observation should not use a photon-counting detector. The UVIS channel on WFC3 is recommended, but observers are encouraged to consult with an STScI instrument scientist.8.10.2 Extended or Multiple TargetsBecause most COS target-acquisition schemes were developed with the implicit assumption that the target is a point source, acquisitions of extended or multiple sources may require more careful planning.If the target is sufficiently uniform and its coordinates are well known, then a target acquisition may not be required. To ensure that the brightest region of an extended source falls into the aperture, an ACQ/SEARCH with CENTER=BRIGHTEST may be sufficient (Section 8.3). ACQ/PEAKXD acquisitions with the NUV detector should be avoided for extended sources, because light from the three spectral stripes will overlap on the detector (Section 8.5). The pattern of dwell points for ACQ/PEAKD and FUV ACQ/PEAKXD should be tuned to the extent of the target.Imaging acquisitions of extended sources are subject to special bright-object screening procedures. If an ETC calculation shows that the source is below the NUV local count rate limit (50 cts/s/pixel) under the assumption that it is a point source (the worst-case scenario), then it passes. If it does not pass as a point source, but the maximum local count rate as an extended source is <5.9 cts/s/pixel, then the visit is safe. Above 25 cts/s/pixel, the visit is unsafe. Between 5.9 and 25 cts/s/pixel, the visit will be checked for safety by multiple reviewers. See this webpage for full details.Complex targets—such as two stars with very small angular separation, multiple bright knots, etc.—may confuse the acquisition algorithms. In such cases, consider an offset target acquisition, discussed in Section 8.10.3. Take care when doing target acquisition for a target in a binary system since it is possible for the pointing to end up between the two stars. The coordinates of the target must be corrected for the orbital phase of the target in the binary system.8.10.3 Offset Target AcquisitionsWhen targets are faint or lie in crowded fields, direct acquisition of the primary science target may be difficult or uncertain. In such cases, an offset acquisition, in which acquisition of a nearby field target is followed by a short slew to the science target, may be appropriate.The size of the offset is limited by the requirement that the guide stars remain within the fields of view of their respective FGSs. Offset acquisition slews routinely involve displacements up to 1 arcmin and can be larger. Offset slews have a typical accuracy of ±0.003 arcsec. The centering of the initial offset target should be refined (via either ACQ/IMAGE or ACQ/PEAKXD+ACQ/PEAKD) before the offset maneuver. For offset acquisitions, bright-object considerations apply over the entire offset region. Refer to Chapter 9 for a discussion of the modest overheads associated with the offset-acquisition spacecraft movement.In unusual cases, including highly uncertain target coordinates or knotty, extended sources for which high wavelength accuracy is required, an offset target acquisition, followed by an additional ACQ/IMAGE or ACQ/PEAKXD+ACQ/PEAKD on the primary target, may be employed.Should any stage of the TA fail or a Local Rate Check (LRC) violation occur during a TA exposure, then the subsequent acquisition procedures in that visit (such as ACQ/PEAKXD or ACQ/PEAKD) will not be executed, but the science exposures will still occur. Note that HST will be left pointing at the last commanded position, which may differ substantially from the initial pointing.Many quantities useful for evaluating the success of COS TAs are recorded in the COS TA data products (the _rawacq and _spt files). Table 5 of COS ISR 2010-14 lists these keywords and their meanings.