COS Target Acquisition Strategies

Quick Guide
COS has four acquisition modes or exposure types that you may use:

• ACQ/IMAGE obtains an NUV image of the field after the initial HST pointing, determines the telescope offset needed to center the object, and secures a second identical NUV image as a confirmation after the telescope movement (details are in Section 7.5 of the COS IHB). This is generally the fastest method of COS target acquisition. This is the recommended acquisition mode for most observations, but some targets may be too bright.

• ACQ/SEARCH performs a search in a spiral pattern by executing individual exposures at each point in a square grid pattern (details are in Section 7.6.1 of the COS IHB). This mode can use either dispersed light or imaging exposures.

• ACQ/PEAKXD determines the centroid of the dispersed-light spectrum in the direction perpendicular to dispersion (details are in Section 7.6.3 of the COS IHB).

• ACQ/PEAKD centers the target along dispersion by executing individual exposures at each point in a closely-spaced linear pattern along dispersion (details are in Section 7.6.4 of the COS IHB).

ACQ/PEAKXD should always precede ACQ/PEAKD and the two should always be performed together in sequence. Typically ACQ/PEAKXD and ACQ/PEAKD sequences should be preceded by ACQ/SEARCH.

These acquisition modes are used in several different ways that depend on the accuracy of the target coordinates and whether or not the target may safely be observed with either MIRRORA or MIRRORB. See Section 7.3 of the COS IHB for more information.

Scenarios
These acquisition modes are used in several different ways that depend on the accuracy of the target coordinates and whether or not the target may safely be observed with either MIRRORA or MIRRORB. These scenarios are described in detail below.

1. The most favorable case is when accurate GSC2 coordinates (to better than 1 arcsec on the GSC2 system) are available and MIRRORA or MIRRORB may be used safely. In this case ACQ/IMAGE may be employed and will result in the most efficient COS acquisition strategy. No peakups are required after an ACQ/IMAGE.
2. In the next case, scenario 1 applies, but a conservative approach concerning target coordinates is desired. An ACQ/SEARCH should be inserted prior to the ACQ/IMAGE.
3. In case 3, accurate GSC2 coordinates are available but neither MIRRORA nor MIRRORB may be used because the predicted count rates exceed the screening limits. In this instance a sequence of an ACQ/SEARCH (in dispersed light) followed by ACQ/PEAKXD and ACQ/PEAKD must be employed.
4. In case 4, the target coordinates are not accurate (i.e., coordinate uncertainties exceed 1 arcsec) but MIRRORA or MIRRORB may be used safely. If the GSC2 coordinates are not sufficiently accurate for the target to fall within the COS PSA or BOA upon the initial HST pointing but the object is safe to observe with MIRRORA or MIRRORB, then the target acquisition sequence should begin with ACQ/SEARCH and be completed with an ACQ/IMAGE (this is a variation on scenario 2).
5. In this last scenario, the target coordinates are not accurate (uncertainties exceed 1 arcsec) and neither MIRRORA nor MIRRORB may be used. If the GSC2 coordinates are not sufficiently accurate for the target to fall within the COS PSA or BOA upon initial HST pointing and the object is not safe to observe with a MIRROR configuration, then the target acquisition sequence should begin with ACQ/SEARCH and be completed with an ACQ/PEAKXD and ACQ/PEAKD sequence.