8.2 Target Acquisition Overview

COS has four TA modes:

  • ACQ/SEARCH performs a spiral search by executing individual exposures at each point in a square grid pattern on the sky (details are in Section 8.3). This mode can use either dispersed-light or imaging exposures.
  • ACQ/IMAGE obtains an NUV image of the target field, moves the telescope to center the object, and obtains a second NUV image as confirmation (details are in Section 8.4). This is generally the fastest and most accurate method of target acquisition, but covers a limited area on the sky.
  • ACQ/PEAKXD determines the location of the target in the cross-dispersion (XD) direction and moves the telescope to center the object in the XD direction (details are in Section 8.5).
  • ACQ/PEAKD centers the target in the along-dispersion (AD) direction by executing individual exposures at each point in a linear pattern along the dispersion axis (details are in Section 8.6). ACQ/PEAKXD should always precede ACQ/PEAKD, and the two should always be performed together.

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:

  1. NUV imaging with the fastest allowable combination of aperture and mirror to achieve the desired S/N, even if the science to follow is performed with the FUV channel.
  2. Dispersed-light acquisition using the same configuration as the first science exposure, if it will use less time overall, if the target violates local count limits, or if the cenwave is not a recommended setting (see Section 2.8).
  3. Dispersed-light acquisition with a different configuration, if it will use less time overall, if the target violates local count limits, or if the central wavelength is not supported for TA.

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.

TypeStep 1Step 2Step 3

Imaging
(if coordinates are good to 0.4")

ACQ/IMAGE

none

none

Imaging
(if coordinates are less accurate)

ACQ/SEARCH

ACQ/IMAGE

none

Dispersed-Light
(coordinates good to 0.4")

ACQ/PEAKXD

ACQ/PEAKD

none

Dispersed-Light
(coordinates less accurate)

ACQ/SEARCH

ACQ/PEAKXD

ACQ/PEAKD

Either Imaging or Dispersed-Light
(e.g., small, extended sources)

ACQ/SEARCH

2 × 2 × 1.767" ACQ/SEARCH

none


Please Note...

  • 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.
  • The blue central-wavelength settings for G130M (1055, 1096, and 1222 Å) and new settings introduced in Cycle 26 (G160M/1533 and G140L/800) cannot be used for target acquisitions.
  • 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.