COS Instrument Handbook

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 centroid of the dispersed-light spectrum 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, or if the target violates local count limits.

3.	Dispersed-light acquisition with a different configuration, if it will use less time overall, or if the target violates local count limits.

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


Type	Step 1	Step 2	Step 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 â 2 â 1.767” ACQ/SEARCH	none

Please Note...

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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.

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If a target falls near the edge of the aperture at the initial pointing, the ACQ/IMAGE and ACQ/PEAKXD algorithms may 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 an extra ACQ/IMAGE or ACQ/PEAKXD in case the observation at the initial pointing was partially vignetted.

•	At the present time, the new central-wavelength settings for G130M (1055, 1096, and 1222 å) 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.