Space Telescope Science Institute
COS Instrument Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 8: Target Acquisitions > 8.2 Target Acquisition Overview

8.2
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 fast and precise, 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.
2.
3.
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
2 2 1.767 ACQ/SEARCH
Please Note...
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.
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.

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 8: Target Acquisitions > 8.2 Target Acquisition Overview

Table of Contents Previous Next Index Print