8.2 Target Acquisition Overview
COS has four TA modes:
ACQ/SEARCHperforms 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/IMAGEobtains 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/PEAKXDdetermines 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/PEAKDcenters 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/PEAKXDshould 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:
- 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.
- 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).
- 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.
|Type||Step 1||Step 2||Step 3|
Either Imaging or Dispersed-Light
2 × 2 × 1.767"
- 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/SEARCHshould 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/SEARCHto 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.
ACQ/IMAGEexposures, 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.
COS Instrument Handbook
- Chapter 1: An Introduction to COS
Chapter 2: Special Considerations for Cycle 28
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Central Wavelength Settings Added in Cycle 26
- • 2.3 Use of the G285M Grating is Discouraged
- • 2.4 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues
- • 2.5 Time-Dependent Sensitivity Changes
- • 2.6 Spectroscopic Use of the Bright Object Aperture
- • 2.7 Non-Optimal Observing Scenarios
- • 2.8 NUV Spectroscopic Acquisitions
- • 2.9 SNAP, TOO, and Unpredictable Source Programs with COS
- • 2.10 Choosing between COS and STIS
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
Chapter 5: Spectroscopy with COS
- • 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
- Chapter 10: Bright-Object Protection
- Chapter 11: Data Products and Data Reduction
Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
Chapter 13: Spectroscopic Reference Material
- • 13.1 Introduction
- • 13.2 Using the Information in this Chapter
- 13.3 Gratings
- • 13.4 Spectrograph Design Parameters
- • 13.5 The Location of COS in the HST Focal Plane
- • 13.6 The COS User Coordinate System
- • Glossary