8.9 Recommended Parameters for all COS TA Modes
As the result of our analysis of the target acquisition (TA) performance of COS, we have adjusted many COS TA parameters and have refined our recommendations and guidelines for routine COS TAs. We present these recommendations and guidelines in the previous sections and summarize the most significant instructions below. Table 8.3 provides our recommended values for optional parameters for each COS TA type.
- All TA modes provide good centering. For optimal wavelength accuracy, use NUV imaging mode; otherwise, use the mode that is fastest, as determined by the Orbit Planner in APT.
- Signal-to-noise (S/N) is important. Use S/N > 40, except for
ACQ/IMAGE, which requires S/N > 20 through the PSA and S/N > 30 through the BOA.
- By default, FUV acquisitions for gratings G130M and G160M take place in the same Lifetime Position used for the grating/cenwave combination. FUV acquisitions for grating G140L take place at LP4 even though science exposures are taken at LP3. In standard cases users do not need to specify a choice of LIFETIME-POS in APT. Users observing with grating G160M at LP4 (Section 9.5.1) should select LIFETIME-POS=LP4 under optional parameters on APT.
- A single
ACQ/SEARCHis not sufficient to
CENTERa COS point-source target in the aperture. Always follow the first
ACQ/SEARCHexposures are not required for targets with coordinate accuracies of ±0.4 arcsec or better in the ICRS/GSC2 frame. Spending extra time to validate target coordinates is the best way to save TA time. On-line catalogs often have coordinate errors that exceed 0.4 arcsec. It is the userʹs responsibility to verify that target coordinates are correct.
- For NUV spectroscopic
ACQ/PEAKXDs, only a few cenwaves have been proven to be reliable. Users should choose one of these cenwaves, which are listed in Section 8.4. The G185M, G225M, and G230L gratings each have three or four recommended cenwaves, while the G285M grating has none. For a full discussion of this issue, see COS ISR 2017-05.
- If at all possible, use STRIPE=MEDIUM (stripe B) for NUV spectroscopic ACQ/PEAKXDs, which is the default. Primarily, this is because the WCA-to-PSA offsets in the flight software are optimized for stripe B in most modes. Choosing a different stripe will result in systematic offsets of the initial spectrum of order of a few pixels, which generally are still within typical target coordinate uncertainties.
- FUV ACQ/PEAKXD uses NUM-POS=3, STEP-SIZE=1.3, and CENTER=FLUX-WT as defaults. In most cases there is no need to change, or enumerate, these parameters.
- There are no defaults for FUV ACQ/PEAKD. Use
ACQ/PEAKDcenterings. For the most accurate AD (
ACQ/PEAKD) centering possible, use
CENTER=FLUX-WT-FLR. When a fast TA is required and lower centering accuracy can be tolerated, use
Table 8.3: COS Acquisition Modes, Options, and Recommended Values.
|Acquisition Type||Description||Optional Parameters||Recommended Values||Recommended S/N|
Spiral pattern; multiple exposures. Can be used with either imaging or spectroscopic ACQs.
40 (PSA and BOA)
Initial and confirmation images (NUV only)
Linear pattern; multiple exposures
Linear pattern; multiple exposures
1 Recommended value. For ACQ/PEAKD, FUV ACQ/PEAKXD, and ACQ/SEARCH, use of special parameter CENTER=DEFAULT is recommended.
2 The G230L grating has special recommendations, see Section 8.5.1.
3 For ACQ/PEAKD, FUV ACQ/PEAKXD, and ACQ/SEARCH, use of special parameter CENTER=DEFAULT is recommended. In the case of ACQ/PEAKD and FUV ACQ/PEAKXD this parameter sets CENTER=FLUX-WT if NUM-POS=3, and CENTER=FLUX-WT-FLR if NUM-POS=5, 7, or 9. For ACQ/IMAGE this parameter sets CENTER=FLUX-WT if NUM-POS=2 and CENTER=FLUX-WT-FLR if NUM-POS > 2.
COS Instrument Handbook
- Chapter 1: An Introduction to COS
- Chapter 2: Proposal and Program Considerations
- 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
- • 5.11 Spectroscopy with Available-but-Unsupported Settings
- • 5.12 FUV Detector Lifetime Positions
- • 5.13 Spectroscopic Use of the Bright Object Aperture
- 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
- • 10.1 Introduction
- • 10.2 Screening Limits
- • 10.3 Source V Magnitude Limits
- • 10.4 Tools for Bright-Object Screening
- • 10.5 Policies and Procedures
- • 10.6 On-Orbit Protection Procedures
- • 10.7 Bright Object Protection for Solar System Observations
- • 10.8 SNAP, TOO, and Unpredictable Sources Observations with COS
- 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
- • 12.16 Cycle 28 Calibration Program
- • 12.17 Cycle 29 Calibration Program
- • 12.18 Cycle 30 Calibration Program
Chapter 13: COS 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