8.9 Recommended Parameters for all COS TA Modes
As the result of our analysis of the target acquisition (TA) performance of COS during SMOV and during Cycles 17 through 25, 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, based on ETC simulations.
- 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. - FUV
ACQ/PEAKXD
exposures in Cycle 25 and beyond take place at LP4 and useACQ/PEAKXD
and the optional parametersNUM-POS
,STEP-SIZE
, andCENTER
. The default pattern ofNUM-POS=3
,STEP-SIZE=1.3
, andCENTER=FLUX-WT
should be used for most targets. At Cycle 29 and beyond, some FUVACQ/PEAKXD
exposures will be at LP5. - A single
ACQ/SEARCH
is not sufficient toCENTER
a COS point-source target in the aperture. Always follow the firstACQ/SEARCH
with anACQ/IMAGE
,ACQ/PEAKXD+ACQ/PEAKD
, or a second 2 × 2ACQ/SEARCH
which usesFLUX-WT
weighting. ACQ/SEARCH
exposures 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/PEAKXD
s, only a few cenwaves have been proven to be reliable. Users should choose one of these cenwaves, which are listed in Section 2.9. 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. (This 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.
- Use
NUM-POS=5
,STEP-SIZE=0.9
, andCENTER=FLUX-WT-FLR
for mostACQ/PEAKD
centerings. For the most accurate AD (ACQ/PEAKD
) centering possible, useNUM-POS=9
,STEP-SIZE=0.6
, andCENTER=FLUX-WT-FLR
. When a fast TA is required and lower centering accuracy can be tolerated, useNUM-POS=3
,STEP-SIZE=1.3
, andCENTER=FLUX-WT
.
Table 8.3: COS Acquisition Modes, Options, and Recommended Values.
Acquisition Type | Description | SCAN-SIZE or NUM-POS | STEP-SIZE (arcsec) | Optional Parameters | Recommended Values | Recommended S/N |
| Spiral pattern; multiple exposures | 2 | 1.767 |
|
| 40 (PSA and BOA) |
31 |
| |||||
4 |
| |||||
5 |
| |||||
| Initial and confirmation images (NUV only) | 20 (PSA) | ||||
| One exposure |
|
| 40 | ||
| Linear pattern; multiple exposures | 31 | 1.3 |
|
| 40 |
5 | 1.0 |
| ||||
7 | 0.8 |
| ||||
9 | 0.65 |
| ||||
| Linear pattern; multiple exposures | 3 | 1.3 |
|
| 40 |
51 | 0.9 |
| ||||
7 | 0.6 |
| ||||
9 | 0.6 |
|
1 Recommended number of acquisition steps.
2 The G230L grating has special recommendations, see Section 8.5.1.
3 For ACQ/PEAKD
and FUV ACQ/PEAKXD
, use of the special parameter CENTER=DEFAULT
is recommended. This parameter sets CENTER=FLUX-WT
if NUM-POS=3
and CENTER=FLUX-WT-FLR if NUM-POS=5
, 7
, or 9
.
-
COS Instrument Handbook
- Acknowledgments
- Chapter 1: An Introduction to COS
-
Chapter 2: Special Considerations for Cycle 29
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Visit Length
- • 2.3 Central Wavelength Settings Added in Cycle 26
- • 2.4 The G285M Grating is Available but Unsupported
- • 2.5 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues
- • 2.6 Time-Dependent Sensitivity Changes
- • 2.7 Spectroscopic Use of the Bright Object Aperture
- • 2.8 Non-Optimal Observing Scenarios
- • 2.9 NUV Spectroscopic Acquisitions
- • 2.10 SNAP, TOO, and Unpredictable Source Programs with COS
- • 2.11 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
- • 12.16 Cycle 28 Calibration Program
- Chapter 13: Spectroscopic Reference Material
- • Glossary