9.5 Science Exposure Overheads
Science-exposure overheads are dominated by the time required to move OSM1 and OSM2 and to read out the on-board memory buffer at the end of each exposure. While the Phase II
overheads computed by APT may be less than the values presented below, it is important to plan Phase I
proposals using the conservative overheads given below to ensure adequate time for each exposure.
The full overhead calculation for science exposures depends upon a number of factors including generic exposure set-ups (which depend on the detector and observing mode), whether an aperture change is required, whether a grating change is required, whether the central wavelength setting for the grating is changed, and the directional sense of any required motion to implement an FP-POS
change. Table 9.5 lists these additional overheads.
When moving to a new grating, you may specify any combination of central wavelength and FP-POS
setting with no additional overhead penalty. The FP-POS
sequence 1,2,3,4 is more efficient than 3,4,1,2, because no backward motion is required.
Table 9.5: Science Exposure Overhead Times.
FUV | NUV | |||
---|---|---|---|---|
Overhead time(s) | TIME-TAG | ACCUM | TIME-TAG | ACCUM |
Exposure set-up | 66 | 70 | 35 | 33 |
Grating change | see Table 9.3 | see Table 9.4 | ||
Central wavelength change | 801 | 75 | ||
| 3 | 3 | ||
| 70 | 70 | ||
PSA − BOA Change | 8 | 8 | ||
WCA − BOA Change | 10 | 10 | ||
| 300 (off); 315 (on) | N/A | ||
Memory readout3 | 114 | 1133 | 114 | 563 |
1 This depends on the cenwaves involved and may vary by up to ±6 s. For G130M, allow up to 105 s if one of the cenwaves is 1222 and up to 132 s if one of the cenwaves is 1055 or 1096 and the other is 1291 or greater.
2 "Forward" refers to the preferred direction of motion of OSM1 or OSM2 (toward larger FP-POS
values) and "backward" to the opposite direction.
3 ACCUM
mode readout overheads can be hidden within subsequent exposures under certain circumstances, but the rules are complex. Use these values as safe upper limits for proposals.
To estimate the overhead for an exposure, round the desired exposure time up to the next whole second and add the generic exposure setup overhead from Table 9.5. If a grating change has occurred from the previous exposure, add the appropriate values from Table 9.3 and/or Table 9.4. If a central wavelength change is made, add the appropriate value from Table 9.5. If an FP-POS
movement is made, add the appropriate value for motion in the preferred direction (toward larger FP-POS
) or non-preferred direction. Note that all dispersed-light target-acquisition exposures are obtained with FP-POS=3
. For all FUV observations except the G140L 800 Å and 1105 Å settings and those impacted by the COS 2025 policy, both detector segments are powered on by default. To turn one of them off, set SEGMENT
to A
or B
and add the associated overhead. Lastly, add the appropriate detector memory readout overhead.
Due to the 100x difference in sensitivity between the COS FUVA and FUVB segments when observing with the G130M/1055 and 1096 CENWAVE
s, it is expected that many observers will need to turn off FUVA when observing bright targets. (We refer to these SEGMENT=B
observations here as either C1055B or C1096B). Only those observers using these two configurations are affected by this issue.
Under these conditions the zero point of the wavelength solution cannot be determined because the MgF2 window on the PtNe lamps (WAVECAL
) blocks light below ~1180Å (all WAVECAL
light falls on FUVA). This results in a degradation of the resolution when FP-POS
are combined by CalCOS and decreases the archival value of the COS data. In these cases, normal TAGFLASH
es are not available and WAVECAL
exposures with FUVA turned ON must be inserted into the observing sequence adjacent to each CENWAVE/FP-POS
setting used. As a result, in these cases FP-POS=ALL
should not be used. Individual FP-POS
science exposures, and their associated WAVECAL
s, should be used instead. For more information, consult with your contact scientist.
Overheads associated with new settings introduced in Cycle 26 (Section 2.3) have not been tabulated, but they are expected to be similar to those of the other cenwaves in their respective gratings.
-
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