5.6 FUV Single-Segment Observations
The FUV detector segments are operated and read out independently. For all FUV gratings Segment A detects the longer-wavelength light and Segment B the shorter wavelengths. Normally both segments are used for a science exposure, but there are circumstances in which operating with one detector segment at the nominal high voltage and the other effectively turned off may be beneficial. The SEGMENT
optional parameter allows this choice. STScI strongly recommends use of both segments (the default for all but the G140L 800 Å and 1105 Å settings as well as G130M cenwaves that are impacted by the COS 2025 policy; see Section 2.1) unless special circumstances exist. Such circumstances include:
- Sources with unusual spectral energy distributions at FUV wavelengths (bright emission lines or rapidly increasing/decreasing continuum slopes), for which the count rate on one detector segment exceeds the bright-object protection limit, while the other segment is safe for observing. In some cases, this problem may be mitigated by adjusting the central wavelength or
FP-POS
setting. - Sources for which the count rate on one detector segment is high but safe, while the other segment has a relatively low count rate. If the science to be done were on the low count-rate segment, operating just that segment would reduce data losses due to dead-time effects and increase the S/N of the resulting spectrum.
The optional parameter SEGMENT
specifies which segment of the FUV detector to use for an observation. A value of BOTH
will activate both segments. This is the default setting. If SEGMENT=A
is selected, only Segment A of the detector will be activated for photon detection, and the spectrum will contain data from only the long-wavelength half of the detector. If SEGMENT=B
is selected, only the short-wavelength Segment B of the detector will be activated and used to generate data. Wavelength and flat-field calibration procedures remain the same for a particular segment whether or not the other segment is operating. Observers should take care to set SEGMENT
appropriately for the G130M/1055, 1096, and 1222 CENWAVE
settings.
If any of the modes are specified for which Segment B spectroscopy is not permitted, then the value defaults to SEGMENT=A
. Switching from two-segment to single-segment operation (or back again) incurs a substantial overhead time; see Table 9.5.
-
COS Instrument Handbook
- Acknowledgments
- Chapter 1: An Introduction to COS
-
Chapter 2: Special Considerations when Observing with COS
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Visit Length
- • 2.3 Central Wavelength Settings Added in Cycle 26
- • 2.4 ORIENT constraints for Extended Sources
- • 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
- • 5.11 Spectroscopy with Available but Unsupported Settings
- 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
- • 12.17 Cycle 29 Calibration Program
- Chapter 13: Spectroscopic Reference Material
- • Glossary