5.7 Internal Wavelength Calibration Exposures
Three types of internal wavelength calibration exposures may be inserted in the observation sequence by the scheduling system or by the observer:
TAGFLASH) lamp flashes (
TIME-TAGobserving with the PSA at LP1-5),
- user-specified GO wavecals, and
SPLITwavecals (TIME-TAG observing with the PSA at LP6).
Note that all wavelength-calibration exposures are taken in
TIME-TAG mode. Wavelength calibration exposure overheads are higher when the BOA is used for science observation or when science observations are performed at LP6 because the aperture mechanism must be moved to place the wavecal spectrum at the appropriate location on the detector.
While it is possible to suppress the taking of any wavelength-calibration spectra, doing so significantly lessens the archival quality of COS data and must be justified on a case-by-case basis.
5.7.1 Concurrent Wavelength Calibration with TAGFLASH
The optional parameter
FLASH indicates whether or not to "flash" the wavelength calibration lamp during
TIME-TAG exposures utilizing the PSA. These flashes provide data used by the CalCOS pipeline to compensate for drifts of the Optics Select Mechanisms. In this mode, when the external shutter is open to observe an external target, the wavecal lamp is turned on briefly at the beginning of, and at intervals throughout, the exposure. Light from the science target and the internal wavelength calibration source is recorded simultaneously on different portions of the detector. Other than the flash at the start, the timing of flashes is determined by the elapsed time since the last OSM motion. As a result flashes may occur at different times in different exposures. The grating-dependent flash durations () and the flash intervals are defined and updated as necessary by STScI. Observers may not specify either flash duration or interval. (Details of
TAGFLASH execution are presented in COS ISR 2011-04.) When flashing is enabled the exposure time must be at least as long as a single flash. Science exposures shorter than the flash durations listed in may be obtained by setting
FLASH=NO, in which case a wavecal exposure will automatically be inserted after the science exposure.
TIME-TAG sequences with
FLASH=YES provide the highest on-target exposure time, as no on-target time is lost to wavelength-calibration exposures. Therefore, we strongly recommend use of the optional parameter
FLASH=YES with all
TIME-TAG observations through the PSA when possible. (Since
FLASH=YES is the default for
TIME-TAG spectroscopic exposures, the observer need not specify it.)
FLASH=YES may not be specified for
ACCUM mode or when the BOA is selected. Users should be aware that the BOA is not well-calibrated at wavelengths below 1200 Å.
FLASH=YES observations are not permitted for spectral elements supported at LP6 (Table 2.0).
SPLIT wavecals are automatically inserted for spectrographic science exposures at LP6 (Section 5.7.6).
AUTO Wavecals (when TAGFLASH is not used)
ACCUM, BOA, or
TIME-TAG exposures a separate wavelength calibration exposure will be automatically scheduled by the APT for each set of external spectrographic science exposures using the same spectral element, central wavelength, and
FP-POS value. These
AUTO wavecals are always obtained in
TIME-TAG mode with the external shutter closed. This automatic wavelength calibration exposure will be added before the first associated science exposure, and after each subsequent science exposure if more than 40 minutes of visibility time has elapsed since the previous wavelength calibration exposure and if the same spectrograph set-up has been in use over that time. The calibration exposure will often use some science target orbital visibility. The calibration lamp configuration and exposure time will be based on the grating and central wavelength of the science exposure. Utilization of a GO wavecal (see below) resets the 40 minute interval timer. Insertion of a
FLASH=YES exposure in the timeline does not affect the 40-minute clock.
AUTO wavecals may not be turned off by the observer. If there is a science requirement to turn off
AUTO wavecals, specific permission must be sought from the STScI Contact Scientist.
FLASH=NO observations will be less efficient than
FLASH=YES observations in terms of on-target utilization of orbital visibility, and in the quality of their wavelength calibration, due to possible OSM residual motions.
5.7.3 GO Wavecals (User-Specified)
Observers may request additional wavelength-calibration exposures, called GO wavecals, by selecting
FLASH=NO. The exposure must be made in
TIME-TAG mode. GO wavecals use the same calibration lamp configuration and exposure times as the automatic wavelength calibrations discussed above. The default modes of operation automatically secure needed wavelength-calibration information to go with your science data, so GO wavecals are rarely required.
5.7.4 No-Cal Wavecals
The COS Pt-Ne wavelength-calibration lamps produce no lines on FUV Segment B in the following observing modes:
- G130M, central wavelength 1055 Å, all
- G130M, central wavelength 1096 Å, all
- G140L, central wavelength 1280 Å, all
To reduce these data CalCOS assigns the wavelength shifts derived from the Segment A spectrum. If no Segment A data are present (i.e., if
SEGMENT=B), then no shift is assigned, and the wavelength calibration is highly uncertain.
5.7.5 Inserting Wavecals in Observations with G130M/1055 and G130M/1096,
Due to the 100x difference in sensitivity between the COS FUVA and FUVB segments when observing with the G130M/1055 and 1096
CENWAVEs, it is expected that many observers will need to turn off FUVA when observing bright targets. 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 ~1180 Å (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
TAGFLASHs 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 should be used instead.
For non-Continuous Viewing Zone (CVZ) science exposures at the start of an orbit not containing a target acquisition (TA) sequence, the WAVECAL can be inserted into the prior occultation with no impact on the timeline. For non-CVZ science exposures at the end of an orbit, the WAVECAL can be inserted into the following occultation with no impact on the timeline. For science exposures that immediately follow TA, or do not start or end an orbit, a WAVECAL must be inserted. This can take anywhere from 500 to 900 s (8–15 minutes) depending on configuration and observing sequence. For more information, consult with your contact scientist.
SPLIT Wavecals (default non-concurrent wavelength calibration at LP6)
For science exposures using spectral elements supported at LP6 (Table 2.0) a non-concurrent wavelength calibration exposure will be automatically scheduled by the APT for each set of external spectrographic science exposures using the same spectral element, central wavelength, and
FP-POS value. These
SPLIT wavecals are always obtained in
TIME-TAG mode with the external shutter closed. This wavelength calibration exposure will be added before and after each science exposure (depending on the length of the exposure and if the same spectrograph set-up has been in use over that time).
SPLIT wavecal exposures are not taken concurrently with the science exposures and are instead executed by moving the aperture to a different position on the detector in order to use the wavecal lamp. The calibration exposure will often use some of the science target orbital visibility. The calibration lamp configuration and exposure time will be based on the grating and central wavelength of the science exposure.
SPLIT wavecals may not be turned off by the observer. If there is a science requirement to turn off
SPLIT wavecals, specific permission must be sought from the STScI Contact Scientist.
FLASH=YES observations are not permitted for science exposures using spectral elements supported at LP6 (Table 2.0).
FLASH=NO is the default for
TIME-TAG spectroscopic exposures at LP6, the observer need not specify it.
COS Instrument Handbook
- 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
- • 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