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:

  1. FLASH=YES (so-called TAGFLASH) lamp flashes (TIME-TAG observing with the PSA only),
  2. AUTO wavecals, and
  3. user-specified GO wavecals.

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 because the aperture mechanism must be moved to place the WCA in the wavelength-calibration beam.

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 (Table 5.2) 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 Table 5.2 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. (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.

5.7.2 AUTO Wavecals (when TAGFLASH is not used)

For ACCUM, BOA, or FLASH=NO, 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 TARGET=WAVE, EXPTIME=DEF, and 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 FP-POS settings;
  • G130M, central wavelength 1096 Å, all FP-POS settings; and
  • G140L, central wavelength 1280 Å, all FP-POS settings.

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.

5.7.5 Inserting Wavecals in Observations with G130M/1055 and G130M/1096, SEGMENT=B

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-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.