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.
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 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.
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.
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.
To 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.
