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.
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.
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.
, 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.
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.
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
, 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 reduce these data calcos
assigns the wavelength shifts derived from the Segment A spectrum. If no Segment A data are present (i.e.,
), then no shift is assigned.
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. 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.