Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.7 Internal Wavelength Calibration Exposures

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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 only),
2. AUTO wavecals, and
3. User-specified 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 as the aperture mechanism must be moved farther to place the WCA in the wavelength calibration beam.

For TIME-TAG observing, we strongly recommend use of the default FLASH=YES mode of wavelength calibration.

5.7.1 Concurrent Wavelength Calibration with TAGFLASH

Optional Parameter FLASH (=YES (default), NO) indicates whether or not to "flash" the wavelength calibration lamp during TIME-TAG exposures. These flashes are needed to provide information used by the calcos pipeline to compensate for the effect of post-move drift of the Optic Select Mechanisms. The default behavior will be that when the external shutter is open, the wavecal lamp is turned on briefly at the start of an externally targeted exposure, and at intervals later in the exposure. In this mode, photons from the external science target and the internal wavelength calibration source are recorded simultaneously on different portions of the detector. Other than the flash at the start of each exposure, the actual timing of flashes is automatically determined by the elapsed time since the last OSM move has occurred. As a result, flashes may occur at different time-points in different exposures. The grating-dependent "flash" durations (discussed below) and the time-since-last-OSM-move-dependent flash intervals will be defined and updated as necessary by STScI. Observers may not specify either flash duration or flash interval. We strongly recommend use of Optional Parameter FLASH=YES with all TIME-TAG observations.

FLASH=YES TIME-TAG sequences provide the highest on-target exposure time per orbital visibility as no on-target time is lost due to required instrumental calibration exposures.

When flashing is enabled, the exposure time must be at least as long as a single flash. FLASH may not be specified, and defaults to NO, when aperture BOA is selected. FLASH also may not be specified for ACCUM mode.

Additional information on how TAGFLASH works may be found in Section 13.5.

5.7.2 AUTO Wavecals (When TAGFLASH is not Used)

For TIME-TAG exposures, specifying FLASH=NO disables automatic flashing for the current exposure. Also, flashes are not performed in ACCUM exposures.

In these cases, unless specifically requested in the Exposure Specification, a separate wavelength calibration exposure will be automatically performed (AUTO wavecal) for each set of external spectrographic science exposures using the same spectral element, central wavelength, and FP-POS value, including each sub-exposure of an exposure specification with Optional Parameter FP-POS=AUTO. These AUTO wavecals are always obtained in TIME-TAG mode with the external shutter closed. This automatic wavelength calibration exposure will be added prior to the first such 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 time-line does not affect the 40-minute clock.

For TIME-TAG FLASH=NO and for ACCUM observations, 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.

ACCUM and TIME-TAG observations with FLASH=NO will be less efficient than with FLASH=YES observations in terms of on-target utilization of orbital visibility and in terms of resultant wavelength calibration due to possible OSM residual motions.

5.7.3 Wavelength Calibration Exposures with the BOA

If the BOA is moved into position for science observations, the WCA is no longer available and light from the Pt-Ne lamps cannot reach the detector. As a result FLASH=YES may not be used when the BOA is specified. If TIME-TAG mode is specified with the BOA as the aperture then separate wavecals will automatically be obtained in the same way as for an ACCUM mode exposure and subject to the same rules. Such wavecals will also be obtained if ACCUM mode exposures are obtained with the BOA as aperture.

5.7.4 User-specified Wavelength Calibration Exposures
(GO Wavecals)

Observers may insert additional wavelength calibration observations in the visit by specifying target=WAVE (so-called GO wavecal exposures). Note that the default modes of operation (TIME-TAG, ordinarily, or ACCUM) automatically secure needed wavelength calibration information to go with your science data, and so the need for user-specified wavecals should be rare. Exposure time must be set to DEF for these exposures, TIME-TAG must be used, and FLASH=NO should be explicitly selected. Exposures specified with the WAVE internal target will use the same calibration lamp configuration and exposure time as the automatic wave calibrations discussed above. Initially, lamp flash durations are identical to the required default wavelength calibration exposure times, however this identity may be changed.