Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.9 Calibrations

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This section discusses calibration data that can be obtained in orbit to support routine reduction of science observations.

5.9.1 Internal calibrations

COS internal exposures for wavelength, flat-field, and dark calibration will be incorporated in routine STScI calibration programs. Internal wavelength, flat-field, and dark calibration data will be obtained in TIME-TAG mode to maximize the scientific content. Doppler corrections will not be made to any internal calibration target data. GO users are not allowed to perform either internal flat-field or dark exposures.

Wavelengths

Wavelength calibration exposures will be routinely obtained with all science exposures (TAGFLASH and AUTO wavecals) and observers also may specify their own additional wavelength calibration exposures (GO wavecals).

Flat fielding

On-orbit flat-fielding using either of the two redundant COS flat-field lamps may only be performed by STScI calibration programs. Basic flat-field calibrations will be obtained during the ground-based calibration of COS. These data should be applicable to either single exposures or multiple exposures made with the FP-SPLIT procedure. We expect them to remain valid until charge depletion causes the pixel-to-pixel variations in the detector response to change significantly.

Observations of the lamp will be used to monitor changes in the detector flat-field response, and to derive updates to keep the calibration data in the pipeline database relevant and useful. We expect that these updates will be infrequent.

Additionally, flat-fields may be determined on-orbit with the use of external targets.

Sensitivity

The sensitivity calibration is the relationship between observed count rates and astrophysical flux. It has been predicted measured during pre-launch Thermal-Vacuum testing, and will be calibrated using HST flux-standard stars on-orbit. There are provisions for time-variability in COS sensitivity built into the calcos procedures, if necessary. The sensitivity will be monitored regularly in routine science cycle calibration programs.

Background rates

Background dark counts will be subtracted from every observation during the reduction process. The dark count rate can either be estimated from a region of the detector far from the optical spectrum in the cross-dispersion direction, Additionally, dark count observations will be a routine part of science cycle calibration programs. GOs may not select target=DARK.