Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.10 Calibrations

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

5.10.1 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 have been obtained during the ground-based calibration of COS. These data should be applicable to either single exposures or multiple exposures made with the FP-POS procedure. We will acquire additional information after COS is installed in HST.

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 will be determined on-orbit with the use of external targets.

Sensitivity

The sensitivity calibration provides a relationship between observed count rates and astrophysical flux. Initial estimates were obtained during pre-launch testing, and will be updated after launch using HST flux-standard stars. The sensitivity of COS will be regularly monitored and the calibration updated. There are provisions for time-variability in COS sensitivity built into the calcos procedures, if the sensitivity changes with time.

Detector Background Rates

Background dark counts will be subtracted from every observation during the reduction process. The dark count rate can 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.

5.10.2 External Calibrations

Flat Fielding

Astrophysical objects such as white dwarfs will be used to obtain improved flat fields once COS is installed.

Sensitivity

Standard stars will be used to evaluate the sensitivity of COS once it is on orbit. One reason for including the BOA in COS is to be able to observe some of the same brighter standards that are used for STIS, to provide an accurate cross-calibration.