The flux calibration procedure for cross-dispersed grism data is similar to that described for long-slit grisms in §8. The approach is to flux calibrate with the best available flux calibrator and to correct for any shortcomings in the flux calibrator by dividing by a `flat spectrum' star after the flux calibrated spectral orders have been merged into a single spectrum. Suitable spectroscopic flux calibrators are listed in Table 24 of Appendix G.
The redxspec task uses the two model flux distributions
discussed in §8; a blackbody distribution
parameterised by a color temperature, and approximations to the Kurucz
flux distributions parameterised by the model effective
temperature. Model flux distributions are calculated using the
cspflux task as described in §8. The
redxspec task searches the same caspirdir$fluxstds.dat file to
locate model parameters for the flux calibrator star using the object
name header entry as the search parameter. Model parameters are
prompted for if they are not found in the
caspirdir$fluxstds.dat file.
Flux calibration is achieved by dividing the object spectrum by the observed spectrum of the flux calibrator and multiplying by the absolute flux spectrum modelled for the flux calibrator. This is done in the redxspec task by setting the fluxcal flag and specifying the base name of the flux calibrator file in the fluxspec parameter. No correction is applied for airmass differences between the object and flux calibrator; it is assumed that the observations were made at similar airmasses in order to optimise terrestrial atmospheric absorption correction. The flux calibrated object spectra are stored in a file named by appending .fos to the base spectrum name and the flux calibrated sky spectra are stored in a file named by appending .fss to the base spectrum name. Comparison spectra are not flux calibrated.