Spectral features in the flux calibrator star, slit losses in the measurement of the flux calibrator, and residual terrestrial atmospheric absorption features are removed by applying a multiplicative wavelength-dependent correction to the merged spectra which we generically call a `flat spectrum star' correction. The cspfltspec task is used to form the `flat spectrum star' correction based on the merged object spectrum (.mos file extension) of a featureless standard. The procedure is described in §8. The correction spectrum is stored in a file named by appending .flt to the base name.
Once the `flat spectrum star' correction has been formed, object spectra can be divided by this correction using the redxspec task by setting the flatdiv flag and specifying the base name of the flat spectrum correction file in the flatspec parameter.