Spectrophotometry: Revised standards and techniques

The telluric features redward of 6700 Å have been removed from the accurate spectrophotometric standards of Hamuy et al. to permit more reliable relative and absolute spectrophotometry to be obtained from CCD spectra. Smooth fluxes from 3300 to 10500 Å are best determined by dividing the raw spectra of all objects taken in a night by the raw spectrum of a " smooth " spectrum star before deriving the instrumental response function using the revised standard star fluxes. In this way the telluric features and any large instrumental variation with wavelength are removed from the raw data, leaving smooth spectra that need only small corrections to place them on an absolute flux scale. These small corrections with wavelength are well described by a low-order polynomial and result in very smooth flux-calibrated spectra.