Flux-calibration of medium-resolution spectra from 300 nm to 2500 nm

While the near-infrared wavelength regime is becoming more and more important for astrophysics there are few spectrophotometric standard star data available to flux calibrate such data. On the other hand flux calibrating high-resolution spectra is a challenge even in the optical wavelength range, because the available flux standard data are often too coarsely sampled. We describe a method to obtain reference spectra derived from stellar model atmospheres, which allow users to derive response curves from 300 nm to 2500 nm also for high-resolution spectra. We verified that they provide an appropriate description of the observed standard star spectra by checking for residuals in line cores and line overlap regions in the ratios of observed spectra to model spectra. The finally selected model spectra are then empirically corrected for remaining mismatches and photometrically calibrated using independent observations. In addition we have defined an automatic method to correct for moderate telluric absorption using telluric model spectra with very high spectral resolution, that can easily be adapted to the observed data. This procedure eliminates the need to observe telluric standard stars, as long as some knowledge on the target spectrum exists.