Flux Calibration for VLT and ELT Spectrographs

ESO offers a range of optical and infrared spectrographs on its telescopes, and more will be available once ESO’s Extremely Large Telescope (ELT) starts operating. Some, but not all, science cases require the flux of spectra to be calibrated in relative or absolute units. The achievable accuracy of such a flux calibration differs with circumstances and instrument. In this article, we provide an overview of the methods and routinely obtained accuracy for current Very Large Telescope and future ELT spectrographs.