Relative Flux Calibration of Keck HIRES Echelle Spectra

We describe a method to calibrate the relative flux levels in spectra from the High‐Resolution Echelle Spectrograph (HIRES) on the Keck I telescope. Standard data reduction techniques that transfer the instrument response between HIRES integrations leave errors in the flux of 5%–10% because the effective response varies during the night. The flux errors are most severe near the ends of each spectral order, where there can be discontinuous jumps. The source of these errors is uncertain but may include changes in the vignetting connected to the optical alignment. Our flux calibration method uses a calibrated reference spectrum of each target to calibrate individual HIRES integrations. We determine the instrument response independently for each integration, and hence we avoid the need to transfer the instrument response between HIRES integrations. The procedure can be applied to any HIRES spectrum or any other spectrum. While the accuracy of the method depends upon many factors, we have been able to flux‐calibrate a HIRES spectrum to 1% over scales of 200 Å that include order joins. We illustrate the method with spectra of QSO 1243+3047, toward which we have measured the deuterium‐to‐hydrogen abundance ratio.