Modeling long-range wavelength distortions in quasar absorption echelle spectra

Spectra observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the European Southern Observatorys VLT exhibit long-range wavelength distortions. These distortions impose a systematic error on high-precision measurements of the fine-structure constant, $alpha$, derived from intervening quasar absorption systems. If the distortion is modelled using a model that is too simplistic, the resulting bias in $Deltaalpha/alpha$ away from the true value can be larger than the statistical uncertainty on the $alpha$ measurement. If the effect is ignored altogether, the same is true. If the effect is modelled properly, accounting for the way in which final spectra are generally formed from the co-addition of exposures made at several different instrumental settings, the effect can be accurately removed and the correct $Deltaalpha/alpha$ recovered.