  Spectra observed with the Ultraviolet and Visual Echelle Spectrograph (UVES)
on the European Southern Observatory's 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 $\Delta\alpha/\alpha$ 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
$\Delta\alpha/\alpha$ recovered.
