Keck constraints on a varying fine-structure constant: wavelength
  calibration errors
  The Keck telescope's HIRES spectrograph has previously provided evidence for
a smaller fine-structure constant, alpha, compared to the current laboratory
value, in a sample of 143 quasar absorption systems:
da/a=(-0.57+/-0.11)x10^{-5}. This was based on a variety of metal-ion
transitions which, if alpha varies, experience different relative velocity
shifts. This result is yet to be robustly contradicted, or confirmed, by
measurements on other telescopes and spectrographs; it remains crucial to do
so. It is also important to consider new possible instrumental systematic
effects which may explain the Keck/HIRES results. Griest et al. (2009,
arXiv:0904.4725v1) recently identified distortions in the echelle order
wavelength scales of HIRES with typical amplitudes +/-250m/s. Here we
investigate the effect such distortions may have had on the Keck/HIRES varying
alpha results. We demonstrate that they cause a random effect on da/a from
absorber to absorber because the systems are at different redshifts, placing
the relevant absorption lines at different positions in different echelle
orders. The typical magnitude of the effect on da/a is ~0.4x10^{-5} per
absorber which, compared to the median error on da/a in the sample,
~1.9x10^{-5}, is relatively small. Consequently, the weighted mean value
changes by less than 0.05x10^{-5} if the corrections we calculate are applied.
Nevertheless, we urge caution, particularly for analyses aiming to achieve high
precision da/a measurements on individual systems or small samples, that a much
more detailed understanding of such intra-order distortions and their
dependence on observational parameters is important if they are to be avoided
or modelled reliably. [Abridged]
