Calibration issues in estimating variability of the fine structure
  constant (alpha) with cosmic time
  Laser Comb Wavelength calibration shows that the ThAr one is locally
unreliable with possible deviations of up to 100 m/s within one order range,
while delivering an overall 1 m/s accuracy (Wilken et al 2009). Such deviation
corresponds to delta alpha/alpha ~ 7E-6 for a FeII-MgII pair. Comparison of
line shifts among the 5 FeII lines, with almost identical sensitivity to fine
structure constant changes, offers a clean way to directly test the presence of
possible local wavelength calibration errors of whatever origin. We analyzed 5
absorption systems, with zabs ranging from 1.15 to 2.19 towards 3 bright QSOs.
The results show that while some lines are aligned within 20 m/s, others reveal
large deviations reaching 200 m/s or higher and corresponding to a delta
alpha/alpha > 1E-5 level. The origin of these deviations is not clearly
identified but could be related to the adaptation of wavelength calibration to
CCD manufacturing irregularities. These results suggest that to draw
conclusions from delta alpha/alpha analysis based on one or only few lines must
be done with extreme care.
