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Laser Comb Wavelength calibration shows that the ThAr one is locallyunreliable 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 deviationcorresponds to delta alpha/alpha ~ 7E-6 for a FeII-MgII pair. Comparison ofline shifts among the 5 FeII lines, with almost identical sensitivity to finestructure constant changes, offers a clean way to directly test the presence ofpossible local wavelength calibration errors of whatever origin. We analyzed 5absorption 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 reveallarge deviations reaching 200 m/s or higher and corresponding to a deltaalpha/alpha > 1E-5 level. The origin of these deviations is not clearlyidentified but could be related to the adaptation of wavelength calibration toCCD manufacturing irregularities. These results suggest that to drawconclusions from delta alpha/alpha analysis based on one or only few lines mustbe done with extreme care.

Calibration issues in estimating variability of the fine structure constant (alpha) with cosmic time