Calibration issues in Δα/α

Laser Comb Wavelength calibration shows ThAr one locally unreliable with deviations up to 100 m s^-1 (or Δα/α ≈ 7 · 10^-6 for a Fe ii-Mg ii pair) while delivering an overall 1 m s^-1 accuracy. Comparison of line shifts of the 5 Fe ii lines with identical sensitivity to Δα/α offers a clean way to test local wavelength calibration errors of whatever origin.