As with the CCD characterisation frames, two wavelength-scale reference (arc) frames should be taken bracketing the science exposures if wavelength scales are required (e.g. for radial-velocity measurements). Normally you should find no significant difference between the two extracted arc spectra; however, if you only take one arc exposure and some shift does occur you won't be able to correct for it. Using both arc spectra, a time-weighted mean wavelength scale can be produced and applied to the science data.
See Wavelength Calibration
for more details.
Simple Spectroscopy Reductions