Internal consistency check of the wavelength solution within a sub-band. The top panel shows a sub-optimal wavelength calibration of sub-band 2A. The misalignment of lines introduced by the uncertainties in the cross calibration of a few iso-α lines and slices is clearly seen as a shift to the right from the main “oversampled” lines. The bottom panel shows the resulting etalon spectrum once the uncertainties in the calibration have been addressed.