Difference between a spectrum corrected for the fringe-like pattern (red solid line) and a spectrum that was not corrected (black solid line). The green solid line shows the correction function across the wavelength range where the scaling is given at the right y-axis. The line strength is measured between the red dashed lines and the continuum band-passes between the dashed-dotted vertical lines. The line-strength measurements for these two lines in the uncorrected and corrected spectrum are for Hγ_A = −4.79 ± 0.33 Å, and −5.07 ± 0.37 Å, and for Mg b = 5.33 ± 0.28 Å, and 5.25 ± 0.32 Å, respectively.