Illustration of the pre-filter correction method using the Hα tables of David (1961). Panel a: average QS profile calculated from the area marked in Fig. 1c. Vertical dashed lines indicate 27 IBIS wavelength positions used. Panel b: reference David’s Hα profile at μ_cal = 0.91. Panel c: correction between panels a and b. Panel d: average QS profile with panel c correction applied, and panel b over-plotted in dashed-line. Panel e: residuals remaining between the corrected profile and the reference profile.