Representative profiles of the weaker Fe I line and its gradual changes. Each profile, which is the average of 26 Fe I lines, is plotted for 14 successive exposures during the exoplanet transit, showing the degree of uniformity reached. To appreciate the slight changes during the transit, this plot should be viewed highly magnified; corresponding line ratios are in Fig. 11. The photometric signal-to-noise ratio here is ~2500.