Stability of the instrumental oxygen edge: comparison of spectra corrected for a change in slope and normalization (top) and ratio (bottom) of Mrk 421 data in orbit 84 (red) and orbit 2002 (green). The spectra show the uncorrected instrumental edge, which is normally corrected for during default data processing. The slight linear trend in the relative difference plot (bottom) is caused by small changes in the source power law spectrum over time.