Diagnostic plots of the night’s data, all plotted with time on the x-axis. Top panel: variation of air mass across the observations. Second panel: the shift in the target’s spectrum (blue crosses) and comparison’s spectrum (red pluses) along the spatial direction. Third panel: the shift in the target’s spectrum (blue crosses) and comparison’s spectrum (red pluses) along the dispersion direction. Fourth panel: rotation of the target’s spectrum (blue crosses) and comparison’s spectrum (red pluses) as measured by the difference in the traces’ locations at red and blue wavelengths. Fifth panel: the variation in FWHM across the night. Sixth panel: the maximum counts recorded in the target’s spectrum (blue crosses) and comparison’s spectrum (red pluses). Seventh panel: the raw white-light curves of the target (blue crosses) and comparison (red pluses). Eighth panel: the normalized sky background. Bottom panel: WASP-39's white-light curve following division by the comparison’s light curve.