Top panel: Ratio of the CELIAS/SEM first-order light curve to a simulated light curve constructed by propagating the SORCE/XPS solar spectral irradiance through the CELIAS/SEM first-order bandpass. We assume that since SORCE/XPS is well-calibrated against independent measurements of the solar spectral irradiance by other instruments that the decline in this plot is evidence of long-term degradation in the CELIAS-SEM light curve. We fit a line (blue) with linear least squares fitting to account for it. Bottom panel: Uncorrected (black) and corrected (blue) versions of the CELIAS-SEM first-order light curve.