TSIS-1 SIM spectral degradation rate as a function of solar exposure. Results here are derived from the SIM A to SIM B spectral ratios (as defined by the form of Eq. (A.5)) and the exponential fits. The individual 1 − σ error bars are representative of the standard deviation of the exponential fits to the cumulative solar exposure time. The dominant degradation occurs near 220 nm and is plotted as a rate based on 24-hours of solar exposure. The SIM channel C is corrected with this degradation rate based on the SIM channel C cumulative exposure time. SIM Channel C experiences 12 h of solar exposure per year (6 h per 6-month calibration cadence).