Ratio between the observed and predicted flux as a function of the stellar effective temperature. The blue dots in the plot represent over 100 measurements conducted during the IOC. Notably, the observed flux is roughly 7% lower than what was predicted by the instrument model. While the exact cause of this discrepancy remains uncertain, it is apparent that the model encompasses several sources of uncertainty that could potentially contribute to this difference. Some of these sources include uncertainties in the CALSPEC flux scale, variations in the measured optical transmission, and discrepancies in the QE, among others.