Difference between the photometric DR1 and the synthetic G magnitudes for the main NGSL+CALSPEC sample in this paper as a function of the synthetic B_T−V_T color. The left panel uses the nominal G passband curve and Vega zero point (0.030 mag) while the right panel uses the corrected G passband curve and a Vega zero point of 0. The horizontal lines in the right panel show the mean value (solid line, the new zero point) and the range spanned by the standard deviation of the data (thick dashed line, the typical uncertainty of an unsaturated star) and by the standard deviation of the mean (thin dashed line, the uncertainty of the zero point).