4 Photometric homogeneity of the two runs

Figure 3: Comparison of the magnitudes and colors of 456 stars in common between the April and December runs, for the GGC NGC 3201. Stars with photometric internal errors smaller than 0.02 mag have been selected. The mean differences are given in each panel. Note: a few NGC 3201 RR Lyrae stars can be identified in the interval 14.2 < V < 15.2 (lower two panels), and between 0.4 < (V-I) < 0.9

In order to check the photometric homogeneity of the data and of the calibration to the standard photometric system, one cluster (NGC 3201) was observed in both runs. Having one common field, it is possible to analyze the individual star photometry, and test if any additional zero point difference and/or color term exist. The latter check is crucial when measures of the relative position of CMD features are going to be done. The comparison between the two runs is presented in Fig. 3, where 456 common stars with internal photometric errors (as given by ALLFRAME) smaller than 0.02 mag are used. Figure 3 shows that there are no systematic differences between the two runs.

The slope of the straight lines best fitting all the points in both the ( $V,~\Delta V^{\rm apr}_{\rm dec}$) plane and the ( $V,~\Delta I^{\rm apr}_{\rm dec}$) plane is $\leq 0.001\pm0.002$, and $\leq 0.002\pm0.003$ in the ( $V-I,~\Delta (V-I)^{\rm apr}_{\rm dec}$) plane. The zero point differences are always $\le$0.01 mag. This ensures the homogeneity of our database, particularly for relative measurements within the CMDs.