2.3. Photometric calibration
The absolute calibration has been obtained from several repeated observations of Landolt (1992) standard fields, including all the stars listed in the extended catalog of calibrators provided by Stetson (2000). The coefficients of atmospheric extinction (Cext) were directly obtained by repeated observations of the same standard field at different air mass. The final calibrating equations and Cext are shown in the upper panels of Figure 2.
To check the accuracy (and reproducibility) of the photometric calibration we reduced one V and one I 30 s frames centered on the globular cluster NGC 6779, acquired during the second night of the run. The catalogue was calibrated with the relations shown in Fig. 2 and the final photometry was compared with the photometry of the cluster provided by Rosenberg et al. (2000). The results of the comparison (shown in the lower panels of Fig. 2) demonstrate that our photometry is in excellent agreement with that of Rosenberg et al. (2000), an the characteristic uncertainty of our absolute calibration is of the order of ± 0.02 mag.
Figure 2. Upper panels: difference between tabulated magnitudes (V,I) and instrumental magnitudes (v,i) vs instrumental color index (v-i) for the observed Landolt (1992) standard stars. The calibrating relations are plotted (solid lines) and the corresponding equations are reported along with the RMS of the linear fits and the value of extintions coefficients. Lower panels: comparison of the photometry of NGC 6779, obtained from data acquired during the same run of the M 33 data and calibrated with the relations above, and the independent photometry by Rosenberg et al. (2000).