The B,V,R,I photometric calibrations were performed using the standard Landolt(1992) fields. For the field Landolt-98, we used the larger set of photometric standards by Stetson (2000).
The transformation equations are as follows:
Some 100-200 standards were used for the linear fits. In Figure 5 we provide the photometric calibration, with the colour terms, for the B,V,R,I broad band filter of the ESO/MPI WFI. The fit residuals are typically less than 2%, but in some cases the residuals are as high as 5% . The relatively high residuals may be a consequence of the non-uniform background illumination over the field of view, that will be discussed in Section 4.3. A few points with particularly high residuals are due to stars falling partially in the CCD gaps and/or to photometric/astrometric errors in the standard catalogue. Such points were rejected by means of a sigma clipping algorithm. Points typically above the 2 level were not considered for the calibration. The zero points of the photometric calibrations are listed in Table 1. Since the second run was partially non-photometric, we report in Table 1 only the zero points of the first and third observing runs. Also, due to technical problems, it was not possible to calibrate the B-band of the field OACDF4 during the third observing run. The latter calibration was done using secondary standard stars. The colour terms for the OACDF run of April 1999 are given in the left panels of Figure 5. The limiting magnitudes reported in Table 1 were calculated as explained in Appendix B.