Optical Photometry
The photometric uncertainties due to the flat fielding are expected to be less than 1% (0.01mag), while those arising from bias removal and linearity correction are likely to be negligible.
Calibration
Following Moles et al.(1985), we assume that, even though the
atmospheric extinction varies night by night, the observing set
remains stable during each observing run. This implies that the
out-of-atmosphere instrumental magnitude of each standard star
measured on a given chip of the mosaic, is constant throughout the
run, being different on different chips. 
Thus, for each observing run
and for each filter, this procedure provides us with an extinction
coefficient for each night and with a set of zero points and color
coefficients (one pair for each CCD of the mosaic) holding for the
entire run.
First, we looked for the extinction coefficients (see table at the left), solving the following system of N generalized Bouger's equations:
where k is the extinction coefficient, Χ is the airmass and N is the number of determinations (m) obtained for the out-of-atmosphere instrumental magnitude (m0) of a given star (s) during a given night (n) on a given chip of the mosaic (c). Then, for each observing run and for each filter, we computed the photometric
zero points Zc and the color coefficients Cc of the different mosaic CCDs(c) (see next Table) by solving a system of equations like this:
Final Magnitudes
After sky subtraction and exposure time normalization, each CCD of each image has been corrected, for atmospheric extinction and gain differences among the different CCDs, multiplying each pixel by the factor:
where again n and c are the night and the CCD, while
and kn
represent the CCD-averaged zero point and the extinction coefficient
of the night, respectively. Then, the final magnitudes have been
obtained by the formula: 
where mSEx comes from SExtractor running with zero point:
Cc represents the color coefficient of the particular CCD and the true color (B-V) can be easily evaluated by the equation:
whith:
The headers of the co-added mosaic frames of each cluster have been updated with keywords giving the proper photometric coefficients, including ZSEx (keyword: K_0) and δCc (keyword: DELTAC) and the subtracted background value (keyword: SKY).
Consistency checks
The typical r.m.s of the residuals we achieved with our calibration
is of the order of 0.025mag (see Figure below). 