On CCD standard stars and flat-field calibration.

Numerical simulations of a CCD flat-fielding calibration procedure on stellar fields have been performed in the monochromatic case. The relations between the photometric conditions, the observational procedure and the calibration accuracy are explored. This analysis shows how and why one must establish high-quality standard sequences in clusters or moderately crowded fields. The advantages of a long-term management of the reduction procedures is stressed.