Data Reduction

The data reduction consisted of the following steps: each individual frame has been flat fielded and sky subtracted; then, for each basic set of flat-fielded and sky subtracted frames, a final image (usually of 1h exposure) has been produced by coadding the frames; the images thus obtained have been rescaled to the same zero pointand airmass and, finally, have been coadded to produce the final image. All the reduction steps have been performed using the Jitter Imaging Data Reduction Algorithms (Devillard 1998) whose procedures are briefly described below.