Flat Fielding

Three different types of flat field have been compared:1) a median sky flat (MSF), obtained for each frame by medianingfrom 9 to 13 time-adjacent frames;2) a differential dome flat (DDF), obtained by subtracting lamp-on and lamp-off images of the illuminated dome;3) a ``superflat'' (SF) obtained for each night by combineing all the images of the observed field (always more than 120).

The various flats have been cleaned of bad pixels (hot andcold) using a bad pixel mask constructed withthe task CCDMASK (Iraf) and a selection threshold of± 6 σ.Then the bad pixels have been replaced by thevalue obtained by linearly interpolating the nearest goodpixels (FIXPIX, Iraf). Finally, flats have been normalized totheir mean value.The comparison among flats has been made on the basis of both high and low frequency systematics, that is on both the pixel-to-pixel variations and the scatter of the magnitude of a star measured on a grid of positions over the field. DDF and SF give comparable results and provide the best accuracy in flat fielding with a scatter in the magnitude estimate of ~ 0.02 mag. However it is worth noting that the scatter of the measured magnitude of the star in the non flat fielded frame is only 0.05 mag. The images here have been flat fielded using the DDF - being easier and faster to construct than the SF.