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Up: A photometric catalogue

2. Flat-field, bias subtraction and correction of MOS distortions

All the data reduction was performed with the IRAF package. Bias and flat-field corrections were made in the usual way. We used the twilight flat-field rather than a median flat produced from the images because the projected density and size of some of the bigger Coma galaxies did not allow to obtain a flat-field totally free of residuals.
We applied the correction for distortion caused by the MOS camera optics (Le Fèvre et al. 1994) that mainly affects the corners of the CCD. This is done by running, for each image, the task GEOTRAN that corrects the distribution of the photon flux in the image pixels by means of a distortion map especially designed for this instrument.
Hot pixels, cosmics and CCD defects (bad columns, dead pixels, ...) were flagged by eye inspection of each image, thus completing the pre-reduction stage.

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