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Up: Optical colour maps

3. Data reduction

The raw CCD images were reduced using the KAPPA package at the STARLINK node in Cambridge (1990 data), and the ESO MIDAS software package at ESO, Garching (1992 data), with standard procedures, i.e. subtraction of the bias current level using a median averaged bias frame, dark current subtraction using dark frames of the same exposure time as the object frame, and flatfielding using a normalized median average of dome flatfield frames. Subtraction of the residual sky was performed by averaging the count levels in several areas away from the galaxy image. Cosmic rays and bad pixels were removed interactively. Multiple exposures were coadded after accurate alignment, to produce the final image. The photometry was calibrated against faint standard stars, observed just before or after the galaxies. An airmass correction was applied to remove the atmospheric extinction, but the frames were not corrected for the small amounts of galactic extinction. The calibrated photometry for NGC 3227, NGC 4151 and NGC 7469 is presented in Kotilainen et al. (1993). For the other galaxies, Table 2 lists the photometry in the B, V, R and I wavebands. We estimate a photometric accuracy of tex2html_wrap_inline2703 based on the standard star dispersion.

Note that at the time of the observations, the CCD camera used suffered from a substantial (up to 10%) non-linearity problem caused by a flaw in the amplifiers after the readout of the chip and before A/D conversion. This non-linearity has no dependence on colour or pixel position or time variation and was corrected for by a polynomial algorithm developed by Kjeldsen (1990). The corrected images are linear to better than 1% and any remaining non-linearity does not affect our conclusions.

Table 2: Photometry

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