Data reduction procedures

One observation run consisted of approx. 1300 images, including calibration images. For the calibrations dark current images of 40 minutes exposure time were made daily, dome and twilight flat frames at the beginning and end of every night. Calibration images of stars with known magnitudes were taken before and after every galaxy exposure, alternatively afte every second galaxy, and the latter were imaged 20 minutes in the Cousins R and 40 minutes in the Johnson B band. This complex procedure limited the acquisitions to around five galaxies per night.

The data reduction was mostly done in the standard manner, but with extreme attention paid to the obtention of very good flat fields, as we were interested in measuring galaxy luminosity profiles down to 1% of the sky luminosity⁷. Sky levels were estimated (as tilted planes) and subtracted from the image frames, stars superimposed on the galaxy images were subtracted and growth curves were determined by circular or elliptical aperture photometry centered on the galaxies.⁸

An example of a faint dwarf galaxy is given in Fig. , where an image from the digital sky survey POSS II is shown for comparison together with a 40 minute exposure in the B band.

Figure: BK5N ( 33#33arcmin (left) and 34#34arcmin 80 minute B exposure (right)

32#32