3 Observations and reduction

The observations were performed with the 2.5 m DuPont telescope at Las Campanas Observatory, Chile, in the nights of 26-29 September, 1994. A Tektronix $2048\times 2048$ pixel chip has been used, with a pixel size of $21~\mu$m or $0\hbox{$.\!\!^{\prime\prime}$}227$ at the sky, corresponding to a total field of view of $7\hbox{$.\mkern-4mu^\prime$}7\times7\hbox{$.\mkern-4mu^\prime$}7$.

Deep exposures in Johnson V and Cousins I were acquired. Typical total exposure times are between 30 min and 60 min per field in both filters. The seeing, given as the full width at half maximum (FWHM) of a Gaussian profile, ranged between $0\hbox{$.\!\!^{\prime\prime}$}9$ and $1\hbox{$.\!\!^{\prime\prime}$}5$.The exposures of the south-west field of the NGC 1399 mosaic were affected by a tracking jump in one long V exposure and the corruption of one long I exposure. Omitting these two exposures the total exposure time was reduced to 15 min in both filters. Nevertheless, this field has been considered in several aspects of our investigations since it contains some interesting bright galaxies. For the photometric calibration typically 15-30 standard stars from the Landolt (1992) list have been observed throughout the nights.

Table 1 summarizes the observations. In addition, the detection threshold $\mu_{\rm limit}$ in mag arcsec^-2 in V and I is given for each field (see also Sect. 4). For more information about the calibration see Kissler-Patig et al. (1997).

  

Table 1: The observations of the Fornax fields obtained with the 2.5 m telescope and the background fields obtained with the 1.0 m telescope on Las Campanas. In addition, the surface brightness threshold $\mu_{\rm limit}$ in mag arcsec^-2, above which objects of at least five connected pixels are detected, is given for each field

Bias subtraction, flat-fielding, long exposures combination, modelling, and subtraction of the galaxy light of the giants were done with IRAF (for details see Kohle et al. 1996 and Kissler-Patig et al. 1997). After these procedures frames with flat backgrounds ready for object search and photometry were obtained.

3.1 Background fields

To get an estimation of the absolute background galaxy counts two additional fields $10\hbox{$^\circ$}$ north (B3) and $15\hbox{$^\circ$}$ south (B4) of NGC 1399 have been observed. The total magnitudes and colors of the galaxies have been determined for a statistical comparison with the photometric properties of galaxies in the Fornax fields. The surface brightness (SB) profiles have not been measured, because the resolution of the images is 3 times lower than in the run at the 2.5 m telescope.

The observations have been performed in a second run with the 1.0 m Swope telescope at Las Campanas Observatory, Chile, in the night of 2/3 December, 1996. A SITe#1 $2048\times 2048$ pixel chip has been used, with a pixel size $0\hbox{$.\!\!^{\prime\prime}$}694$ at the sky, corresponding to a total field of view of $23\hbox{$.\mkern-4mu^\prime$}69\times23\hbox{$.\mkern-4mu^\prime$}69$. Table 1 (at the bottom) gives an observation log of the V and I exposures. As in the first run the photometric calibration was done via standard stars from the Landolt (1992) list.