5 Basic properties of the co-added images

A summary of the properties of the individual co-added images is presented in Table 3. The total integration time for the co-added images is given as well as the number of frames used, the average FWHM measured on 10 stars across the field, the area with 80% weight for each individual image and the 50$\%$ completeness limits for a point source as described in Sect. 6.

 

 

Table 3: Overview of the photometric observations.

Band	Exposure	Frames	FWHM	80% weight	50$\%$ compl. limit
	Time [s]		[ $^{\prime\prime}$]	[ $\hbox{$^\prime$ }^2$]	[mag]
U	44 400	37	0.97	40.7	25.64
B	22 660	44	0.60	40.5	27.69
g	22 145	43	0.87	41.1	26.86
R	26 400	110	0.75	40.8	26.68
I	24 900	83	0.53	40.9	26.37
J	$4800^{\rm a}$	$80^{\rm a}$	1.20	4.2/53.8	23.60/22.85
Ks	$4800^{\rm a}$	$80^{\rm a}$	1.24	4.4/53.7	21.57/20.73

$^{\rm a}$ Minimum exposure time and number of frames for each subset. Due to the overlap of the subsets for some (small)
regions of the FDF the total time was twice or even four times this value. The 80% weight and 50% completeness
levels in J and Ks are given for the 320 (central field) and 80-minutes co-added data, respectively.

The integration times are in total almost a factor of 2 higher than originally planned (except for the U filter). This is due to our strict seeing limits during the first observing runs. It compensates, at least in part, the loss of resolution/depth of the images due to the less than optimal seeing. Still, the completeness limits are somewhat lower than expected for the integration times since the efficiencies of the telescope (reflectivity of the main mirror) and the CCD were below expected at the time of the observations. In general, the zero points remained relatively constant during the observations carried out in 1999, whereas they differed considerably between the observations taken in 1999 and 2000. This resulted in a loss of approx. 0.3 mag (see the ESO-Web page, Paranal zero points).

The area with 80% weight is very similar for all optical bands and 30% larger for the NIR bands. The latter is due to the 4 subsets taken during the NIR observations. The common area with 80% weight in all filters is $39\farcm8^2$.

As an example, the co-added I band image of the FDF is displayed in Fig. 2. The common area of the input images for a $6\hbox {$^\prime $ }\times 6\hbox {$^\prime $ }$ region is shown here. It contains $\sim$6100 galaxies. In general, the galaxies are distributed evenly across the field. There is a poor galaxy cluster (at $z \sim 0.3$) in the southwestern corner of the FDF. The QSO Q 0103-260 is south of the center of the frame and is marked with an arrow. The brightest object in the field is an elliptical galaxy with ${m}_{\rm I} = 16.5$ at $z \sim 0.2$ in the southeastern part of the FDF.