2 Observations

We have imaged the Chandra Deep Field South in all 17 filters for the COMBO-17 project using the Wide Field Imager (WFI, Baade et al. 1998, 1999) at the MPG/ESO 2.2 m telescope on La Silla, Chile. The WFI is a mosaic camera consisting of eight 2k $\times$ 4k CCDs with $\sim$67 million pixels in total, a pixel scale of $0\hbox{$.\!\!^{\prime\prime}$ }238$ and a field of view of $33\hbox{$^\prime$ }\times 34\hbox{$^\prime$ }$. The CCDs are rather blue sensitive and some of them are cosmetically suboptimal since they are only of engineering grade.

Here we discuss data obtained in the broad-band WFI filters B, V and R (see Table 1 for a brief observing log). Our data encompass a total exposure time of 5000 s in B and 8400 s in V with seeing on the order of $\sim$ $1\hbox{$.\!\!^{\prime\prime}$ }2$ and altogether 23700 s in R with $\sim$ $0\hbox{$.\!\!^{\prime\prime}$ }75$ mean PSF. Besides long exposures for efficient light gathering, we included short exposures for the photometry of brighter objects, in particular to avoid saturation of our brighter standard stars.

The long exposures followed a dither pattern with ten telescope pointings spread by  $\Delta \alpha$, $\Delta \delta < \pm 72\hbox{$^{\prime\prime}$ }$. This dither pattern is motivated by the intent to close the gaps in the CCD mosaic, but limited by the requirement of keeping field rotation at a minimum. Due to the gaps in the CCD mosaic the effective exposure time varies within the field. However, dithering was performed such, that each position on the sky falls onto a CCD in at least eight out of ten exposures, while 97% of the area is always recorded in every image.

Twilight flatfields were obtained with offsets of 10'' between consecutive exposures. Exposure times ranged between 0.5 and 100 s per frame (note that the WFI shutter design allows exposures as short as 0.1 s without causing significant spatial variations in the illumination across the CCD mosaic (Wackermann 1999).

   

Table 1: Observing log for B, V and R exposures on the Chandra Deep Field South as part of the COMBO-17 survey.

Epoch (UT)	Band	Seeing	Exposure (s)
1999, Oct. 10	V	$1\hbox{$.\!\!^{\prime\prime}$ }0 \ldots 1\hbox{$.\!\!^{\prime\prime}$ }4$	$14\times 600$ + $3\times 20$
1999, Oct. 13	B	$1\hbox{$.\!\!^{\prime\prime}$ }0 \ldots 1\hbox{$.\!\!^{\prime\prime}$ }4$	$10\times 500$ + $3\times 20$
1999, Oct. 19	R	$0\hbox{$.\!\!^{\prime\prime}$ }6 \ldots 0\hbox{$.\!\!^{\prime\prime}$ }9$	$25\times 420$
1999, Oct. 20	R	$0\hbox{$.\!\!^{\prime\prime}$ }6 \ldots 0\hbox{$.\!\!^{\prime\prime}$ }9$	$11\times 420$ + $1\times 370$
2000, Feb. 6	R	$0\hbox{$.\!\!^{\prime\prime}$ }6 \ldots 0\hbox{$.\!\!^{\prime\prime}$ }9$	$6\times 500$ + $2\times 60$
2000, Feb. 8	R	$0\hbox{$.\!\!^{\prime\prime}$ }6 \ldots 0\hbox{$.\!\!^{\prime\prime}$ }9$	$10\times 500$ + $1\times 200$

We have established our own set of tertiary standard stars based on spectrophotometric observations, mainly in order to achieve a homogeneous photometric calibration for all 17 WFI filter bands. Two stars of spectral types G-F and magnitudes $B_{\rm J} \simeq 16$ were selected in each COMBO-17 field, drawn from the Hamburg/ESO Survey database of digital objective prism spectra (Wisotzki et al. 2000). The spectrophotometric observations for the Chandra Deep Field South were conducted at La Silla on Oct. 25, 1999, using the Danish 1.54 m telescope equipped with DFOSC. A wide (5'') slit was used for the COMBO-17 standards as well as for the external calibrator, in this case the HST standard HD 49798 (Bohlin & Lindler 1992). Two exposures of 45 min were taken of each star, one with the blue-sensitive grism 4 covering the range $\lambda = 3400$-7400Å, and one with the red-sensitive grism 5 covering $\lambda > 5200$Å.

The spectra were reduced by standard procedures and have a final signal-to-noise ratio of >30 per pixel except very near to the low- and high-wavelength cutoffs. The agreement between spectra in the substantial overlap in wavelength between the two grisms is excellent, confirming that contamination from second order was negligible. The absolute spectrophotometric accuracy, estimated from comparing several spectra of the external calibrator HD 49798 obtained during the entire observing run, is better than 10%.

Figure 1: Total system efficiencies for the WFI filters B, V and R(including telescope, instrument and detector).