2 Optical properties of the lens MS 1008-1224

The observations were all carried out by the Science Verification Team at ESO. The $6\hbox{$.\mkern-4mu^\prime$ }{8} \times 6$ $.\mkern -4mu^\prime$8 field of FORS1 was centred on the cD galaxy in MS 1008-1224 and images were obtained in the B, V, R and I bands. ISAAC was used to obtain 2 $.\mkern -4mu^\prime$ $5 \times 2$ $.\mkern -4mu^\prime$5 images of the central region of the FORS1 field in the J and K bands. The details of the data processing, from the image acquisition to co-addition and calibration, may be found at the ESO web site (see URL in the previous section) while a summary of the image characteristics are presented in Table 1.

 

 

Table 1: Summary of the characteristics of the VLT observations of MS 1008-1224 with FORS1 and ISAAC. Columns 2, 4 and 5 are from from the ESO web site while the seeing ( FWHM) was obtained from the images themselves.

Filter	Exp. Time	Seeing	SB-Lim	Scale
	(s)	('')	(mag/arcsec2)	(''/pix)
B	4950	0.72	28.25	0.2
V	5400	0.65	27.90	0.2
R	5400	0.64	27.44	0.2
I	4050	0.55	26.37	0.2
J	2880	0.68	-	0.147
K	3600	0.45	-	0.147

The FORS1 field corresponds to a physical size of 1.2 h^-1 Mpc at z=0.31and provides a global view of the morphology of MS 1008-1224 and of the field of galaxies around it. Together with the X-ray observations of Lewis et al. (1999), these high quality multi-band images are well suited for a detailed analysis of the distribution of mass within the cluster.

Source detection and photometry were performed in a standard manner using SExtractor (version 2.1.0; Bertin & Arnoults 1996). The magnitude distributions of galaxies are shown in Fig. 1.

Figure 1: Histograms of the magnitude distribution of the MS 1008-1224 field: B (top left), V (top right), R (middle left), I (middle right), J (bottom left), K (bottom right).

The photometric accuracy is better than 0.1 mag up to $B \sim 26.5$, $V \sim 26.5$, $R \sim 25.8$, $I \sim 24.5$, $J \sim 22$ and $K \sim 20$. The stars in the field were identified by their location on the magnitude - half-light radius plot (Fahlman et al. 1994). This method easily and unambiguously separates stellar objects from extended ones up to $I \sim 22$ or $R \sim 23$ and provides enough stars to determine the PSF variation across the FORS1 field. We note here that the expected fraction of stars at fainter magnitudes is negligible (Griffiths et al. 1994; Conti et al. 1999) and hence is not a matter of significant concern for the weak-lensing analysis.