The DESCART weak lensing project is a theoretical and observational program for cosmological weak lensing investigations. The cosmic shear survey carried out by the DESCART team uses the CFH12K data jointly with the VIRMOS survey to produce a large homogeneous photometric sample which will eventually contain a catalog of galaxies with redshifts as well as the projected mass density over the whole field (Le Fèvre et al. 2001). In contrast to Van Waerbeke et al. (2000), the new sample presented in this work only uses I-band data taken with the CFH12K camera and is therefore more homogeneous. It is worth noting that only half of the data of the previous CFHT12K sample is reused in our new sample. A comparison of the results will also allow checking the consistency and the robustness of the cosmic shear analysis.

The CFH12K data was obtained during dark nights in May 1999, November 1999 and April 2000 following the standard observation procedure described in Van Waerbeke et al. (2000). The fields are spread over 4 independent $2\times2$ deg² areas of the sky identified as F02, F10, F14 and F22. Each field is a compact mosaic of 16 CFH12K pointings named P[n] with n=1-16. Once the survey is completed, each of them will cover 4 deg². Currently, of the final 16 deg², only 8.38 deg² is available for the analysis - most of the pointings are located in three different fields (F02, F10, F14 listed in Table 1). This total field of view gets significantly reduced by the masking and selection procedures described below. A summary of the data set characteristics are listed in Table 1.

Table 1: List of the fields. All observations were done in I band with the CFH12K camera (Cuillandre et al.2000). The number following the F denotes the field name, and the number following the P denotes the pointing name within the field. The geometry of the survey is detailed in http://terapix.iap.fr/Descart/. The image quality has been measured on each stacked image from a standard fitting of a Moffat profile.
Target Used area Exp. time Period Image quality

F02P1 980 ${\rm arcmin}^2$ 9390 s Nov. 1999 0.75''

F02P2 1078 ${\rm arcmin}^2$ 7200 s Nov. 1999 0.90''

F02P3 980 ${\rm arcmin}^2$ 7200 s Nov. 1999 0.90''

F02P4 1078 ${\rm arcmin}^2$ 7200 s Nov. 1999 0.80''

F10P1 882 ${\rm arcmin}^2$ 3600 s May 1999 0.65''

F10P2 882 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F10P3 490 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F10P4 882 ${\rm arcmin}^2$ 3600 s May 1999 0.65''

F10P5 882 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F10P7 1176 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.75''

F10P8 1176 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.70''

F10P9 98 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.65''

F10P10 784 ${\rm arcmin}^2$ 3600 s Nov. 1999 0.80''

F10P11 294 ${\rm arcmin}^2$ 3600 s Nov. 1999/Apr. 2000 0.90''

F10P12 1176 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.80''

F10P15 686 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.85''

F14P1 882 ${\rm arcmin}^2$ 3600 s May 1999 0.80''

F14P2 882 ${\rm arcmin}^2$ 3600 s May 1999 0.85''

F14P3 686 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F14P4 1078 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F14P5 980 ${\rm arcmin}^2$ 3600 s May 1999 0.70''

F14P6 686 ${\rm arcmin}^2$ 3600 s May 1999 0.80''

F14P7 686 ${\rm arcmin}^2$ 3600 s May 1999 0.70''

F14P8 882 ${\rm arcmin}^2$ 3600 s May 1999 0.85''

F14P9 1078 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.75''

F14P10 784 ${\rm arcmin}^2$ 3600 s May 1999 0.85''

F14P11 882 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.80''

F14P12 784 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.80''

F14P13 882 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.85''

F14P14 882 ${\rm arcmin}^2$ 3600 s May 1999 1.0''

F14P15 882 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.90''

F14P16 1176 ${\rm arcmin}^2$ 2880 s Apr. 2000 0.65''

F22P3 686 ${\rm arcmin}^2$ 3600 s May 1999 0.75''

F22P4 980 ${\rm arcmin}^2$ 3600 s Nov. 1999 0.75''

F22P6 588 ${\rm arcmin}^2$ 3600 s Apr. 2000 0.80''

F22P11 294 ${\rm arcmin}^2$ 2880 s Apr. 2000 0.75''

**Table 1:** List of the fields. All observations were done in I band with the CFH12K camera (Cuillandre et al.2000). The number following the F denotes the field name, and the number following the P denotes the pointing name within the field. The geometry of the survey is detailed in `http://terapix.iap.fr/Descart/`. The image quality has been measured on each stacked image from a standard fitting of a Moffat profile.
Target	Used area	Exp. time	Period	Image quality
F02P1	980 ${\rm arcmin}^2$	9390 s	Nov. 1999	0.75''
F02P2	1078 ${\rm arcmin}^2$	7200 s	Nov. 1999	0.90''
F02P3	980 ${\rm arcmin}^2$	7200 s	Nov. 1999	0.90''
F02P4	1078 ${\rm arcmin}^2$	7200 s	Nov. 1999	0.80''
F10P1	882 ${\rm arcmin}^2$	3600 s	May 1999	0.65''
F10P2	882 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F10P3	490 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F10P4	882 ${\rm arcmin}^2$	3600 s	May 1999	0.65''
F10P5	882 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F10P7	1176 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.75''
F10P8	1176 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.70''
F10P9	98 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.65''
F10P10	784 ${\rm arcmin}^2$	3600 s	Nov. 1999	0.80''
F10P11	294 ${\rm arcmin}^2$	3600 s	Nov. 1999/Apr. 2000	0.90''
F10P12	1176 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.80''
F10P15	686 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.85''
F14P1	882 ${\rm arcmin}^2$	3600 s	May 1999	0.80''
F14P2	882 ${\rm arcmin}^2$	3600 s	May 1999	0.85''
F14P3	686 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F14P4	1078 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F14P5	980 ${\rm arcmin}^2$	3600 s	May 1999	0.70''
F14P6	686 ${\rm arcmin}^2$	3600 s	May 1999	0.80''
F14P7	686 ${\rm arcmin}^2$	3600 s	May 1999	0.70''
F14P8	882 ${\rm arcmin}^2$	3600 s	May 1999	0.85''
F14P9	1078 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.75''
F14P10	784 ${\rm arcmin}^2$	3600 s	May 1999	0.85''
F14P11	882 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.80''
F14P12	784 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.80''
F14P13	882 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.85''
F14P14	882 ${\rm arcmin}^2$	3600 s	May 1999	1.0''
F14P15	882 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.90''
F14P16	1176 ${\rm arcmin}^2$	2880 s	Apr. 2000	0.65''
F22P3	686 ${\rm arcmin}^2$	3600 s	May 1999	0.75''
F22P4	980 ${\rm arcmin}^2$	3600 s	Nov. 1999	0.75''
F22P6	588 ${\rm arcmin}^2$	3600 s	Apr. 2000	0.80''
F22P11	294 ${\rm arcmin}^2$	2880 s	Apr. 2000	0.75''

The data reduction was done at the TERAPIX data center. More than 1.5 Tbytes of data were processed in order to produce the final stacked images. The reduction procedure is the same as in Van Waerbeke et al. (2000), so we refer the reader to this paper for the details. However, in order to improve the image quality prior to correction for the PSF anisotropy and to get a better signal-to-noise ratio on a larger angular scale than in our previous work, all CFH12K images were co-added after astrometric corrections.

The astrometric calibration and the co-addition were done using the MSCRED package in IRAF. Some tasks have been modified in order to allow a fully automatic usage of the package. For each pointing, we first started with the images in the I band. An astrometric solution was first found for one set of exposures in the dither sequence using the USNO-A 2.0 as reference, which provides the position of $\sim\!\times10^8$ sources with an RMS accuracy of 0.3 arcsec (that is 300-500 objects per field). The astrometric solution was then transferred to the other exposures in the sequence. All object catalogs were obtained using SExtractor (Bertin & Arnouts 1996) and a linear correction to the world coordinate system was computed with respect to the initial set. Finally, all images were resampled using a bi-cubic interpolation and then stacked together.

At this stage, each stacked image was inspected by eye and all areas which may potentially influence the later lensing analysis signal were masked (see Van Waerbeke et al. 2000 and Maoli et al. 2001). Since we adopted conservative masks, this process had a dramatic impact on the field of view: we lost 20% of the total area and ended up with a usable area of 6.5 deg².

The photometric calibrations were done using standard stars from the Landolt catalog (Landolt 1992) covering a broad sample of magnitude and colors. A full description of the photometric procedure is beyond the scope of this work and will be discussed elsewhere (Le Fèvre et al., in preparation). In summary, we used the SA110 and SA101 star fields to measure the zero-points and color equations of each run. From these calibrations, we produced the magnitude histograms of each field in order to find out the cut off and a rough limiting magnitude. Although few fields have exposure time significantly larger than 1 hour, the depth of the sample is reasonably stable from field to field and reaches $I_{\rm AB}=24.5$ (this corresponds to a 5 $\sigma$ detection within a 3 arcsec aperture). Up to this magnitude, 1.2 million galaxies were detected over the total area of 8.4 deg², and the final number density of galaxies over the usuable area of 6.5 deg² is ${\sim} 17~{\rm galaxies/arcmin}^2$ . This is about two times less than the number of detected galaxies because of the filtering processes described in the next section.

2 The data set