The shortcomings of photographic plates have been circumvented only in recent years when the introduction of large mosaics of CCD detectors has ensured a compromise between pixels scale, field of view and sensitivity. Large mosaic CCD data thus prompted several pioneeristic projects such as, for instance, the ESO Imaging Survey (EIS) which has provided the whole European astronomical community with extensive catalogues of objects for follow-up's at the VLT. The first public surveys showed the problems posed by the handling (the size of the data sets is in the Terabyte domain), processing (they require a technique which is different from both photographic plates and single CCD data), and mining of the data produced by the new large format CCD detectors.
The Osservatorio Astronomico di Capodimonte (OAC) in collaboration with ESO is currently building the VLT Survey Telescope or VST, a 2.6 m telescope entirely dedicated to imaging work which will become operational in the Fall of 2002 and will be located next to VLT, on the Paranal plateau. VST will be equipped with the OMEGAM camera (a 16k x 16k array of 32 CCD's) which will cover 1 sq. deg. The expected throughput of the system VST+OMEGAM is around 100 GB/night and for many applications (e.g. super novae detection, gravitational lensing, etc.) these data will need to be processed and analysed on a very short time scale. In order to be ready for the scientific exploitation of VST, OAC has started a pilot project, the OAC Deep Field or OACDF, which is a multicolour imaging survey covering 1 sq. deg. The OACDF data were acquired in the course of 15 guaranteed nights at the 2.2 m ESO telescope on La Silla, using the Wide Field Imager (WFI, Baade et al. 1998).
The OACDF observing strategy was driven by three main scientific goals: i) search for rare/peculiar objects and in particular for high-redshift QSO's (3.8<z<5.5); ii) search for intermediate-redshift spheroids to be used as tracers of galaxy evolution; iii) search for high redshift clusters to be used as targets for subsequent spectroscopic follow-up's at larger telescopes. As it always happens with these type of data, the preliminary analysis of the OACDF data has lead to some serendipitous discoveries, namely the discovery of a gravitational lens (Longo et al. 2001, submitted) and of several astrometric (asteroids) and photometric (variable stars) transients (Ortiz et al. 2001, in preparation).
In spite of the large effort which has taken place in the last few years, the problems posed by the processing of mosaic CCD data are still far from being fully solved:
In Section 2 the field selection and observational strategy for the OACDF are described; the OACDF data reduction, including fringing correction, astrometric solution and image stacking, are reported in Section 3; the photometric calibrations, as well as the systematic behaviour of the zero points of the CCDs of the ESO-WFI, are reported in Section 4. In Section 5 the catalogue extraction methods are explained, while Section 6 reports the first results on the galaxy clusters identification and the detection of a new cluster. Finally, our conclusions are presented in Section 7.