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Wide field imaging, and even more whole sky surveys are important tools for most fields of observational astrophysics and cosmology. Solar system history and dynamics, stellar evolution, galactic structure, the evolution of galaxies and of cosmic structures, all call for large statically well defined and homogeneous samples of objects. In the past they were provided only by surveys performed with large format photographic plates (cf. DPOSS) which do not match, however, the requirements (in terms of depth and accuracy) posed by the new generation of 8 m class telescopes such as VLT, Keck, etc.

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:

This paper, far from being the final answer to the above list of problems is intended to i) provide other users with some hints on how to tackle such problems with the WFI data, using publicly available software, ii) characterise the photometric properties of the ESO-WFI (currently the only wide field imaging facility available to the whole European community) and iii) describe the technical background of the forthcoming scientific papers. In addition, the first results of a galaxy cluster identification based on the colour magnitude properties of its population of spheroidal galaxies is outlined and discussed.

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.

next up previous
Next: Field selection and observations Up: The Capodimonte Deep Field Previous: The Capodimonte Deep Field
Juan Alcala