WINGS: a WIde-field Nearby Galaxy-cluster Survey I. Optical imaging

This is the first paper of a series that will present data and scientific results from the WINGS project, a wide- field, multiwavelength imaging and spectroscopic survey of galaxies in 77 nearby clusters. The sample was extracted from the ROSAT catalogs of X- Ray emitting clusters, with constraints on the redshift (0.04 < z < 0.07) and distance from the galactic plane (vertical bar b vertical bar >= 20 deg). The global goal of the WINGS project is the systematic study of the local cosmic variance of the cluster population and of the properties of cluster galaxies as a function of cluster properties and local environment. This data collection will allow the definition of a local, '' zero- point '' reference against which to gauge the cosmic evolution when compared to more distant clusters. The core of the project consists of wide- field optical imaging of the selected clusters in the B and V bands. We have also completed a multifiber, medium- resolution spectroscopic survey for 51 of the clusters in the master sample. The imaging and spectroscopy data were collected using, respectively, the WFC@ INT and WYFFOS@ WHT in the northern hemisphere, and the WFI@ MPG and 2dF@ AAT in the southern hemisphere. In addition, a NIR (J, K) survey of similar to 50 clusters and an H-a + U survey of some 10 clusters are presently ongoing with the WFCAM@ UKIRT and WFC@ INT, respectively, while a very- wide- field optical survey has also been programmed with OmegaCam@ VST. In this paper we briefly outline the global objectives and the main characteristics of the WINGS project. Moreover, the observing strategy and the data reduction of the optical imaging survey (WINGS-OPT) are presented. We have achieved a photometric accuracy of similar to 0.025 mag, reaching completeness to V similar to 23.5. Field size and resolution (FWHM) span the absolute intervals (1.6 - 2.7) Mpc and (0.7 - 1.7) kpc, respectively, depending on the redshift and on the seeing. This allows the planned studies to obtain a valuable description of the local properties of clusters and galaxies in clusters.