In recent years, the operation of large telescopes with wide field detectors - such as the European Southern Observatory (ESO) Wide Field Imager (WFI) on the 2.2 meters telescope at La Silla, Chile - have dramatically increased the amount of astronomical data produced each year. The next survey telescopes, such as the ESO VST, will continue on this trend, producing extremely large datasets. Astronomy, therefore, has become an incredibly data rich field requiring new tools and new strategies to efficiently handle huge archives and fully exploit their scientific content. At the Space Telescope European Coordinating Facility we are working on a new project, code named Querator (http://archive.eso.org/querator/). Querator is an advanced multi-archive search engine built to address the needs of astronomers looking for multicolor imaging data across different astronomical data-centers. Querator returns sets of images of a given astronomical object or search region. A set contains exposures in a number of different wave bands. The user constraints the number of desired wave bands by selecting from a set of instruments, filters or by specifying actual physical units. As far as present-day data-centers are concerned, Querator points out the need for: - an uniform and standard description of archival data and - an uniform and standard description of how the data was acquired (i.e. instrument and observation characteristics). Clearly, these pieces of information will constitute an intermediate layer between the data itself and the data mining tools operating on it. This layered structure is a prerequisite to real data-center inter-operability and, hence, to Virtual Observatories. A detailed description of Querator's design, of the required data structures, of the problems encountered so far and of the proposed solutions will be given in the following pages. Throughout this paper we'll favor the term data-center over archive to stress the need to look at raw-pixels' archives and catalogues in an homogeneous way.