Mining large quantities of uncalibrated archives, for specific sources can prove to be a hard task. Even an automated search engine able to use an archive metadata (instrument, a filter, exposure time...) is not completely sufficient. Indeed, without calibration it is difficult to know whether an interesting source can be seen on images without actually looking. Here, we show how a "reversed" exposure time calculator can be used to efficiently process the database-stored image descriptors of the ESO/Wide Field Imager (WFI) archive, and compute the corresponding limiting magnitudes. The end result is a more scientific description of the ESO/ST-ECF archive contents, allowing a more astronomer-friendly archive user interface, and hence increasing the archive useability in the context of a Virtual Observatory. This method is developed for improving the Querator search engine of ESO/HST archive, in the context of the EC funded ASTROVIRTEL project.