This paper ANDROMEDA, an astronomical data mediation system that enables transparent access to astronomical data servers. Transparent access is achieved by a global view that expresses requirements of community of users (e.g., astronomers) and data integration mechanisms adapted to astronomical data characteristics. Instead of providing an ad hoc mediator, ANDROMEDA can be configured for giving access to different data servers according to different user requirements (data types, content, data quality, and provenance). ANDROMEDA can be also adapted when new sources are integrated to the community or new requirements are specified.