The Extremely Large Telescope (ELT) is a 39 meters optical telescope under construction at an altitude of about 3000m in the Chilean Atacama desert. The optical design is based on a novel five-mirror scheme and incorporates adaptive optics mirrors. The primary mirror consists of 798 segments, each 1.4 meters wide. The architecture of the control system is split in layers and in a high number of subsystems/components developed by different parties. This implies a high number of interfaces that must be designed and maintained under configuration control, to ensure a flawless integration of the different parts. Having interfaces (and data) definitions in a flexible central place allows us to extract several different artifacts (for example Interface Control Documents (ICDs), Interface Definition Language (IDL) files, tabular spreadsheets, help files, other generated code formats like code stubs or state machine implementations). In this paper, we explain how selecting a graphical modeling language like SysML and using graphical and tabular editing features made available by state of the art modeling tools presents a number of advantages with respect to other solutions like spreadsheets, a relational database, or a custom textual DSL. Still, using standard export/import formats (EMF XMI), we do not bind ourselves to a specific vendor. We describe the workflow that we have identified for the definition of interfaces, what artifacts we want to automatically produce and why. We also describe what technologies we are using to reach these objectives. A key aspect of this work is the selection of interface design patterns that are formal enough to allow automatic generation of the artifacts and, at the same time, pragmatic and simple to gain acceptance from all users and not incur in overhead.