The concentric organization of the architectural program resolves the operating constraints and offers a punctuation in the landscape of the Saclay campus located on the site of the Ecole Polytechnique. To reveal the new identity of the observatory, we have implemented a real device supporting scientific research. The choice of concrete was essential for a technical, solid and durable building. Its mechanical and plastic properties made it possible to cast a curved facade in one piece. In the background, the locksmiths of the curtain walls and perfectly curved guardrails bring finesse and precision to a composition regulated as a clockwork.