Final tolerancing approach and the value of short-cutting tolerances by measurement

Within the ESAs 2015 2025 Cosmic Vision framework the 1.2 m aperture EUCLID space telescope addresses cosmological questions related to dark matter and dark energy. Being equipped with two instruments that are simultaneously observing patches of > 0.5 square degree on the sky EUCLID is aiming at major cosmological probes in a large seven years survey scanning the entire extragalactic sky. These two instruments, the visual light high spacial resolution imager (VIS) and the near infrared spectrometer and photometer (NISP) are separated by a di-chroic beam splitter. Its huge field of view (FoV) - larger than the full moon disk- together with high demands on the optical performance and strong requirements on in flight stability lead to very challenging demands on alignment and post launch post cool-down optical element position. The role of an accurate and trust-worthy tolerance analysis which is well adopted to the stepwise integration and alignment concept, as well as to the missions stability properties is therefore crucial for the missions success. While the previous contributions of this series of papers (e.g.[1])was addressing the technical aspects of tolerancing, the mechanical challenges and the answers of the NISP instrument to these challenges, this paper will focus on our concept of shortcutting the tolerance chain by measurement wherever useful and possible. The NISP instrument is only possible, due to the innovative use of technologies such as computer generated hologram (CGH) based manufacturing and alignment. Expanding this concept, certain steps in the assembly process, such as focal length determination before detector placement allow to reduce the overall tolerance induced imaging errors. With this papers we show three major examples of this shortcutting strategy.