A Φ Φ Φ Φ 3.5 M SiC telescope for HERSCHEL Mission Emmanuel SEIN a* , Yves TOULEMONT a , Frédéric SAFA a , Michel DURAN a , Pierre DENY b , Daniel DE CHAMBURE c** , Thomas PASSVOGEL c , Göran PILBRATT c a ASTRIUM(Toulouse, France), b BOOSTEC(Tarbes, France), c ESTEC-ESA (Noordwijk,Netherlands) Since ten years ASTRIUM has developed sintered Silicon Carbide (SiC) technology for space applications. Its unique thermo-mechanical properties, associated with its polishing capability, make SiC an ideal material for building ultra-stable lightweight space based telescopes or mirrors. SiC is a cost effective alternative to Beryllium and the ultra-lighweighted ULE. In Complement to the material manufacturing process, ASTRIUM has developed several assembly techniques (bolting, brazing, bonding) for manufacturing large and complex SiC assemblies. This technology is now perfectly mature and mastered. SiC is baselined for most of the telescopes that are developed by ASTRIUM. SiC has been identified as the most suitable material for manufacturing very large cryogenic telescopes. In this paper we present the development of the Φ 3.5 m telescope for Herschel Mission. Herschel main goal is to study how the first stars and galaxies were formed and evolved. The Herschel Space telescope, using silicon carbide technology will be the largest space imagery telescope ever launched. The Herschel telescope will weight 300 kg rather than the 1.5 tons required with standard technology. The Herschel telescope is to be delivered in 2005 for a launch planned for 2007 Keywords: Large Telescope, Space Application, Lightweighted reflector, SiC 1 INTRODUCTION Over the past ten years ASTRIUM have developed the SiC technology for Space Applications 1, 2 in collaboration with a ceramic company BOOSTEC Industries (Tarbes, France) The joint venture SiCSPACE gathers the expertise of both companies for supplying optics and telescopes for both space and ground applications. The technology is now fully mastered and ESA selected ASTRIUM in 2001 for manufacturing HERSCHEL large telescope. HERSCHEL satellite is part of HERSCHEL/PLANCK program of the European Space Agency (ESA) devoted to far infrared astronomy. Both satellites, HERSCHEL and PLANCK will be launched in 2007 by Ariane 5 in a dual launch and will operate at an orbit about Lagrange L2 point. HERSCHEL telescope is of Cassegrain type with a parabolic primary reflector of diameter 3.5 meters. This unprecedented telescope will be mounted on the top of HERSCHEL helium cryostat vessel that contains three science instruments. The telescope operates at cryogenic temperature (~ 80 K) in the far infrared wavelength range (80 µm to 600 µm) and it will be the largest telescope in orbit in 2007. The overall telescope mass is below 300 kg and this puts in evidence the exceptional lightweighting capability of SiC technology: for comparison, Hubble Space Telescope primary mirror mass is about 900 kg for a diameter 2.4 m During previous study phases (1995–2000), ESA conducted extensive trade-offs on the telescope material, supported by the development of breadboards of CFRP composite and lightweight aluminium mirrors. It was clearly established that ASTRIUM/BOOSTEC Silicon Carbide technology was the only viable solution for meeting mass and performance requirements. After a brief review of the material properties, we present the development status of this telescope and the results of the HERSCHEL mirror demonstrator that was manufactured and tested two years ago.