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From Herschel to Gaia: 3-meter class SiC space optics

HERSCHEL and GAIA are two cornerstone missions of ESA which embark 3-meter class optics. These instruments require so high thermal and mechanical stability than the SiC technology turned out to be indispensable. The BOOSTEC SiC material has been selected first for its high specific stiffness and thermal stability. But it also shows a perfect isotropy of all its physical properties and it is remarkably more stable than the glass-ceramics in time and also against space radiations. This SiC material has furthermore been fully qualified for application at cryogenic temperature (HERSCHEL and also JWST NIRSpec). The BOOSTEC manufacturing technology of very large size SiC components includes i) manufacturing 1.5 - meter class monolithic sintered parts and then ii) assembly based on a brazing process. The former one is a near net shaping process which allows manufacturing at reasonable costs and within short time. HERSCHEL has been successfully operating at Lagrange L2 point since mid of 2009, giving amazing information to astronomers. It includes a 3.5 m primary mirror, a secondary mirror and a hexapod. It weighs only 315 kg and its WFE is kept below 6 μm rms despite an operating temperature of 80 K. GAIA is made of more than 280 SiC parts of 80 different types. The most challenging of them is undoubtedly its highly stable structure, the 3 meters torus. This quasi octagonal and hollow shaped ring is made of 19 SiC elements brazed together. It weighs only 200 kg. All the GAIA hardware has been successfully manufactured and it is now being integrated and tested at ASTRIUM facilities.