SiC is relatively common in space—formed from the combination of silicon and carbon in the absence of oxygen—and small amounts of it have been found within meteorites. On Earth it was first synthesized as an artificial diamond substitute.
Realizing its potential for space, ESA and Airbus (developing Euclid's payload module) entered into a long-term technical collaboration with French company Mersen Boostec, born out of a terrestrial firm which previously manufactured SiC bearings and seals for industrial pumps. The company made the 3.5-m diameter main mirror for ESA's Herschel spacecraft—which when the mission launched in 2009 was then the largest telescope mirror flown to space—and went on to produce mirrors and optical supports for Rosetta, Gaia, the James Webb Space Telescope and now Euclid.
"Gaia's monolithic rectangular main mirror had a wider diameter at 1.5 m across, but Euclid's main mirror represents our company's largest made-in-one circular mirror," explains engineer Florent Mallet of Mersen Boostec.
The company's SiC Product Line Director, Jérôme Lavenac, adds, "We're proud of our contribution to Europe's latest space astronomy mission, which will lead to major advances in fundamental physics."
The main mirror's manufacturing process began with SiC powder which was squeezed into a solid but soft circular block which was then precisely shaped using a computer-guided milling machine. The next step was sintering, or baking it in a 2,100°C oven. The resulting hard ceramic was then coated with additional SiC using chemical deposition, to fill in any residual pores, to a thickness of a few tenths of a millimeter. The mirror was then ground slightly before being passed to the Safran-Reosc company for polishing and silver coating. The final mirror shape is accurate to nine millionths of a millimeter under Earth gravity.
Both of Euclid's instruments will make use of this mirror plus its five smaller ones. Euclid's VISible instrument (VIS) takes very sharp images of galaxies in visible light over a much larger fraction of sky than would be possible from the ground. VIS works alongside the Near Infrared Spectrometer and Photometer (NISP). NISP sifts infrared light coming from these galaxies to derive key data, including their speed of outward expansion—measuring their "redshift," on the same principle as a police radar gun, which will in turn allow astronomers to infer the expansion history of the universe.
Provided by European Space Agency