Last decade EADS-ASTRIUM and its partner Boostec, has become world leader in the field of Silicon Carbide (SiC) optical payloads. In the framework of earth and scientific observation, high and very high-resolution optical payloads have been developed. This leadership allowed EADS-ASTRIUM to propose a large and complete range of space-based system for optical observation. Ceramic mirrors and structures are becoming attractive for high precision light weighted opto-mechanical applications. Developments over the past 15 years by EADS-ASTRIUM and by Boostec have demonstrated the feasibility and versatility of the SiC material for numerous applications. The most favorable characteristics of this material are high stiffness, high thermal conductivity and low thermal expansion (CTE). Furthermore, SiC allows relatively quick and cheap manufacturing of components because the components can be shaped with conventional tools in a milling process of the green body material. Through different joining processes, SiC allows for large size applications and systems. Only the scale of the available production facilities, the largest of which currently is 4 m in diameter, limits size of the structures and mirrors that can be manufactured.
METimage is an advanced multispectral radiometer for weather and climate forecasting developed by Airbus Defence & Space under the auspices of the German Space Administration (DLR) for the EUMETSAT Polar System –Second Generation (EPS-SG). The instrument is equipped with a continuously rotating scan mirror with a 1.7s period followed by a static telescope. The scan mirror permits an extended Earth view of 108° per revolution and regular views to on-board calibration sources. A derotator assembly, which is half-speed synchronised with the scanner, is inserted in the optical beam after the telescope to compensate the image rotation in the focal plane. The derotator optical arrangement is a fivemirror concept that minimises the polarisation sensitivity. The derotator design is constrained by optical performance, mass and compactness, which led to the selection of a full silicon carbide (SiC) concept. This paper describes the preliminary design and verification approach of the derotator optics.
Thermo-elastic distortions of composite structures have been measured by a holographic camera using a BSO
photorefractive crystal as the recording medium. The first test campaign (Phase 1) was performed on CFRP struts with
titanium end-fittings glued to the tips of the strut. The samples were placed in a vacuum chamber. The holographic
camera was located outside the chamber and configured with two illuminations to measure the relative out-of-plane and
in-plane (in one direction) displacements. The second test campaign (Phase 2) was performed on a structure composed of
a large Silicon Carbide base plate supported by 3 GFRP struts with glued Titanium end-fittings. Thermo-elastic
distortions have been measured with the same holographic camera used in phase 1, but four illuminations, instead of two,
have been used to provide the three components of displacement. This technique was specially developed and validated
during the phase 2 in CSL laboratory. The system has been designed to measure an object size of typically 250x250
mm2; the measurement range is such that the sum of the largest relative displacements in the three measurement
directions is maximum 20 μm. The validation of the four-illuminations technique led to measurement uncertainties of 120 nm for the relative in-plane and out-of-plane displacements, 230 nm for the absolute in-plane displacement and 400 nm for the absolute out-of-plane displacement. For both campaigns, the test results have been compared to the predictions obtained by finite element analyses and the correlation of these results was good.
Since several years, EADS-Astrium has developed, in partnership with BOOSTEC, Silicon Carbide (Sic) structural pieces for space telescope applications. This technology has appeared adequate not only for optical elements (mirrors) but also for the complete Telescope structures, thanks to high stiffness, low coefficient of thermal expansion and high thermal conductivity of Sic. At the time being, two space Sic telescopes are operational in observation and scientific missions. Two other monolithic Sic telescopes, among the largest ever built, are in manufacturing progress. The latest innovations in Sic technology have been implemented in the ALADIN Telescope for the AEOLUS mission (LIDAR dedicated to wind speed measurement).