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The Gaia mission will create an extraordinarily precise three-dimensional map of more than one billion stars in our Galaxy. Part of ESA's Cosmic Vision programme, Gaia spacecraft is being built by EADS Astrium and is scheduled for launch in 2011. TNO is developing the all Silicon Carbide Basic Angle Monitoring Opto-Mechanical Assembly (BAM OMA) for this mission.
Gaia spacecraft surveying our galaxy
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The Gaia spacecraft will use the global astrometry concept successfully demonstrated on Hipparcos, which mapped over 100,000 stars in 1989. Gaia is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs. Gaia will provide an unprecedented number of new discoveries. Every day, on average, it will discover about 100 new asteroids in our solar system, 10 new stars possessing planets, 50 new stars exploding in other galaxies, and 300 new distant quasars. Exo-planets will be detected by measuring the tiny movement of the star, caused by the small gravitation pull of the planet. It is estimated that Gaia will discover about 15 000 new exo-planets during its operational lifetime.
The Gaia satellite measures the angles between stars using two telescopes set at a fixed angle of 106.5°, named the Basic Angle. The astrometric measurements will be accurate to 24 microarcsec (at 15 magnitude), comparable to measuring the diameter of a human hair at a distance of 1000 kilometres. This requires ultra high stability, which can only be achieved by using Silicon Carbide for the optical bench and telescopes. In addition the Basic Angle variation shall be measured within a precision of 0.5 microarcsec. Therefore Gaia is equipped with a Basic Angle Monitoring subsystem, a metrology system to monitor the angle between the two telescopes. This system consists of two laser interferometers. Two pairs of parallel laser bundles are sent to the two telescopes, which create two interference patterns on a detector. If the basic angle varies, the interference patterns will shift. With the BAM an Optical Path Difference (OPD) as small as 1.5 picometers RMS can be measured.
The Gaia BAM Opto-Mechanical Assembly
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The BAM OMA consists of two optical benches, a number of flat mirrors, three beam splitters and two fibre collimators. In order to fulfill the stability requirements for such accurate OPD measurements, the entire BAM OMA is constructed from Silicon Carbide (except for the beam splitters). The BAM OMA components have been developed in close cooperation with TU Eindhoven and with support from the Netherlands Space Office (NSO).
Silicon Carbide (SiC) has a number of advantages:
The sintered SiC components will be manufactured by Astrium/Boostec. SiC is brittle and very hard. Therefore machining of SiC components after sintering, shall be minimised as far as possible. This requires a different approach in design, compared to working with conventional materials such as Aluminium. The BAM OMA shall have an extremely small Wave Front Error (WFE), less than 25 nm RMS over the entire optical path. TNO has developed processes for in house polishing of SiC mirrors to very low surface errors.
