Design of a basic angle monitoring system in Silicon Carbide

Due to the 10 microarcsecond accuracy, with which GAIA will measure the positions of stars using 2 astrometric telescopes, stability requirements on the payload module are extremely stringent. In order to achieve the required 10 microarcsecond accuracy, a metrology system could be installed on the satellite to monitor variations on the basic angle: the basic angle monitoring system. This system has high stability requirements of picometers in several hours and is therefore, like the rest of the payload module, constructed of Silicon Carbide. This is a ceramic material with good mechanical and thermal properties. However it also needs special attention in design, because final processing is difficult. The mounting of optical components is performed directly on the optical bench, by tensioning the optical components with spiders against the optical bench. Alignment is performed by shifting the component across a plane using an external and removable alignment mechanism. In the near future experiments will be performed to proof the design.