The Global Astrometric Interferometer for Astrophysics (GAIA) is one of ESA's (European Space Agency) cornerstone missions in the Horizon 2000 Plus long-term scientific program. GAIA's objective is to determine star positions and motions with an accuracy of 10 micro-arcsec. This accuracy can only be realized with sophisticated interferometric measurements where distance variations as small as a few picometers between individual opticalcomponents in the interferometer can be detrimental. For nullification of these variations Alenia Aerospazio, Italy, designed an active tip-tilt mechanism based on three PI Piezo Actuators (model P-844.20, Fig.2). The following tests proved the active control system to be capable of ensuring the ultra-high stability requirements fundamental for the achievement of the GAIA mission goals.
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