We proposed an optical configuration suitable for a micro-arcsecond astrometric interferometer in space enabling direct fringe detection. It overcomes the confusion effect present in an optical configuration such as in the initial scheme of GAIA (Lindegren & Perryman 1996), while considering the expected state-of-the-art CCD technologies and the dimension requirements to fit in the Ariane V fairing.
We also evaluated the sensitivity of the configuration to misalignments. Results show that tolerances require nanometric metrology and less than 20-as tilt to save the astrometric accuracy. The control of the primary mirror is the most stringent.
We have shown that the state-of-the-art optics and detector technology are at the level of the direct fringe detection requirements with an astrometric mission aiming to reach an accuracy level of 10 as. Therefore, it appears that this cornerstone mission could attain, with a less efficient accuracy, the 20th magnitude, i.e. 109 objects.Acknowledgements
The work described was performed during a study conducted under ALENIA's prime contractorship (in the framework of the APLT-AMTS ESA contract) and the activities were carried out in cooperation with the Observatory of Torino (OATo).
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