4 Conclusion

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-$\mu$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 $\mu$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).