Astrometric demonstrator in optical interferometry with the test siderostats at Paranal

We propose a near-IR demonstration experiment with the two VLTI test siderostats (0.4 m) at Paranal, to be used both in wide-angle single-field and in narrow-angle dual-field astrometry. The specifications required to operate an astrometric interferometric are particularly stringent on siderostat rotation axes, pupil transfer and instrument stability. These problems, together with the instrument capabilities in the two operation modes, are detailed in this paper. The present time accuracy of reference stars is evaluated, as well as statistical properties of the dual- field targets. The astrometric testbed instrument will be of great help in analyzing and understanding on site, the main features of the VLTI, in order to optimize its future operation with Auxiliary Telescopes in phase referenced imaging and astrometric modes .