Schematic view of an astrometric interferometer. The light paths are virtual, as seen from the star, and all mirrors are removed, resulting in a straight line from the star to the beam combiner. Only the wide angle baseline (WAB) is represented, but all the baselines limit points are represented. L₁ and L₂ are the pivot points of the telescopes: . The M₁ and M₂ points are the end points of the metrology: . Last, I_P1, I_P2, I_S1 and I_S2 are the barycenters of the effective pupils of each beam combiners: for the first combiner and for the second. Upper panel: the metrology measurement is made between the beam combiners and the metrology end point. The end points are different from the pivot points, which justify in Eq. (13) the use of the narrow angle baseline instead of the WAB. Lower panel: full schematic representation of the interferometer, including the presence of the imaging limit points. Imaging limit points are necessary to account for error terms due to difference in the propagation direction of the metrology (m_1S, m_2S, m_1P and m_2P) with respect to the stellar beams (s and p). It introduces additional error terms that are stated in Eq. (21).