The PRIMA facility phase-referenced imaging and micro-arcsecond astrometry

This article describes dual-field interferometry, in particular PRIMA, the phase-referenced imaging and micro-arcsecond astrometry facility of the very large telescope interferometer. It uses the simultaneous detection of fringes of two stars in a narrow angle and the accurate measurement of their respective positions. PRIMA aim is threefold: (i) to increase the VLTI limiting magnitude with off-axis fringe tracking, (ii) to reconstruct images with a resolution of 2 mas in K-band, 10 mas in N-band, and (iii) to perform differential narrow-angle astrometry with an accuracy of 10 μas. This article exposes the fundamental and technical limitations of such a technique and presents how PRIMA will try to solve the practical problems of measuring 100-m long optical paths with nanometric accuracy in a ground based interferometer.