GRAVITY: The adaptive optics assisted, two object beam combiner for the   VLTI
  We present the adaptive optics assisted, near-infrared VLTI instrument - GRAVITY - for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. Precision astrometry and phase-referenced interferometric imaging will realize the most advanced vision of optical/infrared interferometry with the VLT. Our most ambitious science goal is to study motions within a few times the event horizon size of the Galactic Center massive black hole and to test General Relativity in its strong field limit. We define the science reference cases for GRAVITY and derive the top level requirements for GRAVITY. The installation of the instrument at the VLTI is planned for 2012. 
S. Gillessen, G. Perrin, W. Brandner, C. Straubmeier, F. Eisenhauer,   S. Rabien, A. Eckart, P. Lena, R. Genzel, T. Paumard,...