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We present the installed and fully operational beam stabilization and fiberinjection subsystem feeding the 2nd generation VLTI instrument GRAVITY. Theinterferometer GRAVITY requires an unprecedented stability of the VLTI opticaltrain to achieve micro-arcsecond astrometry. For this purpose, GRAVITY containsfour fiber coupler units, one per telescope. Each unit is equipped withactuators to stabilize the telescope beam in terms of tilt and lateral pupildisplacement, to rotate the field, to adjust the polarization and to compensateatmospheric piston. A special roof-prism offers the possibility of on-axis aswell as off-axis fringe tracking without changing the optical train. Wedescribe the assembly, integration and alignment and the resulting opticalquality and performance of the individual units. Finally, we present theclosed-loop performance of the tip-tilt and pupil tracking achieved with thefinal systems in the lab.

The fiber coupler and beam stabilization system of the GRAVITY interferometer