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This article describes the operation of the near-infrared wavefront sensingbased Adaptive Optics (AO) system CIAO. The Coud\'e Infrared Adaptive Optics(CIAO) system is a central auxiliary component of the Very Large Telescope(VLT) interferometer (VLTI). It enables in particular the observations of theGalactic Center (GC) using the GRAVITY instrument. GRAVITY is a highlyspecialized beam combiner, a device that coherently combines the light of thefour 8-m telescopes and finally records interferometric measurements in theK-band on 6 baselines simultaneously. CIAO compensates for phase disturbancescaused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT)experience during observation. Each of the four CIAO units generates an almostdiffraction-limited image quality at its UT, which ensures that maximum flux ofthe observed stellar object enters the fibers of the GRAVITY beam combiner. Wepresent CIAO performance data obtained in the first 3 years of operation as afunction of weather conditions. We describe how CIAO is configured and used forobservations with GRAVITY. In addition, we focus on the outstanding features ofthe near-infrared sensitive Saphira detector, which is used for the first timeon Paranal, and show how it works as a wavefront sensor detector.

Infrared wavefront sensing for adaptive optics assisted Galactic Center observations with the VLT interferometer and GRAVITY: operation and results