Infrared wavefront sensing for adaptive optics assisted galactic center observations with GRAVITY

We describe the operation of the infrared wavefront sensing based Adaptive Optics system CIAO. The Coude Infrared Adaptive Optics (CIAO) system is a central auxiliary component of the Very Large Telescope interferometer (VLTI). It enables in particular Galactic Center observations using the GRAVITY interferometric instrument. CIAO compensates for phase disturbances caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT) experience during observation. Each of the four CIAO units generates an almost diffraction-limited image quality at its UT, which ensures that maximum flux of the observed stellar object enters the input fibers of GRAVITY. We present CIAO performance data obtained in the first 3 years of operation. We describe how CIAO is configured and used for observations with GRAVITY. We focus on the outstanding features of the infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.