Abstract
This article describes the operation of the near-infrared wavefront sensing based Adaptive Optics (AO) system CIAO. The Coudé 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 the Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly specialized beam combiner, a device that coherently combines the light of the four 8-m telescopes and finally records interferometric measurements in the K-band on 6 baselines simultaneously. 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 fibers of the GRAVITY beam combiner. We present CIAO performance data obtained in the first 3 years of operation as a function of weather conditions. We describe how CIAO is configured and used for observations with GRAVITY. In addition, we focus on the outstanding features of the near-infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.
Highlights
What does a black hole look like? Do black holes really exist? Questions like these can be answered more quantitatively thanks to better and better observations
Recent observations with the GRAVITY instrument allowed for the first time to measure the gravitational redshift in the light of a star orbiting SgrA* and approaching the Galactic Center (GC) to about 120 astronomical units (AU), i.e., 1400 RS [9]
As described in detail in [14,15,16], after passing the delay lines of the VLTI and the GRAVITY fiber coupler unit, the light of the 4 Unit Telescopes (UT) is fed into single-mode fibers and further relayed into an integrated optics unit where pairs of UT beams interfere
Summary
What does a black hole look like? Do black holes really exist? Questions like these can be answered more quantitatively thanks to better and better observations. One of the most interesting observation areas in this context is the center of the Milky Way, where the black hole with the largest angular diameter as seen from Earth is located. Very long baseline radio interferometric measurements, observing the—gravitational lensing magnified—black hole shadow in the emission of SgrA*, give an angular source width of 37 μas [3]. This width is smaller than the expected width of 5 RS = 50 μas because of gravitational lensing. Recent observations with the GRAVITY instrument allowed for the first time to measure the gravitational redshift in the light of a star orbiting SgrA* and approaching the GC to about 120 AU, i.e., 1400 RS [9]. Exoplanet observations to mapping the cores of active galactic nuclei [13]
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