Adaptive Optics for Extremely Large Telescopes 4 – Conference Proceedings

The Adaptive Optics Facility (AOF) at the Very Large Telescope (VLT) will have two ground layeradaptive optics (GLAO) modes of operation. To make the best use of these modes, informationabout the ground layer turbulence prole up to 500 m will be important. External turbulenceproling support will be provided by a robotic slope detection and ranging (SLODAR) instrumentwith 8 resolution elements and a maximum sensing altitude of 100-500 m. When proling at thehigher end of this range the surface layer of turbulence is not resolved so the fraction observed bythe Unit Telescopes (UTs) is ambiguous. Here we present a method of estimating the surface layercontribution seen by the UTs by assuming an exponential decay model for the surface layer. Older,higher resolution, proles from the prototype surface layer SLODAR instrument (from 2011-2012)have been used to determine the most appropriate scale height for the model. By incorporating themodel, a better estimate of the integrated turbulence strength from the UT dome height upwardscan be achieved.