and very low read noise are required. Thus photon noise is the only unavoidable contributor to the WFS noise. Even so, with Natural Guide Stars (NGS) the performance in the K-band of a medium-sized AO system drops significantly when m V ≥15. Figure 2 shows the expected K-band Strehl performance for a 60-element curvature system with the VLT as a function of NGS magnitude and its angular distance from the science target. Note that Strehl curves depend by the seeing at 6th power. Our analytical

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... mputations are for 0.66" seeing at 0.5 µm, 6 msec τ o , and 0.4 Strehl contribution from field anisoplanatism at 20" elongation, in K-band. The analytical simulations allow a parametric exploration of the AO performance. Although this is less accurate than a full numerical simulation, the results are according to our estimates accurate to within 10-15%, and serve well the purpose of this discussion. The effect of anisoplanatism on AO systems is very evident in Figure 2 . For example, a 10th magnitude NGS which is 25 arcseconds away from the science target Figure 1 : Layout of the LGSF. The laser and its controls are mounted under the Nasmyth platform, in a well-insulated laser room. The laser beam is relayed to the 50-cm-diameter launch telescope located behind the UT secondary mirror.