Laser Guide Stars (LGSs) using Sodium mesospheric backscattering resonance as a reference for Adaptive Optics (Foy & Labeyrie 1985; Happer et al. 1994) are becoming a realistic viable solution for diffraction limited, whole sky coverage at large ground-based telescope.
For large aperture telescopes conical anisoplanatism effects (Gardner et al. 1990) due to the finite height of the laser beacon, deteriorates the achievable Sthrel ratio of an LGS-based Adaptive Optics system.
The latter source of error can be corrected using multiple LGSs; this is achieved obtaining a sort of three-dimensional tomography of the wavefront perturbation on the various perturbing layers (Tallon & Foy 1990; TF90 hereafter).
The lack of absolute tilt reference provided by an LGS (Pilkington 1987) can be solved (or at least minimized) in a number of solutions (Belen'kii 1994, 1995, 1996; Foy et al. 1992, 1995; Lukin 1996; Ragazzoni 1996a,b, 1997; Ragazzoni et al. 1995; Ragazzoni & Marchetti 1997; Rigaut & Gendron 1992). The proposed techniques are affected by some technical (Marchetti & Ragazzoni, 1997) or fundamental limitations depending upon the adopted scheme for the absolute tilt determination. Conical anisokinetism is recognized (Esposito et al. 1996; Neymann 1996; Riccardi et al. 1996) as a fundamental limitation affecting any sort of LGS tilt correction other than the ones based solely on natural guide stars (the latter offering a sky coverage far from the 100% at optical wavelenghts). The effects due to the distribution of the turbulence with the altitude (Ragazzoni et al. 1997) are not taken in consideration here and will be the subject of a forthcoming paper.
The interested reader can consult the cited papers for details on
conical anisokinetism. Here we recall that we proposed such a term to
identify the tilt error due to the conical shape of the sampled
atmosphere volume, provided the knowledge of the ground to layer tilt
experienced by the laser beam. The effect can be as severe as to
deteriorate the achievable Sthrel ratio of a factor 
when
a D=8.0 m telescope is observing in the visible
under average seeing conditions
and assuming an Hufnagel-Valley Cn2(z) turbulence
distribution model.