  The new class of large telescopes, as the future ELT, are designed to work
with Laser Guide Star (LGS) tuned to a resonance of atmosphere sodium atoms.
This wavefront sensing technique presents complex issues for an application to
big telescopes due to many reasons mainly linked to the finite distance of the
LGS, the launching angle, Tip-tilt indetermination and focus anisoplanatism.
The implementation of a laboratory Prototype for LGS wavefront sensor (WFS) at
the beginning of the phase study of MAORY, the Multi-conjugate Adaptive Optics
RelaY for the ELT first light, has been indispensable to investigate specific
mitigation strategies to the LGS WFS issues. This paper shows the test results
of LGS WFS Prototype under different working conditions. The accuracy within
which the LGS images are generated on the Shack-Hartmann (SH) WFS has been
cross-checked with the MAORY simulation code. The experiments show the effect
of noise on the centroiding precision, the impact of LGS image truncation on
the wavefront sensing accuracy as well as the temporal evolution of sodium
density profile and LGS image under-sampling.
