\begin{table}%t1
  %\centering
\par
  \caption{List of parameters necessary to compute the {\it SNR}.\label{tab:par}}
        \begin{tabular}{ll}
       \hline\hline
        $N^l_{\rm ph}(r)$ & Number of photons per second per square meter per bandwidth in a given
     layer {\it l} of the\\
     & atmosphere, depending on the guide stars geometry, and  magnitude\\
        $N^l_{\rm gs}(r)$& number of stars whose footprints contribute at {\it r} in
     the layer {\it l}.\\
         $\tau$ & Coherence time of the whole atmosphere\\
          $\tau_l$ & Coherence time of layer {\it l}\\
          $\Delta t$& Integration time employed for the whole atmosphere\\
          $\Delta t_l$& Integration time employed for layer {\it l}\\
        $d$ & Sampling pitch for the wavefront measurements in NUMLO approach.\\
        $d_l$ & Sampling pitch for the wavefront measurements in OPTLO approach,
     layer {\it l}.\\
        $r_0$ & Fried parameter describing the whole atmosphere.\\
        $r_{0,l}$ & Fried parameter associated to layer {\it l}.\\
       $r_0(\lambda_{\rm wfs})$ & Fried parameter describing the whole atmosphere
     at the wavefront-sensing wavelength.\\
        $RON$ & read-out noise of the CCD detector.\\
        $\alpha_l$ & fraction of light arriving to the detector {\it l} in the
     optical layer \\
&oriented measurements due to the beam splitter.\\
        $N_{\rm bg}$ & number of background photons~$\rm s^{-1}~m^{-2}~arcsec^{-2}$ \\
        $Q_{\rm e}$ & quantum efficiency (it includes also optical transmission)       \\
        \hline
       \end{tabular}
\par
   \end{table}