Monitoring of atmospheric sodium: new method to get the telluric spectrum and influence of the water lines in the performances of LGS

The monitoring of the column density of atmospheric sodium could be crucial for the efficiency of LGS systems. However, its detection is affected by the presence of water lines around the sodium doublet lines; this is really dramatic in the D2 line. We have developed a new method to detect the D2 sodium telluric line, obtaining previously the solar spectrum clean of telluric contamination. This method does not need any theoretical supposition on the solar line profile. We analyze the influence of the water lines in the detection of the D2 line and discuss the effects on the efficiency of LGSs.