The mesospheric sodium layer and its impact on the LGS parameters

Abstract

The atmospheric turbulence limits the angular resolution of telescopes of tens of meters to that of a telescope of 20 cm diameter. Large telescopes, such as the VLT and the E-ELT, have adopted the Adaptive Optics (AO) system to reduce the undesirable effects of turbulence. The implantation of the technology of “sodium-LGSs” is essential to increase the performances of the AO. The generation of “sodium-LGSs” is the result of the fluorescence of mesospheric Na atoms located between 80 to 105 km of altitude. The laser beam sent from the ground is tuned to the wavelength 589 nm; excites the D2 line of Na. The backscattered light gives necessary information on atmospheric turbulence and their effects on the incoming wave front of an astronomical object. The mesospheric sodium layer is characterized by the abundance of Na, the centroid height and the thickness of the layer. Their behaviors affect the variations of parameters of the “sodium-LGSs”, such as the Return Flux and the elongation of the spot. The characterization of this layer is necessary in order to optimize the performance of the “LGSs-AO” system. We present semi-empirical models based on experimental measurements. These models explain the variations in the abundance and height of the centroid of the sodium layer.