High-resolution lidar experiment for the Thirty Meter Telescope

Abstract

Adaptive optics (AO) systems of extremely-large optical telescopes (ELTs) will employ laser guide stars (LGS) to achieve wide sky coverage. In these systems the mesospheric sodium layer at 90 km altitude is excited by means of laser-induced fluorescence of the Na I D2 - resonance hyperfine transmission. The finite thickness of the sodium layer and temporal variations in its density structure produce elongation, internal structure, and range variations in the LGS. This degrades the performance of the AO system due to degeneracy between effects of atmospheric and sodium layer variations. In order quantify this and assess the impact on future extremely large telescopes such as the Thirty-Meter Telescope, we have developed a lidar system for the 6-m Large Zenith Telescope. This system provides more than an order-of-magnitude improvement in signal-to-noise ratio and spatio-temporal resolution compared to previous work. Initial observations have revealed new details of the structure and dynamics of the sodium region. These include multiple transitory layers that vary in density and altitude on timescales ranging from minutes to hours, large-scale instabilities, turbulent vortices and coherent short period gravity wave oscillations. The mean sodium altitude exhibits temporal variations having a near-Kolmogorov power spectrum extending over four decades of frequency. The impact of these variations will be significant for all ELTs employing continuous-wave laser beacons.