High-resolution lidar experiment for the Thirty Meter Telescope.

Adaptive optics (AO) systems of next generation optical ground telescopes will employ laser guide stars (LGS) to achieve wide sky coverage. In these systems the mesospheric sodium layer at ~ 90 km height is excited by means of laser-induced fluorescence of the Na I D2 resonance hyperfine transmission. The finite thickness of sodium layer, and temporal variations in its density structure, result in LGS that are elongated and have internal structure that varies with time. This degrades the performance of the AO system due to degeneracy between effects of atmospheric and sodium layer variations. In order to quantify this and assess the impact on future extremely large telescopes such as the Thirty Meter Telescope (TMT), measurements are needed of the density distribution of the sodium layer with high spatial and temporal resolution. We describe the design of a new lidar experiment to investigate the spatio-temporal power spectra of the Na variations at frequencies as high as 50 Hz. This system employs a 5 W pulsed laser and a 6 m liquid mirror telescope, which provide sufficient sensitivity for high-resolution studies. The transmitter is a YAG-pumped dye laser, with an optical collimation system that allows the beam divergence to be controlled over a range from diffraction-limited to several arcmin. This will also allow the investigation of saturation effects, important for the next generation high power LGS systems. Backscattered photons will be collected at the prime focus using four high-efficiency photomultiplier detectors and a fast counting system. The resulting system will provide vertical density profiles with a spatial resolution as small as 2 m. Copyright 2008 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.Science, Faculty ofPhysics and Astronomy, Department ofReviewedFacult