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Mesospheric dynamics and ground-layer optical turbulence studies for the performance of ground-based telescopes Pfrommer, Thomas

Abstract

Modern astronomical instrumentation employs adaptive optics (AO) systems that correct for atmospheric distortion in real time in order to produce sharper images. The design and performance of these systems relies on the knowledge of the atmosphere both at low and high altitude. This thesis investigates the first kilometer of the atmosphere, the ground layer (GL), as well as the sodium layer at ~92 km. Newly-designed lunar scintillometers provide turbulence profiles of the GL, and high spatio-temporally resolved sodium profiles are obtained using a newly-designed lidar system for UBC's 6-m liquid-mirror. For ground layer adaptive optics systems, knowledge of the local height- and time-resolved GL turbulence is crucial to link local topography to optical turbulence and has been obtained with the help of three lunar scintillometers deployed in Chile, Hawaii and in the Canadian High Arctic. Results from measurements inside the Canada-France-Hawaii Telescope (CFHT) dome indicate severe degradation of image quality due to a poorly vented dome and thus provide input for dome modifications. The outside median GL seeing was determined to be 0.48±0.01". Initial results from the Arctic show exceptional GL seeing conditions, better than have been found anywhere on Earth although data quantity is limited. Extremely large telescopes must correct not only for GL turbulence, but also higher atmospheric disturbances in the troposphere. The use of laser guide stars (LGS) increases sky coverage and the field of view, but relies on resonantly excited sodium atoms in the mesosphere. Upper atmospheric dynamics causes varying sodium density, which produces focus-induced wavefront errors in LGS AO systems. The UBC lidar system was built, and its high-resolution data reveal large spatial variability, strong nightly variations and meteor spikes in sodium density on sub-second time scales. The mean altitude power spectrum has been extended to scales approaching the dissipation limit, and its spectral index of -1.95±0.12 and normalization of 30±20 m²/Hz determines AO system wavefront errors of 4 nm for a 30 m, and 8 nm for a 42 m telescope per meter mean-altitude variation. Derived mean altitudes, separated by one arcmin, showed rms fluctuations of order 30 m and could cause AO performance degradation.

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