Departmental Oral Examination (Thesis Title: "Studies of atmospheric properties for optical ground-based Astronomy and methods for laser guide star adaptive optics performance increase")
Physics and Astronomy
Ground-based Astronomy suffers from wavefront distortion by the turbulent atmosphere, preventing telescopes to reach diffraction-limited resolution. Modern large telescopes and next generation extremely-large telescopes use or will use adaptive optics systems with laser guide stars to correct for atmospheric wavefront distortion. The first part of the thesis deals with astronomical site testing and the second part with methods for adaptive optics system improvement.
Meteorological data for 15 observatory sites have been studied. Monthly averages of cloud cover, wind speed at 200 hPa, precipitable water vapour, vertical wind velocity and aerosol index have been compared for the sites. The longterm evolution over 45 years of these five atmopsheric quantities was investigated.
Site testing campaigns aim at the characterization of potential telescope sites in terms of optical turbulence. Using scintillometers, ground layer turbulence profiles can be measured. For an assessment of sites long-term statistics are needed. Two campaigns for daytime and nighttime turbulence profiling have been started and preliminary results will be shown.
Methods for increasing adaptive optics system performance are being studied. Polarization modulation of the adaptive optics laser might be a method for laser guide star return flux enhancement. An experiment has been carried out at the Roque de la Muchachos Observatory on La Palma. As a byproduct of the experiment the Larmor frequency of mesospheric sodium was measured and the magnetic field strength in the mesosphere was determined.
Adaptive optics system could benefit from an estimate of the mesospheric sodium density profile. Sodium density profiles can be retrieved by partial amplitude modulation with pseudo-random binary sequences of continuous-wave lasers. Results for an experiment at the Large Zenith Telescope in Maple Ridge and a feasibility study of this method for extremely-large telescopes will be presented.