UBC Theses and Dissertations
Studies of atmospheric properties for optical ground-based astronomy and methods to enhance laser guide star adaptive optics performance Hellemeier, Joschua Andrea
Ground-based astronomy suffers from waveform distortion produced by the turbulent atmosphere, which prevents telescopes from reaching 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 were studied. Monthly averages of cloud cover, wind speed at 200 hPa, precipitable water vapour, vertical wind velocity and aerosol index were compared for the sites. The long-term evolution over 45 years of the five atmospheric quantities was investigated. Site testing campaigns characterize 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 are described. Methods for increasing adaptive optics system performance are studied. Polarization modulation of the adaptive optics laser might be an useful method. Experimental results are presented. The method also enables measurements of the Larmor frequency and the magnetic field strength in the mesosphere. The average Larmor frequency is 260.4 kHz. We found a maximum LGS return flux enhancement of 18% for a pulsed amplitude modulation at Larmor frequency compared to amplitude modulation offset from the Larmor frequency. For polarization modulation at the Larmor frequency a 6% increase in LGS return flux was found over polarization modulation offset by 30-kHz from the Larmor frequency. Adaptive optics system could also benefit from an estimate of the mesospheric sodium density profile. Such 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, are presented. The method could be used on extremely-large telescopes to estimate the sodium density profile with a temporal resolution of a few seconds.
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