Paul Hickson

Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Studies of atmospheric properties for optical ground-based astronomy and methods to enhance laser guide star adaptive optics performance (2020)

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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Mesospheric dynamics and ground-layer optical turbulence studies for the performance of ground-based telescopes (2011)

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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Lifetimes of states in 19Ne above the 15O+ alpha threshold (2008)

Astrophysical models that address stellar energy generation and nucleosynthesis require a considerable amount of input from nuclear physics and are very sensitive to the detailed structure of nuclei, both stable and unstable. Radioactive nuclei play a dominant role in several stellar environments such as supernovae, X-ray bursts, novae etc. and nuclear data are important in the interpretation of these phenomena. When carbon, nitrogen and oxygen isotopes are present in substantial quantities in a star of sufficient mass, the fusion of four hydrogen nuclei to form a helium nucleus proceeds via the CNO cycles. Energy release in the CNO cycles is limited by the long lifetimes of 14O and 15O. In explosive stellar scenarios such as X-ray bursts, the energy output is very large, suggesting a breakout from the CNO cycles. 15O(α,γ)19Ne is the first reaction that breaks out of the CNO cycle. Nuclear structure information on high lying states in 19Ne is required to calculate the rate of the 15O(α,γ)19Ne reaction. This work focuses on the study of states in 19Ne above 3.53 MeV. The lifetimes of five states in 19Ne above 3.53 MeV were measured in this work. The states in 19Ne were populated via the 3He(20Ne,α)19Ne reaction at a beam energy of 34 MeV. The lifetimes were measured using the Doppler Shift Attenuation Method. The lifetimes of five states were measured and an upper limit was set on the lifetime of a sixth state. Three of the measurements are the most precise thus far. The lifetimes of the other three states agree with the values of the only other measurement of the lifetimes of these states. An upper limit on the rate of the 15O(α,γ)19Ne reaction was calculated at the 90% confidence level using the measured lifetimes. The contributions to the 15O(α,γ)19Ne reaction rate from several states in 19Ne at different stellar temperatures are discussed.

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Master's Student Supervision (2010 - 2018)
Measuring the return flux from laser guide stars (2013)

For future extremely-large telescopes (ELT), operation at or near diffraction limited resolutions will be the norm, rather than the exception. Thus, adaptive optics systems and laser guide star facilities will be a critical component of the ELTs. The UBC Large Zenith Telescope (LZT) has conducted lidar observations to monitor the vertical distribution of sodium atoms with the goal of understanding both the abundance and evolution of sodium in the mesosphere to aid in both AO and laser guide star (LGS) return flux simulations. Access to the LZT's high resolution lidar experiment has lead to a joint collaboration between UBC, Thirty Meter Telescope (TMT), and the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (TIPC), to conduct upgrades of at the LZT site for sodium laser characterization tests, specifically TIPC's prototype pulsed sodium laser. The additional facilities and instrumentation at the LZT site include: 1) a new building to be used as a laser room to house visiting groups' lasers, corresponding control equipment and power systems, 2) optical equipment (scanning Fabry-Pérot interferometer, fast photodiode sensor, and miscellaneous optical filters) to measure the characteristics of the laser spectral format and pulse-shapes of pulsed lasers, and finally, 3) the site is now capable of directly imaging both natural stars and the LGS sodium spot with a 30 cm Ritchey-Chrétien equipped with a SBIG CCD camera to help determine the e ciency of LGS lasers. This document describes the new ancillary equipment for sodium laser characterization tests as well as a successful campaign conducted in the summer of 2012 on the UBC lidar laser. The summer campaign measured the laser pulse profile and spectral profile as well as LGS sodium spot measurements. The combined measurements which subsequently lead to an estimated sodium column density ranging from 5.1 × 10¹³ atoms/m² to 1.1 × 10¹³ − 1.5 × 10¹³ atoms/m² depending on the number of laser spectral modes used in the model.

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