Characterising Dynamic Aberrations for Ground-Based Optical Telescopes.

Abstract

For ground-based telescopes, atmospheric turbulence is often the factor limiting resolution. To recover the resolution capability of a telescope, adaptive optics is used to measure and correct the dynamic aberrations caused by atmospheric turbulence. Conversely, a telescope can perform scientific observations that do not require resolution above the natural seeing, negating the need for adaptive optics. Regardless of whether a telescope aims to perform state-of-the-art scientific observations at the resolution limit or make more modest observations, the expected performance limit must be quantified to ensure the science goals can be achieved. This requires the characterisation of all the significant sources of dynamic aberrations. There are several individual sources of dynamic aberrations, which fall into two broad categories: refractive index dependent and optical surface shape dependent. For many telescopes, extended turbulence profiling and weather analysis still need to be conducted. Follow-up monitoring also needs to be done to account for changes in climate or the local built environment. The new generation of extremely large telescopes will have additional phasing characterisation as the segmented mirrors that make up the large apertures need to be aligned (and maintain alignment) within a fraction of the observing wavelength. This thesis presents the results from characterisation studies filling three areas of current dynamic aberration research gaps. Firstly, a decadal weather analysis at Siding Spring Observatory was conducted. Although Siding Spring Observatory does not perform as well as leading global optical telescope sites, it still has reasonably favourable weather conditions. Of particular concern for Siding Spring Observatory are the high relative humidity, occasional strong gusts and higher temperatures compared to other sites. These results have informed the observation goals and the required dome engineering for potential new telescopes. Secondly, a dome turbulence study was conducted for the Anglo-Australian Telescope at Siding Spring Observatory-- the first dome turbulence study for this telescope to be fully disentangled from atmospheric turbulence. Temperature differentials (particularly convection conditions) and wind speed had the most significant effect on the dome turbulence, with a minor contribution from the azimuth angle (due to vent placement). This study gives weight to some operational recommendations for the Anglo-Australian Telescope that aim to optimise the balance between improved dome seeing and minimising operational costs. Lastly, a phasing testbed and accompanying phasing algorithm were designed for the Giant Magellan Telescope. Pocket-GMT was designed as a miniature optical emulation of the Giant Magellan Telescope, complete with matched F-number and correct pupil geometry. An innovative design was developed, where the first three Zernike modes are designed to be corrected on the segmented mirrors, and the Zernike modes 4-12 are corrected on a deformable mirror. Additionally, a novel fast phasing algorithm for the segmented pupil array metrology was tested for accuracy under varying signal-to-noise ratios. The algorithm performed well at signal-to-noise ratios above ten but will require further development to have an acceptable accuracy at lower signal-to-noise ratios. The phasing testbed will provide validation to the current phasing procedures and a platform to test new phasing procedures, ultimately saving valuable on-sky time for the telescope. There is great capacity for future work to build off the research in this thesis. The preliminary results of a site characterisation campaign with a Ring Image Next Generation Scintillation Sensor show the potential of this relatively new profiler instrument to greatly improve the site characterisation and monitoring process. Show more