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Title: Lucky exposures : diffraction limited astronomical imaging through the atmosphere
Author: Tubbs, Robert Nigel
ISNI:       0000 0000 7927 4098
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2003
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The resolution of astronomical imaging from large optical telescopes is usually limited by the blurring effects of refractive index fluctuations in the Earth's atmosphere. By taking a large number of short exposure images through the atmosphere, and then selecting, re-centring and co-adding the best images this resolution limit can be overcome. This approach has significant benefits over other techniques for high-resolution optical imaging from the ground. In particular the reference stars used for our method (the Lucky Exposures technique) can generally be fainter than those required for the natural guide star adaptive optics approach or those required for other speckle imaging techniques. The low complexity and low instrumentation costs associated with the Lucky Exposures method make it appealing for medium-sized astronomical observatories. The method can provide essentially diffraction-limited I-band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and large isoplanatic patch size for the Lucky Exposures technique at the Nordic Optical Telescope means that 25% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%-10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and full width at half maximum flux (FWHM) as small as 90 milliarc seconds. Within the selected exposures the isoplanatic patch is found to be up to 60 arc seconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I 16 are presented. A new generation of CCDs (Marconi L3Vision CCDs) were used in these observations, allowing extremely low noise high frame-rate imaging with both fine pixel sampling and a relatively wide field of view. The theoretical performance of these CCDs is compared with the experimental results obtained.
Supervisor: Not available Sponsor: Particle Physics and Astronomy Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral