Robert Nigel Tubbs

	Dissertation submitted in candidature for the degree of
	Doctor of Philosophy in the University of Cambridge

St Johns College

Cambridge University

September 2003

Preface

This dissertation is the result of work I have undertaken between October 1999 and September 2003 as a research student in the Institute of Astronomy and Cavendish Astrophysics Group in the University of Cambridge.

This dissertation is my own work and contains nothing which is the outcome of work done in collaboration with others, except as specified in the text and Acknowledgements.

This dissertation is not substantially the same as any that has been submitted for a degree or diploma or other qualification at this or any other University.

This dissertation does not exceed 60 000 words.

Robert Nigel Tubbs

Originality

Chapter 1 is a review of recent developments in high resolution ground based optical imaging, and is mostly the work of other authors.

Chapter 2 includes some results from previous authors and some of my own work as indicated in the text.

Chapters 3-6 and Appendix A are my own work except where indicated in the text.

Summary

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 $90$  $milliarcseconds$. Within the selected exposures the isoplanatic patch is found to be up to $60$ $arcseconds$ 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 $I\sim16$ 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.

Acknowledgements

I would first like to acknowledge the people who made this project possible: Craig Mackay for building a low noise camera and knowing how to use it to its best advantage; and John Baldwin for developing the original concepts on which this work is based. Both John and Craig have guided me through this project, tactfully correcting my conceptual errors, providing a mathematical background for the work, building instrumentation and helping to turn exciting experiments into valuable science.

Current and former members of the COAST group in Cambridge provided essential input to this project. I would like to thank Dave Buscher for useful insights, many interesting conversations and for providing atmospheric modelling algorithms; Donald Wilson for helping to design and build our instrumentation; James Keen, Ali Bharmal and Ali Basden for many useful conversations and input to atmospheric modelling; and Peter Warner for getting me interested in interferometry in the first place, and then guiding me through the first few months. Thanks to Graham Cox for assisting with arrangements on La Palma, and to Richard Wilson and Peter Tuthill for useful conversations and data which helped in understanding the atmosphere. I am grateful to the whole COAST team for providing all the support I needed during my time in the group; to Chris Haniff for keeping things in perspective; to Natalie Thureau and John Young for providing various pieces of software useful for my COAST observing; and to Donald Wilson, Roger Boysen, Bodie Seneta, John Baldwin, John Young and Dave Buscher for keeping COAST running. Long nights observing at COAST would have been much duller without the humour of Josh Eisner, James Keen, Debbie Pearson and Bodie Seneta, and long days and nights at the NOT would have been much less bearable without the wit and insight of John Baldwin, Craig Mackay, Graham Cox and on the latest run Richard Wilson. I hope the COAST group continues to flourish in the coming years. Outside the COAST group, Dave Titterington, Helen Brimmer and Andrew Dean provided much software help.

I would like to give particular thanks to Helen for supporting me throughout this work, and in particular during the stressful writing-up period. Life would have been very difficult without you. I would also like to thank Ian for many relaxing evenings in The Castle, Neil, Andrew, Tim, Stian, Sarah R, Sarah B, Karen, Hannah, Dave, Dan, Lisa, and Andy for many enjoyable parties, Steve, Wendy and Haley for many fun trips to London, and Paula and Andy for always giving me somewhere pleasant to stay and doing my proof-reading.

I would like to acknowledge the support of a PPARC fellowship. This work is based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The staff at the NOT provided much useful support, particularly Graham Cox, Peter Brandt, Johannes Andersen, Markku Verkkoniemi and Paco Armas. This project has been supported by the European Commission through the Access to Research Infrastructures Action of the Improving Human Potential Programme, awarded to the Instituto de Astrofísica de Canarias to fund European Astronomers' access to the European Northern Observatory, in the Canary Islands. This research has also made use of the SIMBAD database, operated at CDS, Strasbourg, France, and NASA's Astrophysics Data System.

This document was typeset by the author in L^ATEX.

	To all amateur and professional astronomers
	with short exposure cameras. Good luck!