Infrared Imaging with COAST

John Stephen Young

St John's College, Cambridge
Cavendish Astrophysics

A dissertation submitted for the degree of
Doctor of Philosophy in the University of Cambridge

26 March 1999

See below to download the thesis, or to use the bibliography with hyperlinks.

Outline of thesis

The first optical/IR aperture synthesis image from separated telescopes was made at visible wavelengths, in 1995 (Baldwin et al. 1996), using the Cambridge Optical Aperture Synthesis Telescope (COAST). COAST is the first optical interferometer specifically designed to produce images, by measuring closure phase as well as visibility amplitude.

COAST was designed to work at visible wavelengths. The aim of this project was to add the capability of imaging at near-infrared wavelengths, following on from the work of Beckett. The project was made possible by the arrival of new detector technology, discussed in Chapter 2.

This work describes the problems encountered in modifying COAST to operate as an imaging telescope in the near-infrared, and the solutions employed in overcoming these problems. Many of the solutions have more general applications in interferometry.

The effectiveness of the solutions is clearly demonstrated by the number of astronomical results obtained with the working COAST IR system. These results, described in Chapters 5-8, include high-quality images of Capella and Betelgeuse, and a large number of diameter measurements of Mira variables. The astrophysical implications of these results, combined with high angular resolution observations at visible wavelengths, are also discussed.

Chapter 2 is a brief review of the principles of operation of modern infrared array detectors, required in order to follow the discussion in later chapters.

Chapter 3 describes the optical systems of a typical stellar interferometer, with particular emphasis on beam-combining schemes which could be used at infrared wavelengths. The relative merits of the different schemes when a noisy detector is used are discussed. The pupil plane beam combiner chosen for IR operation at COAST is described, as are the fundamental problems in aligning such a system. Possible solutions to these problems are discussed, including the methods finally adopted at COAST. These procedures have been used at COAST to accurately align the four-way IR beam combiner and maintain its alignment throughout the observing season.

Chapter 4 discusses the readout mode used for the NICMOS3 camera at COAST to sample pupil plane fringes at up to 2.5 kHz. The functionality of the software used to control the NICMOS array and store fringe data is described.

Chapter 5 describes the procedures used when observing with COAST in the infrared, illustrated by observations of the binary star Capella. I discuss the methods for extracting visibility amplitudes and closure phases from the raw data, and subsequently using them to reconstruct an image of the astronomical target.

Chapter 6 is concerned with contemporaneous high resolution imaging of the M-type supergiant star Betelgeuse in a number of wavebands from 0.7-1.3 micron, performed with COAST, and with the William Herschel Telescope by the technique of non-redundant masking. The implications for the possible origin of the "hotspots" frequently detected on Betelgeuse are discussed.

Chapter 7 presents the results of a programme to monitor the angular diameter of the Mira variable star chi Cygni. This programme was carried out with COAST and the William Herschel Telescope, over a period of 17 months.

Chapter 8 describes a programme to make near-continuum (1.3 micron) diameter measurements of a sample of Mira variables with COAST, and the implications of the measurements for the effective temperatures, physical diameters and pulsation modes of the sample stars.

Chapter 9 is a summary of the most important ideas from each chapter. Developments planned for the IR instrumentation at COAST are discussed, with suggestions for future astronomical programmes.

The thesis is available as a PDF file (2.4Mb), a single PostScript file (6.7 Mb), or as a number of smaller PostScript files, one for each chapter:

Please note that the thesis is formatted for A4 paper.

Below is a version of the bibliography with links to the text of most of the cited articles.


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