Abstract
This report discusses several new techniques for studying the properties of interference fringes formed in astronomical optical interferometers. It looks in detail at the tracking of fluctuations in the path difference between incoming light beams introduced by atmospheric turbulence and measurement of the effects of optical dispersion on the light beams. Also discussed are the consequences of these effects for closure phase measurements. Many of the procedures suggested were tested experimentally, and the results are presented within the report. Several new techniques for closure phase and complex closure amplitude measurements are also suggested.
Introduction
The development of closure phase optical imaging over the past decade represents one of the most important advances in high resolution optical astronomy. The use of closure relations allows small bright astronomical sources to be mapped using methods that are not significantly affected by the random fluctuations in optical path length produced within the Earth’s atmosphere. The Cambridge Optical Aperture Synthesis Telescope (COAST) is a prototype instrument designed specifically for closure phase imaging, and has already generated a wealth of astronomical and astrophysical data.
One of the primary aims of the COAST project is the experimental testing of new techniques, many of which may be employed in larger interferometric arrays planned or currently under construction. The new techniques presented in this report are designed to allow more accurate measurement of certain observational parameters. It is hoped that some of these will be tested more thoroughly at COAST over the coming months.
Section 3 of the report introduces some of the principles of optical interferometry. This is followed by discussion of techniques for tracking the changes in the phase of the interference fringes and the results of experimental trials of one phase tracking procedure. Sections 6 and 7 investigate a new method for the measurement of the position of the fringe envelope, and give some experimental results.
The second half of the report discusses some of the effects of optical dispersion on closure phase measurements. This study was stimulated in part by plans to expand the size of the COAST array, which if implemented would increase dispersive effects substantially. In the final section of the report three new techniques for closure phase measurements are explained. These are designed to reduce the systematic effects of dispersion in closure phase measurements. They are not presented as ideal solutions to the problems associated with optical dispersion, but rather as the starting points for future studies.
The Principles of Optical Interferometry
Tracking Phase Fluctuations at COAST
Experimental Phase Tracking at COAST
Tracking the Position of the Fringe Envelope
Experimental Tracking of the Fringe Envelope
The Measurement of Complex Closure Amplitude at COAST
Closure Phase Measurement Techniques not affected Systematically by Linear Optical Dispersion
Experimental Measurement of Optical Dispersion
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