An Acousto-Optical Correlation Spectrometer for Radio Astronomy

Citation

Cheung, Kwok-wai (1987) An Acousto-Optical Correlation Spectrometer for Radio Astronomy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pvms-m577. https://resolver.caltech.edu/CaltechETD:etd-08282008-094735

Abstract

The objective of this thesis research is to develop and construct a wide-band, high resolution, two-channel prototype acousto-optic correlation spectrometer (AOCS) to analyze signals received by the three element millimeter-wave interferometer at Caltech's Owens Valley Radio Observatory (OVRO) which may be used to study the distribution of carbon monoxide and other molecules in galaxies.

The proposed correlation spectrometer has the main advantage of large time-bandwidth products and hence of low cost per frequency channel. Thus, it is suitable for many scientific objectives that require both large bandwidth and high resolution, such as mapping the distribution and temperature of the interstellar gases of galactic sources and extragalactic sources or studying the atmospheric conditions of planets in the solar system.

Phase switching has been used to reduce the zero level variation of this instrument, and is found to be more effective than other schemes used by the single dish acousto-optical spectrometers.

Both the frequency resolution and the frequency coverage of this instrument can be changed easily, and give it a flexibility not attainable by the filter-bank spectrometers. The relative light weight and compactness of this instrument make it a good candidate for outer space applications.

An absolute calibration of the instrument has been attempted by using the system temperature as a scale for both the correlated signal received and the noise fluctuation of the instrument. A statistical method has been used to measure the various noise contributions of the instrument, which allows a very precise characterization of the zero level stability and the noise degradation of the system. The visibility loss of the present instrument was measured to be about 50%, and the noise degradation was about 40%. These losses were not due to any theoretical limitations but were the results of the imperfections in the present setup. They can be improved in the future versions of this instrument.

This instrument can be used to measure any RF spectrum and the cross spectrum of any two RF signals. Since it is a time integrating device, its sensitivity can be as high as one desires. With phase switching, one can compare the frequency characteristics of two closely matched microwave devices with great sensitivity. Because of its compactness, low cost and high sensitivity, it could be a useful and practical instrument for microwave measurements.

Thesis Files