The development and use of a high-speed astronomical photometer

This dissertation describes the development and implementation of a computer-based photometric data acquisition system for the Mt Canopus telescope. The high-speed photometric system allows continuous acquisition of data, for up to —69 min with a maximum sample rate of 9.8 kHz, while providing a real time display of the counts. The system was thoroughly tested with a pulsing LED source and with observations of the Crab pulsar. The results from several observational programs, suited to the system's capabilities, are also presented. The observational programs involved a study of the DQ Her type cataclysmic variable EX Hydrae, a fast photometric study of geostationary spin-stabilised satellites, a monitoring program on SN1987a, and an occultation program which culminated in a successful observation of a Pluto occultation and the detection of an atmosphere on this planet. The Mark I version of the fast-photometer system was used for the photoelectric study program on the eclipsing cataclysmic variable EX Hya. This program demonstrated the continued secular decrease in the period of its 67 min modulation. The observed rate of change of the 67 min period implies a mass transfer rate of the order of —3x10-11Mo yr-1and a magnetic moment around 8x10 32G cm3 . Two variations in the eclipse timings are also identified. Firstly, one sees a long-term variation that has been fitted with an —20 yr period sinusoid. Secondly, one sees a short-term variation which is correlated with the phase of the 67 min cycle. This second variation has been interpreted in terms of the eclipse of an accretion column or arc in the region above the magnetic pole of the white dwarf. Supplementary to the photometric data, high time resolution spectroscopic data from an eclipse of EX Hya were obtained with the Anglo-Australian telescope. These data are compatible with the general spectral characteristics noted by other workers. The V/R ratio of the spectral lines, around orbital phase 098 = 0, is consistent with the eclipse of an accretion disk. A sharp peak in the equivalent width of the Balmer lines is seen at the time of the eclipse. It is comparable in width to the X-ray eclipses, and narrower than the optical eclipses. Finally, the accretion curtain model of EX Hya is shown to accommodate, in a qualitative sense, all the observational constraints that must be imposed on any model of the system