Investigations of low levels of stellar polarimetric variability
The aim of this thesis is the investigation of stars which show very low levels of polarimetric variability. A sample of such stars has been observed and assessed statistically to determine small differences, and temporal changes in polarisation. A statistical test has been improved upon, and used in a more rigorous, but more conservative fashion, emphasising the need for great care and thorough statistical assessment of such sources, to detect true temporal change. A new idea for an astronomical polarimeter has been introduced and discussed. The device, incorporating liquid crystals as phase modulators, has been theoretically modelled, designed and developed. Finally the instrument has been tested to ascertain its sensitivity and to investigate the repeatability of any results accrued from its use. Chapter 2 is concerned with measurements taken over seven nights in May 1996 on the 0.75m telescope at Sutherland, South Africa, using the Cape Town Polarimeter. The concern of Chapter 3 is to investigate the use of the Kolmogorov statistical technique (Conover (1980)) as a means of detecting low levels of polarimetric variability. The development of a Twin Liquid Crystal Polarimeter is the theme of Chapter 4. The need for a polarimeter with no mechanically moving components is explained, and to this end, the possible use of liquid crystals is discussed. Initially a device incorporating just one liquid crystal is described, but inevitably this system still requires some mechanical rotation. The subsequent theory behind developing a system with two liquid crystals, to eliminate the need for any rotation, is then developed. It is made apparent that for successful operation for such a device, the cells need be aligned very precisely to each other. Procedures for doing this are considered and assessed.