The use of optical fibres as light guides in astronomy promises to increase the efficiency of data collection in certain types of observing programmes. Fibres can also be used to simplify the construction of telescopes and associated instrumentation replacing complicated optical systems. At the Anglo-Australian Observatory (AAO) we have developed a number of fibre systems which spatially rearrange images at the 3.9m Anglo-Australian Telescope's (AAT) focal plane so as to optimise the input of light into the spectrograph. One system known as a fibre optically coupled aperture plate (FOCAP) allows the spectra of 50 objects to be gathered simultaneously. This unique application of fibres has revealed a number of problems not normally encountered in fibre data transmission systems. When fibres are inputted with a slow f-ratio beam, transmission along the fibre results in a spreading of the output beam beyond that of the input. This so-called focal ratio degradation is the major source of loss in these fibre systems.