Further developments of optical fibre techniques for astronomical spectroscopy

Abstract

This thesis describes instrumental techniques pursued to optimise the quality of the spectroscopic data which will be obtained via the new generation of multifibre systems. The technical aspects discussed include the instrumental sources of systematic errors on sky subtraction and new methods to evaluate the fibre performance in terms of throughput and preservation of the input focal ratio. The limiting factor in faint multifibre spectroscopy is the accuracy and reliability of sky subtraction. Instrumentality the precision to which sky subtraction can be done is limited by systematic errors. The conditions that have to be satisfied to subtract the sky properly at faint levels, the sources of errors and possible ways of eliminating them are analysed. A comprehensive discussion of the effect of deficient wavelength sampling and of poor fibre flatfield calibration on sky subtraction is provided via computer simulations of fibre data. Several laboratory fibre evaluation techniques and data processing software packages have been developed. These techniques have been used to measure the through put of silica fibres at optical wavelengths. Also presented in this thesis are preliminary results obtained from a series of experiments implemented at infrared wavelengths. As a result of these developments a new method for determining the focal ratio degradation (FRD), based on a known theoretical model for microbending induced beam- spreading is proposed. This theory characterises the FRD by a single parameter which can be determined by a very simple experiment. The model successfully predicts real experimental results. This new technique will allows us to accurately model the complete performance of any fibre. Such modelling can include complicated phenomena such as tilted input beams and central obstructions without the need to setup experiments for specific cases. Furthermore, it can be used to evaluate the performance of infrared fibres. Finally, the results of an exhaustive laboratory evaluation of the Autofib-1.5 fibre bundle are presented. The evaluation was carried out after a multiway fibre connector prototype was incorporated into the Autofib-1.5 fibre bundle, with the emphasis being placed on the performance of the connector.