Kim, Sug-Whan; (1993) Active production of large aspheric optics for astronomy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is devoted mainly to tackling the unsolved problem of producing secondary mirrors for 8 m telescopes, which will be up to 2-2.5 m in diameter and in excess of 1000 waves aspheric. This cannot be done by traditional methods. The project directly addresses the problem and forms part of the UK's R and D contributions to the Gemini USA/UK/Canada/Brazil/Chile/Argentina project to produce two 8 m telescopes. Its aim was to develop a new active method. The thesis starts with a review of astronomical implications of 8 m telescope projects currently being undertaken or planned worldwide and continues with discussions on technological challenges specifically in main optics production. The first stage of producing the mirror is generation of the aspheric surface profile by diamond milling. This has been directly addressed by developing a new computer-controlled profiler based on the existing manual hardware of the Grubb-Parsons 2.5 m machine. An essential part of the development also includes a computer controlled contact profilometer. The system performance is presented, including calibration, error profile and convergence of error compensation. The main part of the project was to develop polishing using a full size active lap, by which the pressure distribution and hence ablation rate are modulated in real time. The progress of the project is described, starting with a review of other approaches being developed world wide. The overall philosophy and design of active components are presented. Following this, experiments with a sub-diameter polisher and a prototype active lap of 85 cm in diameter, as built, are also described, including methods of testing, ablation algorithm and control theory. The final part of this section discusses the performance of the active polishing lap in terms of functionality at component and system levels. The conclusion briefly summarises the evaluation of the active method and its impact on large optics production. It also gives ideas for future improvements of performance, research work still needed and viable applications for the technique.

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