Bibby, M; (2015) The Development of Automatic On-Machine Metrology. Doctoral thesis , UCL (University College London).

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Abstract

The manufacture of large off-axis aspheric optics for the next generation of extremely large telescopes presents a number of unique challenges. For example, the European Southern Observatory (ESO) Extremely Large Telescope (E-ELT) requires the manufacture of one 1.4 m class ultra-precision mirror segment per week in order to satisfy the first-light deadline. One of the factors limiting the pace of manufacture is metrology. Many of the tasks associated with measurement, such as optic positioning, alignment and acquisition are carried out manually. It is also common for the optic to be removed to a laboratory for measurement, which can be time consuming and risk damage. This thesis presents research into the development of new on-machine metrology techniques, which allow measurement to be carried out in the manufacturing environment. This work is supported by a software application developed by the author to allow the design and control of on-machine metrology. This application uses the computer numerical control (CNC) polishing system as part of the positioning and alignment system. The inclusion of CNC has enabled the development of a close-loop control system which facilitates automatic alignment and acquisition of metrology data. The software presented uses a modular architecture, allowing many different types of metrology to be planned and control using a single application. This is demonstrated using two case studies, which allow automatic on-machine sub-aperture stitching metrology using a metrology tower placed over the machine, and automatic on-machine texture measurement. The use of a closed loop software application to control automatic on-machine texture measurement is a novel step. It is also demonstrated that on-machine metrology in the manufacturing environment can produce measurement data of comparable quality to that of the laboratory. Automatic measurements systems such as those presented are likely to play an increasing role in the large and high-volume optical fabrication sectors.

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