Design considerations for a hybrid swing-arm profilometer to measure large aspheric optics.

This thesis presents issues regarding the design and implementation of a profilometer based on the swivelling motion of a pivot arm. The main advantage of such a method, over conventional profiling techniques, is that it can be easily adapted to measure a range of optics including both spherics and aspherics, convex and concave as well as a variety of optic diameters and radii of curvature. The initiative for the development of the new aspheric metrology technique comes from the need for fabrication and measurement of hundreds of aspheric segments to enable the future Extremely Large Telescopes (ELTs). A detailed investigation of currently available aspheric metrology techniques concluded with the proposal of a novel SAP with a hybrid opto-mechanical sensor as a way to tackle issues of ELT segment metrology that remain in doubt. The successful submission of the proposal to the Department of Trade and Industry resulted in the allocation of funding for the development of a functional full scale prototype. Initially a basic error model was constructed in order to forecast the error of a generic SAP and to estimate allowed sub-component tolerances. Subsequently a first, scaled- down model of an SAP was used as a test platform which guided the construction of a full-scale prototype. The prototype is capable of measuring optics up to 1 m in diameter with a minimum radius of curvature of 1.75 m for concave and 1.25 m for convex surfaces. The SAP performance was evaluated through the measurement of a 680 mm diameter optic and measurements of surface profiles with accuracy better than 20 nm were realised. The thesis describes in detail the design process followed from the conceptual idea to the implementation and evaluation of the instrument.

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