Sub-surface damage issues for effective fabrication of large optics

A new ultra precision large optics grinding machine, BoX®has been developed at Cranfield University. BoX®islocated at the UK's Ultra Precision Surfaces laboratory at the OpTIC Technium. This machine offers a rapidand economic solution for grinding large off-axis aspherical and free-form optical components.This paper presents an analysis of subsurface damage assessments of optical ground materials produced usingdiamond resin bonded grinding wheels. The specific materials used, Zerodur®and ULE®are currently understudy for making extremely large telescope (ELT) segmented mirrors such as in the E-ELT project.The grinding experiments have been conducted on the BoX®grinding machine using wheels with grits sizes of76 μm, 46 μm and 25 μm. Grinding process data was collected using a Kistler dynamometer platform. Thehighest material removal rate (187.5 mm3/s) used ensures that a 1 metre diameter optic can be ground in lessthan 10 hours. The surface roughness and surface profile were measured using a Form Talysurf. The subsurfacedamage was revealed using a sub aperture polishing process in combination with an etching technique.These results are compared with the targeted form accuracy of 1 μm p-v over a 1 metre part, surface roughnessof 50-150 nm RMS and subsurface damage in the range of 2-5 μm. This process stage was validated on a 400mm ULE®blank and a 1 metre hexagonal Z

Testing of optical components to assure performance in a high acerage power environment

Evaluation and testing of the optical components used in the Atomic Vapor Laser Isotope Separation (AVLIS) plant is critical for qualification of suppliers, development of new optical multilayer designs and monufacturing processes, and assurance of performance in the production cycle. The range of specifications requires development of specialized test equipment and methods which are not routine or readily available in industry. Specifications are given on material characteristics such as index homogeneity, subsurface damage left after polishing, microscopic surface defects and contamination, coating absorption, and high average power laser damage. The approach to testing these performance characteristics and assuring the quality throughout the production cycle is described