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An ultra precision large optics grinding machine, BoX®, was developed andproduced at Cranfield University. BoX® offers a rapid and economic solution forgrinding large off-axis aspherical and free-form optical components. Grindinghigh accuracy surfaces with low subsurface damage reduces subsequent polishingtime. This efficient grinding process provides the capacity to grind 1.5 mparts. This paper presents an analysis of Astrositall® optical ground parts: ahexagonal 84 m radius of curvature mirror of 1 m across corners and an off-axis350 mm diameter mirror. The 1 m hexagonal part is representative of segmentsunder study for making extremely large telescope (ELT) segmented mirrors. Thesecond part was machined off-axis to demonstrate free-form fabricationcapability. These operations demonstrate the scalability of the rapid grindingprocess developed for large free-form optics. The use of an error compensationprocedure improved an initial ground form accuracy to +/- 1 μm p-v over 1 metresurface. The results highlighted the effect of grinding parameters and machinedynamics on form accuracy and fabrication

Precision grinding for rapid fabrication of segments for extremely large telescopes using the Cranfield BoX