The New Robotic Telescope will be a fully autonomous, rapid reaction, primarily spectroscopic facility for the classification of astronomical transients. The 4.18m diameter primary mirror is to be composed of 18 hexagonal mirror segments, arranged with a secondary mirror that feeds the Cassegrain focal stations with an F/10.635 beam. The final telescope design does not follow an established prescription, although both primary and secondary remain hyperbolic. However, the tube length is retained from an earlier F/7.5 RC design and secondary mirror size reduced to minimise obscuration of the primary. The optimisation process involved considering the M2 / fold size trade-off while solving the surfaces for image quality, contrast and wavefront error after speeding up the primary mirror. The final effective focal ratio is then slower to allow for workable tolerances through manufacture, installation and operations. In this presentation the optimisation process, trade-offs, tolerances and final design will be summarised.