The requirements for position, orientation and performance of the primary mirror active support system have been optimised through extensive FEA to minimise the wavefront slope error. The output of this optimisation has been a detailed performance specification which also takes into account telescope control and wind rejection requirements. The FE model has also been used to calculate the active force eigenmodes based on the static actuator patterns rather than approximations to the vibration modes. In addition significant development and prototyping has been undertaken in the actuator and definer design including control. Interesting aspects of this development include use of flexures in the mirror definers in order to meet the stiffness requirements and control of a pneumatic astatic system. This paper describes the process of requirement optimisation for mirror performance and also the development and design of the support system.