Flexure based fibre positioners: design optimisation to follow arbitrary focal plane curvature

The use of flexures to achieve fibre positioner motion is being actively investigated by several institutes, for example at the UK Astronomy Technology Centre (UKATC) and Leibniz-Institute for Astrophysics Potsdam 1. One challenge when designing with flexures is the large number of degrees of freedom available which makes it difficult or impossible to optimise their motion by hand. In this paper we demonstrate two approaches for optimising flexure geometry to follow arbitrary focal surface curvature and to orient the optical fibre with arbitrary tilt. These approaches are: analytical using MATLAB models and FEA based using Ansys. The approaches are complementary allowing the designer to efficiently explore the parameter space and then do precise optimisation of the flexure geometry. We demonstrate the applicability both to the UKATC's preferred design for WST, and to flexure-based fibre positioner designs generally. We also present a sensitivity analysis relating small changes in design parameters to changes in fibre tip motion. Finally we briefly present the UKATC's preferred geometry for the WST fibre positioner.