Design and realisation of a position actuator for the E-ELT primary mirror

The European Extremely Large Telescope (E-ELT) primary mirror has a diameter of approximately 39 m, and consists of 798 near-hexagonal shaped segments. Three position actuators (PACTs) are responsible for the active control of the degrees of freedom that define the optical plane of a segment: piston (z), tip (φ), and tilt (ψ). In this extended abstract, the concept design and realisation of such an actuator is presented. The PACTs are designed to have a stroke of 15 mm with a position accuracy of 1.7 nm RMS. To overcome the large dynamic range, a dual-stage design has been adopted. A cost-effective PACT with a low axial stiffness is proposed and realised, with a voice coil as fine stage actuator, and a coarse stage driven by a BLDC-motor and a leadscrew. A flexure straightguide, dimensioned such that it has a long lifetime (≥30 years), is applied to minimise hysteresis and friction. PACT must operate at different angles with respect to vertical, to keep track of sky targets during observation, while keeping the power dissipation below 1.5 W. As the axial load during operation can vary between 0 and 900 N, an off-loading mechanism is integrated into the flexure straightguide. The results of the first measurements look promising, and it is expected that the performance can be shown during the conference.