An active telescope secondary mirror control system

One of the main constraints for a modern astronomical telescope is the acti ve control of the secondary mirror, mainly for the correction of the decent ering coma and defocus induced by thermomechanical distortions. The mirror movements should be smooth, as precise as the optical design requires, and restricted to within predefined limits. It should be possible to perform mi rror alignment and focus corrections without pausing the exposure (online c ontrol). The Galileo telescope achieves all this by using a support structu re driven via six actuator bars (a hexapod system or Stewart platform) and a real-time control system based on a transputer network that allows parall el control of each actuator. Both the hexapod secondary support and the con trol system have been built and tested at the telescope. The results show t hat the errors introduced during mirror positioning lead to optical aberrat ions well below the diffraction figure of the telescope, and the systems wo rk smoothly enough to allow online control. (C) 1999 American Institute of Physics. [S0034-6748(99)01106-5].