Performance of the VST secondary mirror support system

Abstract

The VST telescope is equipped with an active optics system based on a wavefront sensor, a set of axial actuators to change the primary mirror shape and a secondary mirror positioner stage. The secondary mirror positioning capability allows the correction of defocus and coma, caused by incorrect relative positions of the two mirrors arising from the deformation of the telescope tube and of the optical train under the effect of gravity and thermal espansion. Periodically the image analyser calculates the deviation of the image from the best quality and the telescope control software decomposes the deviation into the single optical contributions. The new position and orientation of the secondary mirror is computed by the telescope control software and transmitted to the secondary mirror support system for execution. The secondary mirror positioner is a hexapod, i.e. a parallel robot with a mobile platform moved by six linear actuators acting simultaneously. This paper describes the secondary mirror support system and the qualification test campaign performed both in laboratory and at the telescope.