Position-control for active secondary mirror of a two-mirror telescope

In an aplanatic two mirror telescope, spherical and coma aberrations may be introduced if a misalignment of the secondary mirror is present. That misalignment may be intrinsic to telescope, due to small errors in optical design or manifacturing; however it will always be present during the life of the telescope, due to maintenance errors, thermal distorsions of the mirror and the structure, mechanical distorsions of the mirror mount, mechanical arrangement of the structure and other. An active control of the secondary mirror, allowing free positioning of the secondary, is important to correct such unwanted effects. The Italian Galileo National Telescope is equipped with a secondary mirror supported by an ''hexapod'' structure, allowing a complete positioning control. In this paper a strategy for handling the positioning and movement of the exapod support of a secondary mirror will be investigated from two point of view: an analitical and a neural network approach.