On-line control of an active telescope secondary mirror

Sometimes during a scientific data acquisition the focus of a telescope may change, especially for medium- and long-time exposures. This is mainly due to temperature changes inducing an expansion of the serrourier truss and, more generally, due to elastic structural flexures. Modelisation and control of defocussing effect is an important topic in telescopes with great magnification ratio. In the Telescopio Nazionale Galileo case, a 3.5 meters class telescope located in La Palma island, a temperature change of 1 degree induces a 0.9 arcsec image blur. In such a case a control of defocus during an acquisition is essential to obtain sub-arcsec images, thus exploiting the site seeing. The secondary mirror of Galileo telescope is actively controlled via an hexapod system, which fulfils two basic requirements: precision in positioning and smoothness of the movement, the latter being necessary to maintain a perfect alignment of the secondary mirror during detector exposures. The hexapod control system is of real-time type, based on a transputer network, and allowing parallel control on each hexapod actuator. Both systems, the hexapod secondary support and the control system, have been completed and tested at the telescope. The results show that positioning of the mirror is smooth enough, introducing an image movement of about 0.1 arcsec and allowing in such a way the on-line correction also during scientific operations.