Software Design Aspects and First Test Results of VLT Survey Telescope Control System

The 2.6 m VLT Survey Telescope (VST) is going to be installed at Cerro Paranal (Chile) as a powerful survey instrument for the ESO VLT. The tightest requirements to be respected for such a telescope, (large field of view of 1°x1°, pixel scale of 0.21 arcsec/pixel, and hosted in a one of the best worldwide astronomical sites), are basically very high performances of active optics and autoguiding systems and an excellent axes control, in order to obtain the best overall image quality of the telescope. The VST active optics software must basically provide the analysis of the image coming from the 10x10 subpupils Shack Hartmann wavefront sensor and the calculation of primary mirror forces and secondary mirror displacements to correct the intrinsic aberrations of the optical system and the ones originated for thermal or gravity reasons. The algorithm to select the guide star depends on the specific geometry of the adapter system. The adapter of the VST hosts many devices handled by the overall telescope control software: a probe system to select the guide star realized with motions in polar coordinates, a pickup mirror to fold the light to the image analysis and guiding cameras, a selectable reference light system and a focusing device. All these devices deeply interface with autoguiding, active optics and field rotation compensation systems. A reverse engineering approach mixed to the integration of new specific solutions has been fundamental to match the ESO commitments in terms of software re-use, in order to smoothen the integration of a new telescope designed and built by an external institute in the ESO environment. The control software architecture, the simulation code to validate the results and the status of work are here described. This paper includes also first results of preliminary tracking tests performed at the VST integration site for azimuth, altitude and rotator axes, that already match system quality requirements