Proceedings PaperVLT tracking and guiding software
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The VLT must provide high quality images in a wide range of operating conditions and for a number of different modes. Performances of tracking and guiding impacts heavily on the final image quality. Control of the tracking axis is implemented in VME controllers, one per axis. They are accurately synchronized with a central time distribution system. This allows each axis to operate independently from the others, deriving its own position from the star coordinates and the time. This coordinate transformation includes basic corrections like refraction and pointing model compensations. To improve tracking performances, guiding devices (guiding cameras located in the adapter, or even instrument detector itself) can be used to measure position errors and derive small corrections for tracking. These corrections can be applied either to the secondary mirror, or to the tracking axis. To coordinate all these activities, the VMEs are connected through several LANs together with supervising workstations. While the basic functionality is controlled by applications implemented in the VMEs running a real time commercial operating system: VxWorks, the presentation layer and the non time critical operations can be implemented on workstations running standard Unix. The distribution of control software on VMEs or workstations is then dictated by real time constraints and/or availability of proved solutions for one of the operating system. This paper presents the choices in the hardware and software architectures, as well as the design concepts, made to support the different operational modes and fulfill the performance requirements.