Time to go H_∞?

Traditionally telescope main axes controllers use a cascaded PI structure. We investigate the benefits and limitations of this and question if better performance can be achieved with modern control techniques. Our interest is mainly to improve disturbance rejection since the tracking performance normally is easy to achieve. Comparison is made to more advanced controller structures using H-infinity design. This type of controller is more complex and needs a mathematical model of the telescope dynamics. We discuss how to obtain this model and also how to reduce it to a more manageable size using state of the art model reduction techniques. As a design example the VLT altitude axis is chosen.