Control of the secondary mirror unit for the SOFIA telescope

SOFIA is a 2.5-m telescope to be carried on a special Boeing 747 for astronomic observations at altitudes of about 15,000 m. The paper describes the main features of the secondary mirror unit. The SOFIA secondary mirror needs active control for alignment along five degrees of freedom as well as for very fast chopping with a frequency up to 20 Hz. The SOFIA secondary mirror chopping mechanism and control system are described. A state space model derived from the mechanical and electrical equations of the system is then introduced. This model can be used for the design of a state space controller based on a pole placement algorithm, making the system to behave as a fourth order Bessel filter. To avoid overshoots due to current and voltage limitations, a variable speed limitation which is function of the position error is added to the controller.