Active focusing system for an earth imaging reflecting telescope

The M.Sc. degree project, which is presented in this paper, was undertaken at the University of Surrey under the financial support of TUBITAK-BILTEN, as a part of the remote sensing micro-satellite project: BILSAT-1. This micro-satellite was designed and constructed by Surrey Satellite Technology Limited (SSTL) in the UK in collaboration with TUBITAK-BILTEN. The core of the project was to build a system to illustrate an active focusing concept for a high spatial resolution Earth imaging reflecting telescope, in order to practically comprehend the possible difficulties that would be encountered. This paper gives a brief overview of the design concept of the imaging system, together with details of the system constructed to demonstrate and evaluate various active focussing techniques. It then, proceeds with the evaluation of focus measure performance under various optical conditions. Hence, the emphasis is not on the theoretical verifications of the focus measures, but rather is on the practical evaluation based on the experiments presented. Finally, the paper concludes that the energy of the image gradient is the best practical measure of focus for the control system, in terms of the noise sensitivity and the ability to function for a wide range of image types, when compared with the variance of the image, and the energy of the image Laplacian.