In our laboratory, an 8 kg nano-scale remote sensing satellite "PRISM" has been developed, which has an extensible
boom with a telephoto lens on the tip, being a refracting telescope in the orbit. Imaging using such flexible optical
systems bears the problem of structural deformation which results in low quality of the captured images. To compensate
for it, the satellite has to have a focusing function in limited techniques. This paper proposes a practical way of auto
focusing for such nano-scale remote sensing satellites using fuzzy image processing. The proposed method is
independent of photographed objects and does not require tracking of the contrast evaluation areas to be used for
conventional auto focusing algorithm, which reduces the on-board computational load and requirement of attitude
determination and control very much.