High-speed telescope autofocus for UAV detection and tracking

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This paper presents the analysis, implementation and experimental evaluation of a high-speed automatic focus module for a telescope-based UAV detection and tracking system. An existing optical drone detection system consisting of two telescopes and deep learning-based object detection is supplemented by suitable linear stages and passive focus algorithms to enable fast automatic focus adjustment. Field tests with the proposed system demonstrate that UAVs flying at speeds of up to 24 m/s towards the system are successfully tracked and kept in focus from more than 4500 m down to 150 m. Furthermore, different search functions and contrast measures are evaluated and it is shown that the Tenengrad operator combined with the Hill Climbing search function achieve the best performance for focusing on fast moving small UAVs.