RESUMO
In this study, an online refocusing algorithm is proposed for a satellite camera performing an Earth observation mission. Satellite cameras are vulnerable to misalignment in orbit because of their severe launching environments and the thermal vacuum environment in space. The proposed online refocusing algorithm is able to guarantee high quality images by aligning the satellite camera in real time. This alignment is achieved by precisely adjusting the movement mechanism of the secondary mirror (M2) and the focal plane. The target optical system used in this study was originally designed for the purposes of algorithm development. The system uses a Schmidt-Cassegrain-type satellite camera with a 200-mm diameter primary mirror (M1). The ground sampling distance (GSD) is 3.8 m from an altitude of 700 km. A fourth-order equation model is derived for the modulation transfer function (MTF) variation tendency for M2 de-spacing. Following this, the proposed online refocusing algorithm for the target optical system is developed. The algorithm is able to assess the de-space position from the MTF measurements using stellar sources. It is determined from the simulation that any misaligned satellite camera can be refocused within a ± 0.5µm M2 de-space error by applying the proposed refocusing algorithm in real time.