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In-flight refocusing and MTF assessment of SPOT5 HRG and HRS cameras

The MTF (Modulation Transfer Function) is a means of characterizing the spatial resolution of the instruments. So, the MTFs of HRG and HRS cameras are parts of image quality parameters assessed during the in-flight commissioning phase. Vibrations during the launch and transition from air to vacuum may defocus the HRG cameras and degrade their MTF. Therefore, SPOT5 HRG cameras are refocused before measuring their MTF. The paper first describes the HRG focusing procedure that uses both cameras viewing the same landscape: the focus of one camera is changed while the other is fixed and used as a reference. Results are given for each camera in terms of best focus and focus variation in the field of view. These results are compared to those provided by an autotest system, on-board each HRG camera, that images a high frequency periodic pattern while the focus is changed. Then, MTF measurements are presented. The MTF of HRG cameras is measured by imaging a spotlight that aimed at the satellite; the results are compared with pre-flight measurements. Besides, the MTF of HRS cameras is assessed by imaging landscapes with edge patterns; the main objective is to compare the two HRS cameras.