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Fig. 3


Selected processing steps of Ko observation taken on 14/09/1912: panel a: original density image, panel b: first estimate of background map (after Step 1.4), panel ch: results of each step of the iterative process (Steps 2.1–2.5), panel e and panel f: correspond to Step 2.3. The black square in panel f shows the dimensions of the window used for the median filter in Step 2.4 (see Sect. 3.1 for details).

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