A successful in-flight refocusing experiment based on image processing is described. Each of the two SPOT1 HRV cameras was refocused with respect to the other by analyzing the image spectrum taken simultaneously by both cameras. The experiment was carried out during the autumn of 1994 and its results are also presented: (1) the estimated optimal position for the rear corrective lens of the camera, (2) MTF improvement and its relative quality in the field of view in terms of homogeneity and astigmatism, (3) the validation of a theoretical geometric defocusing model that gives the changes in MTF as a function of corrective lens position. We conclude with the high accuracy of our method (2%) and its sensitivity to the spectral content of the viewed landscapes.
"In-flight refocusing of the SPOT1 HRV cameras", Proc. SPIE 2758, Algorithms for Multispectral and Hyperspectral Imagery II, (17 June 1996); doi: 10.1117/12.243225; https://doi.org/10.1117/12.243225