Hi Philipp, The two FFTs introduce a scale factor equivalent to the number of grid points. Please try adding this code: x = abs(new_array).data() y = abs(very_new_array).data() lc = flex.linear_correlation(x, y) lc.show_summary() lr = flex.linear_regression(x, y) lr.show_summary() The correlation coefficient should be 1.0 exactly, and the linear-regression slope is the same as the "total number of grid points" shown in the output: total number of grid points: 345600 ... is_well_defined: True mean_x: 43.9292797741 mean_y: 15181959.0899 coefficient: 1.0 is_well_defined: True y_intercept: 5.1687812285e-07 slope: 345600.0 Ralf ____________________________________________________________________________________Ready for the edge of your seat? Check out tonight's top picks on Yahoo! TV. http://tv.yahoo.com/