Example of the construction of the calibration matrix for Winkler (secondary observer, G) to RGO (primary, G^∗) over 1900–1910. Panel a) shows the original distribution matrix G^∗ vs. G: the black line has a slope of unity. Panel b) shows the difference, G^∗−G vs. G. Panel c) is the same as b) but the empty columns for G^∗> 8 have been filled with the results of the MC simulation. The red circles with error bars depict the mean G^∗ values for each G column and their 1σ uncertainty. The yellow line shows the k–factor used in Hoyt & Schatten (1998).