Average decorrelation of OPD maps between 1 and 2 c/p over time differences up to 1.5 h in our sequences. The curves are computed as the average along the diagonals of Ɗ_f (i.e., at fixed time differences, see Sect. 2.2). Two distinct regimes are visible: a fast decorrelation on timescales below a few seconds and a much slower, quasi-linear decorrelation on timescales of minutes to hours. The dotted line at 60 s corresponds to the limit for the fitting of the parameters of the decorrelation (Eq. (2)).