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Those are the lines on the plot above.
A few other comments.
1) the assumption of isotropy does not hold. Motion in y is more than motion in x:
2) If image motions are totally uncorrelated, we expect the RMS of their separation to be sqrt(2) times the RMS of a single spot. So asymptotic value should be around 5.5 pixels or so?
3) We need to span a wider separation in order to reach asymptotic value, where relative motion becomes independent of separation, or else have smaller amounts of turbulence, or both.
MATLAB code DIMSUM.m is here: DIMSUM.m
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