Correlations of delay measurement errors for the quad lens analyzed in Sect. 8. The distribution of delay measurement errors on 1000 synthetic curves is shown for each quasar image pair against each other pair, marginalizing over the true delays of the synthetic curves. The central crosshair of each panel indicates zero error, and the ticks are days; i.e., each axis shows errors from − 5 to + 5 days. Displacements of the distributions with respect to the crosshairs indicate bias, while the width of the distributions indicates variance of the time delay estimators. For clarity, only single contours at half of the maximum density are shown for two of the techniques. The measurement errors shown in this figure are exactly the same as used in Fig. 10.