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Fig. 3.

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Reference images derived by various methods. All images are plotted with the same colorbar, which is shown on the right. The orange box denotes the reference that was used for the on-sky tests: (a) Reference image calculated by taking the mean of 40 short exposure images taken in quick succession; (b) Reference image measured by the calculating the mean of all images used for deriving the on-sky RM; (c) Reference image calculated by taking the mean of 4000 short exposure images taken over ∼5 min.

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