Offsets measured for various parts of the shell (R1–R5) defined in Fig. 1, as described in the text. The grey band shows the 1σ expectation for the offset as a function of distance, assuming a uniform expansion as obtained by a joint fitting of the regions R1–R4 (that is, excluding the outlier region, R5). The horizontal lines correspond to zero offset and a 0.8″ estimated uncertainty, from the alignment of the two observations.