these radii gives a linear expansion rate of $3500\pm100$ . The mean radius at each observing epoch is listed in Table \[tab\_params\] and plotted in Figure \[fig\_expand\].

The signal-to-noise of the data presented by Gaensler et al. (1997) was too low to justify more complex fits to the data. However, we are now in a position to compare a thin spherical shell to other models. We first note that other simple spherically-symmetric models, such as face-on rings and gaussians, produce significant residuals, and are clearly inconsistent with the data. In Figure \[fig\_models\] we show three possible fits to the data from epoch 1998.0. In each case, we have generated an input model (shown in the first column). We have then simulated an observation of the sky-distribution corresponding to this model as follows: we Fourier-transform the model, then multiply this transform by the transfer function of the ATCA observations from this epoch to produce a set of $u-v$ tracks identical to those of the real data. We then image these visibilities in the same way as for the real data, and similarly deconvolve and super-resolve the image, to give the result shown in the second column. We also subtract the $u-v$ 