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The positional accuracy available from a WFC observation depends on both the significance of the detection and the systematic uncertainty of the WFC coordinate system, mainly determined by the accuracy of the attitude solution derived from the WFC star trackers.
For an on-axis source (off-axis angle ), the estimated positional uncertainties (90% confidence radii, ) are: strong source (signal-to-noise :)
weak source (signal-to-noise :) In the ``strong source'' case, where the positional uncertainty is dominated by the accuracy of the attitude reconstruction, the value is expected to improve with in-orbit experience.
The spatial resolution for the WFC is shown in Figures 5.4 and 5.5. Note that the HEW is roughly constant over the central of the field of view, but then increases rapidly towards the edge. For studies of structure in extended sources, the FWHM of the instrument is more relevant than the HEW. The behaviour of the FWHM as a function of off-axis angle is shown in Figure 12.13 . The degradation of spatial resolution at large off-axis angles should be taken into account for studies of extended sources, and for position determinations of off-axis point sources.
For studies of very extended objects, the detailed behaviour of the WFC background on large angular scales is important. Although initial in-orbit experience is encouraging, the capability of the WFC to measure extended low-surface brightness emission is still under evaluation.