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The uncertainties intrinsic in reconstructing the true sky position of an event detected by the WFC come from two main sources. Firstly, the resistive anode image readout system of the MCP detectors has a small but significant intrinsic distortion. This is determined largely by the physical parameters of the system (ie. anode resistance and capacitance) but asymmetries are introduced by variations in the resistivity of the anode as a result of the manufacturing process. Consequently, the distortion needs to be removed during the data processing. This task, termed `linearisation', is performed by means of a look-up table that gives the relationship between the detector coordinate system and that of the telescope. Positional errors introduced by this process are approximately 12'. In-orbit experience demonstrates that the linearisation parameters have remained stable over the operational life of the detector.
A second source of uncertainty is the reconstruction of the WFC aspect from the star tracker data, which is subject to an error of around 12'. Combined with the linearisation errors the limiting positional accuracy of the WFC is around 30', however this limit is achievable only for bright sources close to the optical axis. For other cases the positional uncertainty is dominated by the PSF and by source statistics.
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