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The WFC focal plane detector has no intrinsic spectral capability. Nevertheless the WFC can be used to measure XUV `colours' with a set of observations through different filters, and thus place constraints on source spectra. The WFC filters have been designed to define as set of relatively narrow passbands giving good sensitivity to a wide range of spectra (see Figure 5.2 and Table 5.3 ).
The sensitivity of the WFC for filter spectroscopy can be evaluated using the predicted count rates given in Figures 12.3 - 12.7 . To illustrate the ability of the WFC to make such measurements in a more convenient form, Figure 12.8 shows some examples of the filter count rate ratios predicted as a function of spectral parameters for two selected filter pairs. The spectral sensitivity is clearly a strong function of the filter combinations used.