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At low energies (< 0.2 keV) the response of the WFC is similar to that of the EXOSAT LE telescope with CMA detector. Thus it is useful to be able to compare the two instruments. Figures 12.9 - 12.11 show the predicted WFC to CMA count rate ratios for optically thin thermal, power law and black-body spectra. (Ratios were computed for the WFC with the S1 filter and the CMA with the thin Lexan filter: these 2 filters produce the most similar response.)
As can be seen, for modest column densities ( cm ), the WFC count rate is comparable with the CMA count rate, but as the column increases the ratio becomes very small. This behaviour results from the gross differences between the CMA and WFC response at high energies: the WFC has no sensitivity above keV whilst the CMA passband extends up to keV. Care must be taken, therefore, in using EXOSAT CMA count rates to predict WFC count rates, especially for spectra with significant absorption.