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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.