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The energy-to-counts conversion factors in Figures 10.2 - 10.13 apply only to on-axis observations. For off-axis observations the effective area and consequently the expected count rates will be reduced. In addition the source counts will be spread over a larger area in the detector due to the telescope blur. The reduction in effective area does not depend sensitively on energy for energies below keV. In Figure 10.18 the ratio of on-axis to off-axis effective area is plotted as a function of off-axis angle for energies 0.2 keV, 1 keV, 1.5 keV and 2 keV. With the information provided in Figures 10.2 - 10.13 and Figure 10.18 the calculation of off-axis count rates is possible.
In order to estimate the reduction in sensitivity due to the increasing spread of the point response function we show in Figure 10.19 the radius of the circle containing 50 percent of the counts vs. off-axis angle for a soft source (curve a) and a hard source (curve b). While the 50 percent circle radius depends on energy in the center of the field of view, at off-axis angles larger than the point response function does not depend sensitively on energy.
For guest investigators wishing to perform more detailed calculations we provide in Figure 10.20 the effective on-axis area of the XRT+PSPC combination with and without the boron filter.