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On-axis point response
In Figure 4.3 the point response function measured in the MPE test facility is shown. As can be seen from Figure 4.3 , the measured point response in terms of encircled energy features a half energy width of and no extended wings. This number should improve in flight (the test facility measurement is an overestimate because the test X-ray beam has its own intrinsic width) but, due to current uncertainties in the telescope attitude solution, the in-flight point response has not yet been properly measured.
Off-axis point response
The off-axis point response of grazing incidence optics is rather complicated; a convenient measure of the image spread is the so-called blur radius. Figure 4.4 shows the expected ROSAT XMA rms blur circle radius (in arc minutes) as a function of off-axis distance. The blur circle radius is independent of energy and, as is obvious from Figure 4.4 , the blur radius increases rapidly with off-axis angle.
Microroughness of the reflecting mirror surfaces leads to non-specular reflection of X-rays, i.e., scattering. For the ROSAT XMA, the total scattered power is less than 6 percent at X-ray energies below keV for a detect cell radius of 10'' and less than 3.8 percent for a detect cell radius of 100''. Therefore, no scattering corrections need to be applied. Since the scattered power increases with the square of photon energy, even at higher energies the scattering corrections will be very small. This represents a considerable improvement over previously available X-ray mirrors and allows very high contrast imaging.