next up previous contents
Next: The PSPC Up: Performance Previous: Mirror Areas

Angular resolution

On-axis point response

In Figure 4.3 gif the point response function measured in the MPE test facility is shown. As can be seen from Figure 4.3 gif, the measured point response in terms of encircled energy features a half energy width of tex2html_wrap_inline1904 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.

 fig4-3 figure165

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 gif 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 gif, the blur radius increases rapidly with off-axis angle.

 fig4-4 figure174


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 tex2html_wrap_inline1906  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.

Michael Arida
Tue Jun 11 16:18:41 EDT 1996