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Comparisons between the Einstein and ROSAT HRI's must account for the different detector photocathodes and band passes of the two telescopes. As shown by the effective area comparison given in Figure 11.8 of Appendix F, the HRI effective area of ROSAT exceeds that of Einstein over most of the ROSAT energy band (because of the increased area of the ROSAT mirrors and the greater sensitivity of its CsI photocathode). As can be seen from Tab. 5.5, observed ROSAT count rates are typically a factor of 3 to 4 times those measured with the Einstein HRI (the values of this ratio, of course, depending upon interstellar absorption and the intrinsic source spectra).
The better scattering performance and a smaller point spread function of the ROSAT XRT, vis a vis the Einstein telescope, should ultimately permit the use of a 5'' detect cell, as assumed above for feasibility calculations, provided the aspect contribution to the PSF can be reduced.