The mirror design for the International X-ray Observatory (IXO) is currently following two paths: a segmented slumped glass shell Wolter-I design, and a Silicon Pore Optics (SPO) conical approximation to the Wolter-I design. The conical approximation used for the SPO imposes a lower limit to the angular resolution which puts this option at a potential disadvantage. In this paper we describe ways in which this can be circumvented. We analyse the surface profile modifications that can be made to lift this limitation and show that a much closer approximation to the Wolter I ideal is possible. We describe several ways in which a much tighter angular resolution limit could be achieved in practice and discuss ways in which this can be implemented in the manufacture of the SPO.
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