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Different optical designs are under consideration for the International X-ray Observatory (IXO). In this paper we show
results of simulations of the segmented shell Wolter-I design, of the Silicon Pore Optics (SPO) conical Wolter-I
approximation and of the Silicon based Kirkpatrick-Baez design. We focus particularly on the issue of stray light. When
a source is off axis, such that it is not imaged on the detector, some of its light may still be directed by the optics onto the
detector plane. Sources close to the pointing direction can thereby introduce an extra background radiation level in the
detectors. This phenomenon is investigated by numerical ray tracing of the three designs, yielding detector images of the
stray light, and an indication of which part of the mirror that light originates. Results show the similarities and
differences of the designs with respect to stray light, and give a quantitative indication of the level of background
radiation in different cases. Furthermore, for the Silicon Pore Optics design, two different ways of partially blocking the
stray light have been modelled, indicating that a reduction of the stray light can be achieved. In general, the results that
have been found indicate that for the simulated set-ups the stray light levels are compliant with the design specifications
of the International X-ray Observatory.
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