The ATHENA X-ray observatory is a large-class ESA approved mission, with launch scheduled in 2028. The technology of silicon pore optics (SPO) was selected as baseline to assemble ATHENA's optic with hundreds of mirror modules, obtained by stacking wedged and ribbed silicon wafer plates onto silicon mandrels to form the Wolter-I configuration. In the current configuration, the optical assembly has a 3 m diameter and a 2 m 2 effective area at 1 keV, with a required angular resolution of 5 arcsec. The angular resolution that can be achieved is chiefly the combination of i) the focal spot size determined by the pore diffraction, ii) the focus degradation caused by surface and profile errors, iii) the aberrations introduced by the misalignments between primary and secondary segments, iv) imperfections in the co-focality of the mirror modules in the optical assembly. A detailed simulation of these aspects is required in order to assess the fabrication and alignment tolerances; moreover, the achievable effective area and the angular resolution depend on the mirror module design. Therefore, guaranteeing these optical performances requires: a fast design tool to find the most performing solution in terms of mirror module geometry and population, and an accurate point spread function simulation from local metrology and positioning information. In this paper, we present the results of simulations in the framework of ESA-financed projects (SIMPOSiuM, ASPHEA, SPIRIT) to prepare the ATHENA X-ray telescope: we deal with a detailed description of diffractive effects in an SPO mirror module, show ray-tracing results including mirror module misalignments, study in detail diffractive effects in different configurations, and assess the focal spot correspondence in X-rays and in the UV light, an important aspect to perform the mirror module alignment and integration. We also include a proton tracing simulation through a magnetic diverter in Halbach array configuration.
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