Modelling diffractive effects in silicon pore optics for the ATHENA X-ray telescope

Silicon Pore Optics (SPO) are the technology selected for the assembly of the mirror module of the ATHENA X-ray telescope. An SPO mirror module consists of a quadruple stack of etched and wedged silicon wafers, in order to create a stiff and lightweight structure, able to reproduce in each pore the Wolter-I geometry required to image X-rays on the telescope focal plane. Due to the small pore size (a few mm2), aperture diffraction effects in X-rays are small, but not totally negligible to the angular resolutions at play. In contrast, diffraction effects are the dominant term in the UV light illumination that will be used to co-align the 600 mirror modules of ATHENA to a common focus. For this reason, diffractive effects need to be properly modeled, and this constitutes a specific task of the ESA-led SImPOSIUM (SIlicon Pore Optic SImUlation and Modelling) project, involving INAF-Brera and DTU. In this context, a specific software tool (SWORDS: SoftWare fOR Diffraction of Silicon pore optics) has been developed to the end of simulating diffraction effects in SPO mirror modules. This approach also allows the user to effectively predict the effects of various imperfections (figure errors, misalignments) in a self-consistent way, in different experimental configurations (X- ray source off-axis or at finite distance), as a fast and reliable alternative to ray-tracing, also at X-ray wavelengths.