Optical simulations for the Wolter-I collimator in the VERT-X calibration facility

The VERT-X X-ray calibration facility, currently in prototypal realization phase supported by ESA, will be a vertical X-ray beamline able to test and calibrate the entire optical assembly of the ATHENA X-ray telescope. Owing to its long focal length (12 m), a full-illumination test of the entire focusing system would require a parallel and uniform X-ray beam as large as the optical assembly itself (2.5 m). Moreover, the module should better be laid parallel to the ground in order to minimize the effects of gravity deformations. Therefore, the ideal calibration facility would consist of a vertical beam, with the source placed at very large distance (>> 500 m) under high vacuum (10-6 mbar). Since such calibration systems do not exist, and also appear to be very hard to manufacture, VERT-X will be based on a different concept, i.e., the raster scan of a tightly (≈ 1 arcsec) collimated X-ray beam, generated by a microfocus source and made parallel via a precisely shaped Wolter-I mirror. In this design, the mirror will be made of two segments (paraboloid + hyperboloid) that, for the X-ray beam collimation to be preserved, will have to be accurately finished and maintain their mutual alignment to high accuracy during the scan. In this paper, we show simulations of the reflected wavefront based on physical optics and the expected final imaging quality, for different polishing levels and misalignments for the two segments of the VERT-X collimator.