SIMBOL-X: the problem of the calibrating a 0.5-80 keV 20m focal length focussing telescope

Simbol-X is a hard X-ray telescope being developed by France and Italy, with the participation of Germany, that will operate in the 0.5-80 keV. Simbol-X will allow, in the key 10-40 keV band, a two order of magnitude leap both in sensitivity and angular resolution, by means of a hybrid focal plane coupled with a 20m focal length multi-layer coated X-ray mirrors. The novel telescope architecture and its three-decade extended energy band translate however into technical issues never addressed before for what concerns the on-ground calibrations. In fact, at least in the full-illumination geometry, the unprecedented focal length causes, due to the calibration source finite distance, two main problems: (a) a non-negligible increase of the fraction of incident photons undergoing only one reflection (in the parabola); (b) a variation of the reflectivity curve between parabola and hyperbola, due to a corresponding variation of the incidence angle of incoming photons. In this work, the issue of calibrating a 20m X-ray focussing telescope is addressed in details, and technical solutions are envisaged and proposed to tackle and minimized the above mentioned problems.