Elliptically shaped X-ray monocapillaries belong to imaging single-bounce optical systems. The spot size is influenced by manufacturing accuracy of the imaging optics (e. g. slope error, straightness etc.) and the characteristics of the source. The main parameters of ellipsoids have to be optimised according to the particular application. Only a definite part of the ellipsoid is used for focussing, because the reflection angle should be lower than the critical angle. For image magnification this part lays between the first focal point where the source is positioned and the maximum diameter of the ellipsoid. For image demagnification the reflection part of the ellipsoid lays behind its maximum diameter.