Source and detector are mounted on a cylinder axis with the Bragg crystal on
a cylindrical surface . The crystal is bent with a curvature .
The distance of source to detector is given by
As from the horizontal focussing condition the distance source
is required to be
the focal length is given by
. As these two
quantities form a rectangular triangle together with (
the relation between and is given by
In practice is chosen because the otherwise necessary ellipsoidal curvature faces major technical difficulties. The loss in vertical focussing is still tolerable.