Bent perfect crystals as X-ray focusing polychromators in symmetric Laue geometry

Figure 1
Illustration of the optical setup under consideration, showing the different systems of coordinates used in the text. The O [tau]

axis is perpendicular to Os_o; the [Omega]

u axis is perpendicular to [Omega]

q and to Os_h. The shape of the caustic curve is schematically shown on the right-hand side.