Tri-crystal fixed exit monochromator
  It is proposed a novel design of an X-ray monochromator which uses three crystals. The exit beam turns in horizontal plane on the angle $2\alpha$ with respect to the incoming beam. So, this kind of monochromator is suitable for side beam-lines; several tunable beam-lines can be built for a one fan beam of synchrotron radiation (from wiggler or bending magnet).   In contrast to the double-crystal scheme of general position considered previously (which also suitable for side beam-lines), the new variant does not lead to a tilt of exit beam profile. 
I.L. Zhogin