A fixed-exit double-crystal x-ray monochromator has been constructed for use at the macromolecular crystallography beamline at the Sincrotrone Trieste. For its design a new concept has been developed, which is based on a single rotational stage to control the θ or Bragg angle for both the first and second crystal. The stage rotation axis lies in the center of the surface of the first crystal, thus the incoming photon beam remains centered on it over the whole tuning range, which opens the possibility to, e.g., optimize the crystal cooling system. The fixed-exit aspect of the monochromator is achieved by translating the second crystal on one slide only forming an angle α with the surface of this crystal. In this way the translation distance can be kept small thus minimizing detuning effects of the monochromator due to not-perfect slides. The cost is, that during energy scans the beam footprint will wander somewhat on the second crystal surface. It is demonstrated that, by choosing a suitable angle α, both the translation distance T and the length of the second crystal can be reasonably minimized. In fact, e.g., a monochromator having a vertical beam offset of 17 mm and an angular tuning range 5°-30°, T can be kept as small as 25 mm by not even doubling the length of the second crystal with respect to the first one. © 1995 American Institute of Physics.
