+Present address: European XFEL, Notkestrasse 85, Hamburg 22607, Germany.

1For the ID06 monochromator the centre of rotation is located at half the height between the white and monochromatic beams. Therefore the beam footprint moves across the surface of the first crystal when the Bragg angle is changed. In this geometry it is not necessary to translate the second crystal parallel to the beam. Experiments were carried out at large Bragg angle, where the beam footprint is small and is located relatively close to the centre of rotation, i.e. the downstream edge of the crystal.