We develop an idea of fixed exit double-crystal monochromator of generalposition, in which the lines of entering and exit beams (both being fixed) donot lie in one plane, i.e., are in general position. Both cases of identical and not identical crystals are considered, and exactsolutions are obtained describing valid positions and orientations of bothcrystals. Special attention is paid to the case of small lattice mismatchbetween the first and the second crystal (e.g. due to temperature difference)and small angle of `skewness' (that is to say, an angle between skew lines ofentrance and exit beams is much less than unity). We derive also some useful formulations describing beam profile change aftertwo reflections (whose reflection planes are non-parallel). At last, possibledesigning approach for such a skew DC-mono is briefly touched upon.
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