In the following the influence caused by crystal and source dimensions is discussed.
We follow closely the paper by J. Eggs and K. Ulmer  which gives
much more details. The horizontal and vertical opening angle of the crystal
as seen from point O is denoted by and
respectively. The height of the source is given by the opening angle
with z being the geometrical height of the source. The change in angle
caused by the finite extension of source and crystal is given by
The angular offset caused by the height of the crystal is of the order