In the doc on CBCT xray sources, it says "The direction of the generated photons always point to the center of the world. The source has its own local axis." and an accompanying diagram (also attached) shows the source axes (x'-y'-z') in green. Is one of these axes (x', y', or z') meant to represent the "direction of generated photons"?
In the diagram, it looks like the local axis is to be understood as always oriented with x' parallel to the world z-axis, y' parallel to world y-axis, and z' parallel (and antipodal) to the world x-axis. If that is the case, then it would seem that there is no plane in the local axis that is always perpendicular to the direction of photon propagation. And if that's the case, it seems hard to understand how to set the focal spot size dimensions.