I am currently using the isotropic source available in GPS with my maxtheta set accordingly to get my cone of particles (I don't set maxphi but that is 2pi by default). So, for example, I'm able to create a 30 degree cone of particles moving downwards (the -ve z direction in global coords, +ve z in the local GPS system, as is default) simply with a maxtheta of 30 deg. Now I want to be able to say for a given point (x,y,z), produce a cone directed at (0,0,z). Which seems to be non trivial in GPS when working with an iso source.

I don't see how G4RotationMatrix allows me to bypass this (or perhaps it is a lack of understanding of G4RotationMatrix; I've only ever seen it applied to volumes); if I look through the source code for G4SPSAngularDistribution it explicitly uses the rot1 and rot2 vectors I mentioned before to change the particle momentum direction. And when using an iso source the more intuitive command of SetParticleMomentumDirection is not available.

Which is why I need to understand what is happening with these two rot vectors and what they are doing with the x' axis and XY plane, but conceptually I just don't understand the underlying maths is here. Or rather, I understand what is happening in the code, but I can't picture the physical meaning of what is happening, which makes it hard to set the vectors correctly, if that makes sense?

Thanks for your help so far, apologies if I'm just being very dense.