Full control over orientation dispersion requires ψ on rotationally symmetric models

[pkienzle]

Playing with the orientation viewer, I was comparing a long square rod with Δψ = 90° ~ uniform to a cylinder, both with Δφ=20°

$ python -m sasmodels.jitter -s 20,20,100 -j 0,20,90 -d uniform -v 30,60,0 &
$ python -m sasmodels.jitter -s 10,10,100 -j 0,20,0 -d uniform -v 30,60,0 cylinder &

If I now change ψ on the square rod we can see the fan shape moving relative to the beam and the pattern changing on the detector. Notice in particular that the pattern is no longer symmetric about any axis (though it is mirror symmetric).

Now if I change ψ on the cylinder, the fan again rotates but the pattern does not change. Notice that no matter how you select (θ, Δθ, φ, Δφ) the pattern always has two axes of symmetry.

We could probably address this by providing ψ parameter to the cylinder model. This would allows us to orient the fan arbitrarily, and presumably generate a similarly asymmetric pattern.

This issue only arises if the rods are constrained within a single plane and the plane is at an arbitrary angle to the beam. It is likely not a problem in practice, but it is worth noting the orientation viewer is misleading in this case.

[pkienzle]

Alternative: disable ψ, Δψ in the orientation viewer for shapes such as cylinder which do not have a ψ parameter in their model.

Then all orientation dispersions that we can access would be consistent with the displayed scattering pattern.

This change also needs to happen for the orientation viewer in SasView.