Doubts about Onion Doc

[Joachim Wuttke]

quite generally:
to facilitate citation and discussion, I'd suggest to number equations
in doc pages that contain lots of them.

concerning ​http://marketplace.sasview.org/models/73:

The introduction announces that the SLD of each shell may be »described
by an exponential, linear, or constant function«. In the following,
however, only only the exponential and the constant case are explicitly
covered. For a linear SLD, the reader is left with the hint that
the exponential function with small A is a good approximation.

rho_shell(r) is defined in terms of six parameters. One of them is redundant
since rho_in=B+C. The parameters B and C have no obvious physical meaning,
and they do not appear in the parameter list of Iq. Two equation blocks later,
the paremeter B is defined a second time, now in terms of rho_in, rho_out, and A.
This makes the logic very obscure.

I would suggest that rho_shell(r) be defined in terms of the parameters
A, rho_in, rho_out, r_in, r_out. Then one would introduce Delta t_shell,
B, and C as abbreviations, which makes them categorically distinct from
the API parameters.

• Joachim

[richardh]

I did work out the equations for sld linear increasing or decreasing spherical shells, so one day we could add them properly.

Richard

[smk78]

Joachim's comment about B & C not appearing in the parameter table is invalid because they are defined by parameters that do; as the docs clearly state.

But I have reworked the docs a little to try and make the different conditions a bit clearer.

His remaining comments are essentially suggesting a reparameterisation of fthe model which is not a documentation issue.

[pkienzle]

There are equations for linear shells in spherical_sld:

​https://github.com/SasView/sasmodels/blob/62dc8895694e8dcacf8cb65a82b543b1efa56f42/sasmodels/models/spherical_sld.c#L46

I suspect there is a Taylor series for low qr that improves accuracy and handles q=0.

Don't know if they are valid for large steps, or if they are only an approximation.