Hello Severin,
However if I want to choose a point inside the spherical part (outside of the cartesian grid) I can not gather any data.
I assume that the output thorn is only aware of the cartesian coordinates? Do you (or anyone else) know if there is a way to bypass this issue?
The output thorn is CarpetIOASCII, though I am not familiar enough with it to know exactly how it finds out which point to output. It may well not be aware of Llama (it would then use the "local" coordinates instead of the "global", Cartesian ones).
You could write some C code to use Cactus's interpolator to get interpolate the data. If the coordinate given is the location of an actual grid point then there is (effectively) no interpolation. For an example on how to use it, please see the Cactus docs:
https://www.einsteintoolkit.org/referencemanual/ReferenceManualch2.html#x4-1...
You could (ab)use CactusNumerical's InterpToArray thorn which lets you do the same without writing code. For an example use, please see its tests though for your use you will want to use the "scalar_vars" etc parameters instead of "array1d_vars".
Yours, Roland