McStas: representation of polarisation

[Per-Olof Astrand]

Dear McStas user,

After some discussions with Kristian Nielsen, we have finally sorted out
how polarisation is supposed to work in McStas. Only components actually
modifying the spin vector explicitly (modifying the (sx,sy,sz) vector)
requires the POLARISATION PARAMETERS(sx,sy,sz) line. If the instrument
contains at least one component with this line it will handle polarization
correctly, i.e. the spin vector is correctly transformed to the local
coordinate system of the components and components containing this line
are allowed to modify (sx,sy,sz).

It is also of importance that non-polarising components do not contain
this line because a simulation of an instrument without polarisation is
more efficient if the transformation of the spin is excluded. This will be
cleaned up in the components of McStas 1.5 (but it already works in the
McStas 1.4 kernel) and it will also be properly documented in the manual.

You can follow the development of McStas 1.5 at
http://neutron.risoe.dk/mcstas/developments/developments.html . We have a
quite long list of things to fix, but further comments on problems and
extensions are welcome.

Best regards,

Per-Olof 


Per-Olof Åstrand
Dept. of Chemistry, University of Copenhagen 
and Materials Research Department, Risø National Laboratory 
per-olof.aastrand at theory.ki.ku.dk or per-olof.aastrand at risoe.dk
http://theochem.ki.ku.dk/~peo/