Improved near optimal angular quadratures for polarised radiative
  transfer in 3D MHD models
  Accurate angular quadratures are crucial for the numerical solution of
three-dimensional (3D) radiative transfer problems, especially when the
spectral line polarisation produced by the scattering of anisotropic radiation
is included. There are two requirements for obtaining an optimal quadrature and
they are difficult to satisfy simultaneously: high accuracy and short computing
time. By imposing certain symmetries, we were recently able to derive a set of
near optimal angular quadratures. Here, we extend our previous investigation by
considering other symmetries. Moreover, we test the performance of our new
quadratures by numerically solving a radiative transfer problem of resonance
line polarisation in a 3D model of the solar atmosphere resulting from a
magneto-hydrodynamical simulation. The new angular quadratures derived here
outperform the previous ones in terms of the number of rays needed to achieve
any given accuracy.
