If you are using McStas 2.3 we recommend you to install the updated component library for that version - distributed as a deb/rpm package or downloadable through the updates/ folder for your relevant platform at the site
http://downloads.mcstas.org/mcstas-2.3/
The component is a new development with an easier-to-use geometry, centred on the so-called "moderator focal points".
The component DOES NOT implement historical brilliance formulations, it only realises a model of the so-called BF1 design.
For more information, including extended usage instructions and benchmark datasets, please consult this website:
http://ess_butterfly.mcstas.org
The website also contain (currently draft) documents written with Ken Andersen and Luca Zanini, explaining the reasoning behind changing from the BF2 design to the BF1 design, neutronical calculations and benchmarks. Please note that these are still in DRAFT status, are expected to be in their final shape around 2016-09-22.
Peter Willendrup, 2016-09-15