The axis of rotation of the switch mirror is aligned
with the beam axis between F2CAL and F3CAL (the switch
mirror). In this configuration, the switch mirror is set at the
position angle 59.4 which corresponds to the reference beam pointing
This is the tricky one because the beam has to pass
by F2CAL without truncation. It is for this reason that a bite has
been machined out of F2CAL.