Transforming the directional response function to μ-space. Panel a: physical opening cone (gray cone) of a particle telescope allows incoming particle trajectories originating over a spherical cap of a unit sphere. The top of each trajectory is color coded according to the corresponding μ-value which is determined for the magnetic field vector (black arrow). Panel b: top view of panel a. The black dot indicates the direction of the magnetic field vector. Panel c: The corresponding directional response function in μ (the cosine of the pitch angle) space.