Directional response function of a simple particle telescope. (a): cross-section of a simple particle telescope, where the origin of the normalized coordinate system in panel b is represented by the intersection of the two red lines. (b) and (c): general directional response function of a planar detector to isotropically distributed incident particles within the physical opening cone, which is indicated by the red lines in (a) or by the red cone in (b). The directional response function is given as the color-coded response factor for a spherical grid (black lines) in a coordinate system using normalized axes.