Directional electron response function of SEPT. (a): Simplified cross-section of the SEPT telescope, where the origin of the normalized coordinate system in panel (b) is represented by the intersection of the two lines that connect the detector’s edges to the edges of the physical opening. (b) and (c): Directional response of one of the electron telescopes of STEREO/SEPT to isotropically distributed incident electrons with primary energy between 65 and 105 keV in a coordinate system using normalized axes. The response extends over the physical opening cone of the instrument, which is indicated by the red lines in (a), the red cone in (b), or the red circle in (c). The black lines in (b) and (c) represent the spherical grid for which the directional response factors are given.