  The Solar Electron and Proton Telescope (SEPT) aboard the Solar Terrestrial
Relations Observatory (STEREO) is designed to provide the three-dimensional
distribution of energetic electrons and protons with good energy and time
resolution. Each SEPT instrument consists of two double-ended magnet/foil
particle telescopes which cleanly separate and measure electrons in the energy
range from 30 keV to 400 keV and protons from 60 keV to 7000 keV. Anisotropy
information on a non spinning spacecraft is provided by two separate but
identical instruments: SEPT-E aligned along the Parker spiral magnetic field in
the ecliptic plane along looking both towards and away from the Sun, and
SEPT-NS aligned vertical to the ecliptic plane looking towards North and South.
The dual set-up refers to two adjacent sensor apertures for each of the four
viewing directions SUN, ANTISUN, NORTH, and SOUTH: one for protons, one for
electrons. In this contribution a simulation of SEPT utilizing the GEANT4
toolkit has been set up with an extended instrument model in order to calculate
improved response functions of the four different telescopes. This will help to
understand and correct instrumental effects in the measurements.
