Electrons are measured from 0.2 to 10 MeV, primarily providing time markers for injections of solar particles. The collection powers and energy ranges for heavier nuclei up to iron are ideal for observations of quiet-time populations such as particles accelerated by interplanetary shocks and the anomalous cosmic rays. The large collecting power available is ideal for observing 3He, 4He, and heavier nuclei in impulsive solar events. Finally, there is also a telescope designed to measure isotopes from He to Fe, which is intended for solar particles, anomalous cosmic rays, and galactic cosmic rays. The orbit is designed to make several passes through the magnetosphere and then move into a halo orbit around the earth-sun L1 Lagrange point.
STEP contains two identical telescopes.
STEP was designed and built by the University of Maryland. Each STEP telescope includes a start and stop microchannel plate detector as well as a solid-state detector to measure the total energy. It is subdivided into five 13.3 mm wide strips on each side. The primary purpose of the APE telescopes is to provide coverage of the lowest charge particles over a wide range of incident energies up to very high rates. The APE telescopes do ovserve heavy nuclei up through iron, however, and can resolve isotopes of the lighter elements without trajectory information.