STEP Flight Model of EPD without MLI. Two similar sensor heads are installed on a common electronics box (EBox). The sensor heads each contain a segmented SSD detector, an ASIC, a pinhole, and support structures. The right sensor head also includes the magnet assembly for electron deflection. The blackened apertures serve for stray-light reduction and passive cooling. In order to maintain the SSDs inside the sensor heads at a sufficiently low temperature, the EBox has a poor thermal coupling to the sensor heads. Optical Solar Reflectors (OSRs) are installed on top of the sensor heads and in the front of the EBox for heat emission purposes. The EBox contains all the other electronics required to operate and run the unit.