incident ENA. If the ENA strikes the SSD instead, the secondary electrons ejected by the impact are steered to the "coincidence" MCP, which provides the TOF stop signal; the position of impact is registered by the SSD. The start and stop signals are processed by the analog ToF electronics in the MEU and digitized for input into the DPU. The start and stop pulses give the ENA's time of flight, while the position measurements reveal its trajectory and thus its path length within the sensor. With these two pieces of information, time of flight and path length, HENA can calculate the ENA's velocity. The energy of the incident ENAs is measured with the SSD. When an ENA strikes the SSD, it generates a current pulse. The amplitude of this pulse (the pulse height) is directly proportional to the amount of energy that the ENA deposits in the SSD crystal. Thus, by analyzing the pulse height, HENA can determine the energy of an ENA incident on the SSD. And as mass is equal to the twice the energy divided by the velocity squared, once the energy and velocity of the ENA have been determined, its mass can be calculated. Calculating mass from the velocity and the SSD energy measurement is the primary technique used by HENA to determine composition of the ENAs. A second technique uses the pulse height of the MCP signal to distinguish between oxygen and hydrogen, the two most common neutral atoms expected in the magnetosphere. Low-Energy Neutral Atom Imager (LENA) The Low-Energy Neutral Atom (LENA) imager on IMAGE is one of three instruments designed to make observations of the Earth's magnetospheric environment using neutral atom imaging. The objectives of LENA are to: (1) measure neutrals without interference from electrons, ions or UV; (2) distinguish neutral protons from oxygen; (3) determine ion outflow on five minute time scales over broad range of local times; and, (4) measure energies as low as 10 eV with high counting statistics. The LENA instrument consists of a collimator, conversion unit, extraction lens, dispersive energy analyzer and time-of-flight mass analyzer with position-sensitive particle detection. Neutral particles enter the instrument through a collimator which filters charged particles. LENA converts neutrals to negative ions through a near specular glancing reflection from a tungsten surface. Negative ions from the surface are then collected by an extraction lens which focuses all negative ions with the same energy