be able to track and destroy its target without assistance from the gunner. This is done by coupling an onboard imaging IR system (separate from CLU imaging system) with an onboard tracking system. The gunner uses the CLU's IR system to find and identify the target, then switches to the missile's independent IR system to set a track box around the target and establish a lock. The gunner places brackets around the image for locking. The seeker stays focused on the target's image, continuing to track it as the target moves or the missile's flight path alters, or attack angles change. The seeker consists of three main components: focal plane array image sensor, cooling and calibration, and stabilization. Focal plane array (FPA) The seeker assembly is encased in a dome that is transparent to long-wave infrared radiation. The IR radiation passes through the dome and then through lenses that focus the energy. The IR energy is reflected by mirrors on to the FPA. The seeker is a two-dimensional staring FPA of 64×64 MerCad (HgCdTe) detector elements. The FPA processes the signals from the detectors and relays a signal to the missile's tracker. The staring array is a photo-voltaic device where the incident photons stimulate electrons and are stored, pixel by pixel, in readout integrated circuits attached at the rear of the detector. These electrons are converted to voltages that are multiplexed out of the ROIC on a frame-by-frame basis. Cooling/calibration To function effectively, the FPA must be cooled and calibrated. In other applications, a CLU's IR detectors are cooled using a Dewar flask and a closed-cycle Stirling engine, but there is insufficient space in the missile for a similar solution. Prior to launch, a cooler mounted on the outside of the launch tube activates the electrical systems in the missile and supplies cold gas from a Joule-Thomson expander to the missile detector assembly while the missile is still in the launch tube. When the missile is fired, this external connection is broken and coolant gas is supplied internally by an onboard argon gas bottle. The gas is held in a small bottle at high pressure and contains enough coolant for the duration of the flight of approximately 19 seconds. The seeker is calibrated using a chopper wheel. This device is a fan of six blades: five black blades with low IR emissivity and one semi-reflective blade. These blades spin in