Helicopter mounted optical systems require compact packaging, good image performance (approaching the diffraction-limit), and must survive and operate in a rugged shock and thermal environment. The always-present requirement for low weight in an airborne sensor is paramount when considering the optical configuration. In addition, the usual list of optical requirements which must be satisfied within narrow tolerances, including field-of-view, vignetting, boresight, stray light rejection, and transmittance drive the optical design. It must be determined early in the engineering process which internal optical alignment adjustment provisions must be included, which may be included, and which will have to be omitted, since adding alignment features often conflicts with the requirement for optical component stability during operation and of course adds weight. When the system is to be modular and mates with another optical system, a telescope designed by different contractor in this case, additional alignment requirements between the two systems must be specified and agreed upon. Final delivered cost is certainly critical and "touch labor" assembly time must be determined and controlled. A clear plan for the alignment and assembly steps must be devised before the optical design can even begin to ensure that an arrangement of optical components amenable to adjustment is reached. The optical specification document should be written contemporaneously with the alignment plan to insure compatibility.
The optics decisions that led to the success of this project are described and the final optical design is presented. A description of some unique pupil alignment adjustments, never performed by us in the infrared, is described.