Some years ago QinetiQ introduced a short-range reconnaissance unmanned air vehicle (UAV), known as OBSERVER, which carried a visible three-camera sensor. To increase its versatility, a compatible infrared (IR) thermal imaging (TI) sensor was developed for the vehicle for operation in the 8-12mm waveband with a dual field of view function. The sensor incorporates a specially designed camera board, employing two IR lead scandium tantalate (PST) detectors based on UK un-cooled TI technology. Since no cooling engine is required for the detectors, the sensor module is very lightweight and hence well suited to its UAV application. So as to achieve the minimum possible payload for the vehicle, in addition to the lightweight detectors and electronics board, compact low mass optical solutions were devised for the camera objectives. These functioned at a relative aperture of f/1.0 and were designed to provide stable focus and imaging performance over a comparatively large temperature span (-10°C to + 50°C) to enable all weather operation. In order to achieve an athermalisation scheme devoid of elaborate electro-mechanical drives, thermally passive solutions were developed for the objectives in which the differing thermal characteristics of the components were designed to self-cancel optically. In this paper, the design and performance limitations of the optics are discussed and the procedure employed for establishing a thin lens pre-design for one of the objectives is described.
In high performance optical systems, a frequent requirement is 'athermalization' or the stabilization of optical performance with environmental temperature. This is particularly relevant in the case of refracting infrared systems for which the variations of refractive index as a function of temperature are relatively large. In this paper, as an example of an athermalization problem, the approach adopted for the stabilization of an infrared multi- magnification 'zoom' telescope objective is reviewed and the mechanism devised for introducing the necessary adjustments is described. Finally, the variation in performance of an actual athermalized system, following heat soak over a temperature range of 130 degrees Celsius, is discussed. The telescope, which was designed for use in the 8.0 micrometer to 13.0 micrometer waveband, provides a set of four fixed magnifications ranging from X3.5 to X20. It employs germanium and zinc selenide as refracting materials.