The development of a compact, low cost, high performance thermal imager is described. This imager is based on a novel coaxial scanning technique designed at RSRE. The paper reviews the essential design requirements for thermal imagers. These include the avoidance of Vignetting and pupil wander in the scanning elements and objective lenses, and the maximising of detector cold shielding. These aims are met by (a) choice of array technology, as exemplified in the UK development of SPRITE IR detectors, (b) use of either the pupil relay optics (as in the UK TICM II Scanner) or the alternative coincident pupils (as in the coaxial scanner). The optical design of the scanner and its implementation in hardware are given along with details of the telescopes, detectors and electronic modules used in the coaxial imager. Direct View and TV compatible outputs are discussed. The paper concludes with examples of the imager in various military applications including RPV's, airborne pods, helicopters and land based sights. Examples of the imagery obtained and the thermal performance of the imager are also described.