Detector developments raise the possibility of achieving high resolution images, employing linear arrays of over 3000 pixels, in any of the atmospheric windows extending from the visual to the far infra-red. Simultaneous capture with fusion of the VNIR, MWIR & LWIR images requires a reflective optical system. Reasons are given for the choice of a 4 mirror anastigmatic telescope for the basic imaging system which is combined with another 4 mirror afocal telescope to cover a 60 degree field in wide angle mode. To permit ground based trials, a mirror scanner covers a 40 degree field in either mode. Choice of a telescope with projected entrance pupil was essential for a compact system. Focus and mode changes are all made by the movement of flat mirrors. A particular arrangement of these folds contributes towards a rational structure with the mirror groups mounted on optical axes perpendicular to a common base. The methods used to ensure dimensional stability including the design and mounting of the beamsplitters are described.