Infrared afocal refractor telescope

An infrared afocal refractor telescope comprising a zoom system, a collecting system and an eyepiece system aligned on a common optical axis, the zoom system being arranged to accept from object space radiation in the infrared waveband, the collecting system being arranged to form a real image from radiation delivered thereto by said zoom system, and the eyepiece system being arranged to deliver radiation from said image to a pupil in image space wherein said collecting system is formed by a single lens component which is fixedly located on said optical axis and said zoom system is formed by an objective lens component fixedly located on said optical axis and by first and second other lens components mounted on respective carriages and separately selectively positionable along said optical axis between said objective lens component and said single lens component, said first and second other lens components each having the same sign of optical power, said objective lens component having positive optical power, the variation in telescope magnification provided by selective positioning of said first and second other lens components being at least 5:1 in range, the locus of movement of said first and said second other lens component being substantially free of curl back, the arrangement being such that relatively small positional adjustments of said first and second other lens components renders the telescope athermalised.