Abstract: An afocal dual magnification refractor telescope is formed by a fixed focus achromatic telephoto objective system (21) formed by a primary objective lens element (H) and a secondary objective lens element (G) aligned on a common optical axis (19) with a fixed focus collimation system (22) and interchangeable high and low magnification lens systems (23,24) each of which is arranged to provide an internal real image (25,26). The high magnification lens system (23) is formed by two lens elements (B,C) and the low magnification lens system (24) is formed by three lens elements (D,E,F). The eight lens elements (A,B,C,D,E,F,G and H) are made of materials which have a useful spectral bandpass in the infrared wavelength region and the refractive surfaces of these lens elements which intercept the optical axis (19) are substantially spherical except for one or both refractive surfaces (15,16) of the primary objective lens element (H).

Abstract: An afocal zoom refractor telescope 20 is formed by a variable magnification achromatic objective system 18 and a fixed focus eyepiece system 19 aligned on a common optical axis 17 and arranged to provide an internal real image I. The objective system 18 is formed by a primary lens element F and three other lens elements C,D,E, and the eyepiece system 19 is formed by two lens elements A,B. The six lens elements A-F are made of a material which has a useful spectral bandpass in the infrared wavelength region and all refractive surfaces (1-12) intercepting the optical axis 17 are substantially spherical. Objective lens system element E which is proximal the primary lens element F is color corrective with a V-value of not less than 120, negatively powered and with a lower refractive index than the remaining objective lens elements (C,D,F). Element E is fixedly coupled to the adjacent lens element D the pair being mounted for movement in a first locus along optical axis 17.

Abstract: An eyepiece or collimation lens system (22) is formed by three optically-powered lens elements (A, B, C) aligned on a common optical axis (23) and arranged to accept radiation in the infrared wavelength region from a real image (I) and provide a bundle of parallel rays at an exit pupil (.0.), each of the lens elements (A, B, C) is positively powered and made of a material which has a useful spectral bandpass in the infrared wavelength region and the refractive surface (6) of lens element (C) which lies adjacent the real image (I) is flat whereas the other refractive surfaces, (1, 2, 3, 4, 5) of the lens elements (A, B, C) are substantially spherical. A graticule may be mounted on or adjacent the flat refractive surface (6) and each refractive surface may be anti-reflection coated. Conveniently lens element (c) is made of zinc selenide whereas lens elements (A, B) are each made of germanium.

Abstract: An afocal refractor telescope (10) is formed by a fixed focus achromatic objective system (11) and a fixed focus eyepiece system (12) aligned on a common optical axis (13), the objective system (11) being formed by two lens elements (C, D) and the eyepiece system (12) being formed by two lens elements (A, B), each of the four lens elements (A, B, C, D) being made of a material which has a useful spectral bandpass in the infrared wavelength region, and having refractive surfaces (1,2,3,4,5,6,7,8) intercepting the optical axis (13) which are substantially spherical, lens element (c), which is proximal the eyepiece system (12) being color corrective, with a V-value not less than 120, negatively powered and having a lower refractive index than lens element (D) which is positively powered. Conveniently lens element (C) is made of Chalcogenide glass and each of lens elements (A, B and D) is made of germanium.

Abstract: An infrared radiation detecting system comprises a detector element 13A forming part of a detector 13 incorporating a cold shield 14 and an optical system 12, 15, for imaging infrared radiation from a field of view O onto a real image surface 19 spaced from the detector element 13A and relaying the image I from the surface 19 to the detector element 13A is provided with a graticule 20 having markings which are reflective to infrared radiation emitted by the detector 13. Graticule 20 is located at the image surface 19 so that the graticule markings are imaged onto the detector element 13A in superimposition with the infrared radiation from the field of view O.