# US 4,397,520 A · Afocal refractor telescopes > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,397,520 A - Document ID: US4397520A - Status: Granted - Kind: A - Application number: 06/237,981 - Filed: 1981-02-25 - Granted: 1983-08-09 - Earliest claimed: 1980-03-05 - Assignees: Barr & Stroud Limited - Inventors: Iain A. Neil - Web page: https://patentgrep.com/patent/US4397520A ## CPC - G02B 1/00: Optical elements characterised by the material of which they are made; Optical coatings for optical elements - G02B 13/14: for use with infrared or ultraviolet radiation ({G02B13/008, } G02B13/16 take precedence) - G02B 13/16: for use in conjunction with image converters or intensifiers {, or for use with projectors, e.g. objectives for projection TV} - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS ## 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. ## Claims ### Claim 1 An afocal refractor telescope formed by a fixed focus achromatic objective system and a fixed focus eyepiece system aligned on a common optical axis and arranged to provide an internal real image, said objective system being formed by two lens elements and said eyepiece system being formed by two lens elements, each of the four lens elements being made of a material which has a useful spectral bandpass in the infrared wavelength region, and having refractive surfaces intercepting said optical axis which are substantially spherical, that one of the objective system lens elements which is proximal the eyepiece system being colour corrective, with a V-value of not less than 120, negatively powered, and having a lower refractive index than the other objective system lens element which is positively powered. ### Claim 2 (depends on 1) A telescope as claimed in claim 1, wherein said other objective lens element has a refractive index at a temperature of 20.degree. C. of not less than 4.0. ### Claim 3 (depends on 1) A telescope as claimed in claim 1, wherein said one objective system lens element is of low optical power and is mounted for movement along the optical axis, each of the other lens elements being fixedly mounted, and means are provided for moving said one objective system lens element whereby the telescope can be compensated against thermal variations or adjusted for focus. ### Claim 4 (depends on 1) A telescope as claimed in claim 1, wherein said one objective system lens element at a temperature of 20.degree. C. has a refractive index at a wavelength of 10 microns not less than 2.45, each of the other lens elements having a refractive index at a temperature of 20.degree. C. not less than 4.0. ### Claim 5 (depends on 1) A telescope as claimed in claim 1, wherein one or more of said refracting surfaces has an anti-reflection coating. ### Claim 6 (depends on 5) A telescope as claimed in claim 5, wherein said one objective system lens element is made of Chalcogenide glass and each of the other lens elements is made of germanium and at a wavelength of 10 microns the telescope transmits at least 60% of the incident radiation. ### Claim 7 (depends on 1) A telescope as claimed in claim 1, wherein the radius of curvature of each refractive surface is given together with the aperture diameter of each surface and of the telescope exit pupil the position of which pupil is used as a datum from which the separation of successive refractive surfaces is defined, together with the nature of the material relevant to each such separation interval in the following table: ______________________________________ Sur- Radius of Aperture Item face Separation Curvature Material Diameter ______________________________________ Exit -- -- Flat Air 15.50 Pupil 1st surface 36.00 -169.06 Air 46.31 1st lens element 2nd surface 5.74 -77.22 Ge 47.77 1st surface 7.60 44.96 Air 42.83 2nd lens element 2nd surface 20.20 32.77 Ge 27.37 1st surface 207.56K -549.66K Air 128.30K 3rd lens element 2nd surface 12.50K -1,440.92K BSl 133.76K 1st surface 116.20K -452.37K Air 226.38K 4th lens element 2nd surface 18.00K -303.95K Ge 232.22K ______________________________________ wherein numerical data is given in millimeters and negative radius of curvature denotes that the center of curvature is on the exit pupil side of the surface whereas positive radius of curvature denotes that the center of curvature is on the side of the surface remote from the exit pupil, and K is a constant scale factor which is preselected in numerical value within the range 0.40 to 2.25. ### Claim 8 (depends on 1) A telescope as claimed in claim 1, wherein the radius of curvature of each refractive surface is given together with the aperture diameter of each surface and of the telescope exit pupil the position of which pupil is used as a datum from which the separation of successive refractive surfaces is defined, together with the nature of the material relevant to each such separation interval in the following table: ______________________________________ Sur- Radius of Aperture Item face Separation Curvature Material Diameter ______________________________________ Exit -- -- Flat Air 11.30 Pupil 1st surface 23.66 -76.99 Air 43.79 1st lens element 2nd surface 9.85 -51.00 Ge 49.06 1st surface 1.05 49.89 Air 47.91 2nd lens element 2nd surface 19.86 42.57 Ge 33.75 1st surface 167.03K -549.66K Air 98.12K 3rd lens element 2nd surface 16.00K -2,921.41K BSl 103.86K 1st surface 125.11K -369.82K Air 193.92K 4th lens element 2nd surface 19.65K -261.56K Ge 201.40K ______________________________________ wherein numerical data is given in millimeters and negative radius of curvature denotes that the center of curvature is on the exit pupil side of the surface whereas positive radius of curvature denotes that the center of curvature is on the side of the surface remote from the exit pupil, and K is a constant scale factor which is preselected in numerical value within the range 0.55 to 2.65.