Afocal telescopes

Claim 2

An afocal telescope comprising an objective system and an eyepiece system having a common optical axis, the objective system being arranged to accept radiation in the infrared waveband from a distant scene and to form a real image thereof within the telescope the eyepiece system being arranged to receive radiation from said image and to provide at a real pupil a magnified view of the scene, wherein said objective system has an f-number of the order of 2.5 or less and said eyepiece system is formed by a triplet of three powered lens elements each being made of a material having a refractive index of the order of 2.4 or greater and a dispersive V-value of the order of 140 or greater, two of said eyepiece lens elements being of high positive power, the eyepiece lens element adjacent the objective system being of very low power due to its thickness and having a concave refractive surface towards the objective system and a convex refractive surface remote from the objective system, said convex surface being separated from the adjoining refractive surface of the central lens element of the triplet by an air space which in the axial direction is substantially zero on said axis and which progressively increases in magnitude as the distance off axis increases.

Claim 3

A telescope as claimed in claim 1, wherein the six refractive surfaces of the eyepiece system are free from aspherics.

Claim 4

A telescope as claimed in claim 1, wherein of the six refractive surfaces of the eyepiece system at least that one which is proximal the real pupil is coated with an antireflection coating.

Claim 5

An afocal telescope as claimed in claim 1, wherein the lens elements of said eyepiece system have refractive surface curvatures, separation distances between refractive surfaces (RS) as measured successively from the refractive surface proximal to said pupil, and materials as set forth in the following table: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperature Item face (mm) (mm) rial Diameter ______________________________________ Eyepiece System 1st 1st RS 23.86 -70.13 Air 37.34 lens 2nd RS 5.25 -47.97 Ge 39.73 element 2nd 1st RS 1.00 344.97 Air 40.19 lens 2nd RS 5.00 -482.88 Ge 39.92 element 3rd 1st RS 1.00 35.65 Air 36.44 lens 2nd RS 13.95 24.97 Ge 24.20 element ______________________________________

Claim 6

An afocal telescope as claimed in claim 1, wherein the lens elements of said eyepiece system have refractive surface curvatures, separation distances between refractive surfaces (RS) as measured successively from the refractive surface proximal to said pupil, and materials as set forth in the following table: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Eyepiece System 1st 1st RS 33.46 -60.72 Air 41.82 lens 2nd RS 5.25 -49.49 Ge 44.61 element 2nd 1st RS 1.00 250.34 Air 45.27 lens 2nd RS 4.75 -2087.02 Ge 44.87 element 1st 1st RS 1.00 38.68 Air 41.09 lens 2nd RS 14.70 27.57 Ge 27.81 element ______________________________________

Claim 7

An afocal telescope as claimed in claim 1, wherein said objective system is formed by a primary lens element and a secondary lens element and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperature Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 11.00 Eyepiece System 1st 1st RS 23.86 -70.13 Air 37.34 lens 2nd RS 5.25 -47.97 Ge 39.73 element 2nd 1st RS 1.00 344.97 Air 40.19 lens 2nd RS 5.00 -482.88 Ge 39.92 element 3rd 1st RS 1.00 35.65 Air 36.44 lens 2nd RS 13.95 24.97 Ge 24.20 element Objective System Secondary 1st RS 152.11 -610.07 Air 94.21 lens 2nd RS 9.00 -1432.15 (BSl) 97.48 element Primary 1st RS 47.29 -269.04 Air 127.69 lens 2nd RS 11.35 -184.85 Ge 131.52 element ______________________________________

Claim 8

An afocal telescope as claimed in claim 1, wherein said objective system comprises a high magnification system formed by a primary lens element and a secondary lens element, and a low magnification system formed by first second and third lens elements of which the first is proximal said secondary lens element, and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 11.0 Eyepiece System 1st lens 1st RS 23.86 -70.13 Air 37.34 element 2nd RS 5.25 -47.97 Ge 39.73 2nd lens 1st RS 1.00 344.97 Air 40.19 element 2nd RS 5.00 -482.88 Ge 39.92 3rd lens 1st RS 1.00 35.65 Air 36.44 element 2nd RS 13.95 24.97 Ge 24.20 Objective System Low Magnification System 3rd lens 1st RS 40.00 Flat Air 34.22 element 2nd RS 3.50 594.41 Ge 34.68 2nd lens 1st RS 5.25 -61.88 Air 36.30 element 2nd RS 4.80 -41.62 Ge 38.15 1st lens 1st RS 71.313 179.52 Air 42.45 element 2nd RS 4.75 121.31 Ge 42.08 Objective System High Magnification System Secondary 1st RS 22.50 -610.07 Air 59.66 lens 2nd RS 9.00 -1432.15 (BSl) 63.32 element Primary 1st RS 47.29 -269.04 Air 105.39 lens 2nd RS 11.35 -184.85 Ge 109.80 element ______________________________________

Claim 9

An afocal telescope as claimed in claim 1, wherein said objective system is formed by a primary lens element and a secondary lens element and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 11.00 Eyepiece System 1st 1st RS 23.86 -70.13 Air 37.34 lens 2nd RS 5.25 -47.97 Ge 39.73 element 2nd 1st RS 1.00 344.97 Air 40.19 lens 2nd RS 5.00 -482.88 Ge 39.92 element 3rd 1st RS 1.00 35.65 Air 36.44 lens 2nd RS 13.95 24.97 Ge 24.20 element Objective System Secondary 1st RS 152.11 -1108.90 Air 94.67 lens 2nd RS 7.50 -2232.29 Ge 96.36 element Primary 1st RS 48.79 -269.04 Air 127.41 lens 2nd RS 11.35 -184.85 Ge 131.24 element ______________________________________

Claim 10

An afocal telescope as claimed in claim 1, wherein said objective system comprises a high magnification system formed by a primary lens element and a secondary lens element, and a low magnification system formed by first second and third lens elements of which the first is proximal said secondary lens element, and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 11.00 Eyepiece System 1st lens 1st RS 23.86 -70.13 Air 37.34 element 2nd RS 5.25 -47.97 Ge 39.73 2nd lens 1st RS 1.00 344.97 Air 40.19 element 2nd RS 5.00 -482.88 Ge 39.92 3rd lens 1st RS 1.00 35.65 Air 36.44 element 2nd RS 13.95 24.97 Ge 24.20 Objective System Low Magnification System 3rd lens 1st RS 40.00 Flat Air 34.22 element 2nd RS 3.50 594.41 Ge 34.68 2nd 1st RS 5.25 -61.88 Air 36.30 element 2nd RS 4.80 -41.62 Ge 38.15 1st lens 1st RS 71.313 179.52 Air 42.45 element 2nd RS 4.75 121.31 Ge 42.08 Objective System High Magnification System Secondary 1st RS 22.50 -1108.90 Air 59.94 lens 2nd RS 7.50 -2232.29 Ge 61.82 element Primary 1st RS 48.79 -269.04 Air 105.19 lens 2nd RS 11.35 -184.85 Ge 109.60 element ______________________________________

Claim 11

An afocal telescope as claimed in claim 1, wherein said objective system is formed by a primary lens element and a secondary lens element and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 15.00 Eyepiece System 1st 1st RS 33.46 -60.72 Air 41.82 lens 2nd RS 5.25 -49.49 Ge 44.61 element 2nd 1st RS 1.00 250.34 Air 45.27 lens 2nd RS 4.75 -2087.25 Ge 44.87 element 3rd 1st RS 1.00 38.68 Air 41.09 lens 2nd RS 14.70 27.57 Ge 27.81 element Objective System Secondary 1st RS 156.23 -610.07 Air 93.00 lens 2nd RS 9.00 -1432.15 (BSl) 96.22 element Primary 1st RS 47.29 -269.04 Air 126.59 lens 2nd RS 11.35 -184.85 Ge 130.66 element ______________________________________

Claim 12

An afocal telescope as claimed in claim 1, wherein said objective system comprises a high magnification system formed by a primary lens element and a secondary lens element, and a low magnification system formed by first second and third lens elements of which the first is proximal said secondary lens element, and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- .0. Flat Air 15.00 Eyepiece System 1st lens 1st RS 33.46 -60.72 Air 41.82 element 2nd RS 5.25 -49.49 Ge 44.61 2nd lens 1st RS 1.00 250.34 Air 45.27 element 2nd RS 4.75 -2087.25 Ge 44.87 3rd lens 1st RS 1.00 38.68 Air 41.09 element 2nd RS 14.70 25.57 Ge 27.80 Objective System Low Magnification System 3rd lens 1st RS 44.53 Flat Air 35.06 element 2nd RS 3.50 594.41 Ge 35.53 2nd lens 1st RS 5.25 -61.88 Air 37.08 element 2nd RS 4.80 -41.62 Ge 38.92 1st lens 1st RS 71.31 179.52 Air 42.94 element 2nd RS 4.75 121.31 Ge 42.58 Objective System High Magnification System Secondary 1st RS 22.09 -610.07 Air 60.27 lens 2nd RS 9.00 -1432.15 (BSl) 64.02 element Primary 1st RS 47.29 -269.04 Air 107.19 lens 2nd RS 11.35 -184.85 Ge 111.64 element ______________________________________

Claim 13

An afocal telescope as claimed in claim 1, wherein said objective system is formed by a primary lens element and a secondary lens element and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 15.00 Eyepiece System 1st 1st RS 33.46 -60.72 Air 41.82 lens 2nd RS 5.25 -49.49 Ge 44.61 element 2nd 1st RS 1.00 250.34 Air 45.27 lens 2nd RS 4.75 -2087.25 Ge 44.87 element 3rd 1st RS 1.00 38.68 Air 41.09 lens 2nd RS 14.70 27.57 Ge 27.81 element Objective System Secondary 1st RS 156.23 -1108.90 Air 93.47 lens 2nd RS 7.50 -2232.29 Ge 95.11 element Primary 1st RS 48.79 -269.04 Air 126.34 lens 2nd RS 11.35 -184.85 Ge 130.41 element ______________________________________

Claim 14

An afocal telescope as claimed in claim 1, wherein said objective system comprises a high magnification system formed by a primary lens element and a secondary lens element, and a low magnification system formed by first second and third lens elements of which the first is proximal said secondary lens element, and the refractive surface curvatures, separation distances and materials of the telescope are as set forth in the following table, separation distances being measured successively from said pupil: ______________________________________ Separ- ation Radius of Sur- Distance Curvature Mate- Aperture Item face (mm) (mm) rial Diameter ______________________________________ Pupil -- 0 Flat Air 15.00 Eyepiece System 1st lens 1st RS 33.46 -60.72 Air 41.82 element 2nd RS 5.25 -49.49 Ge 44.61 2nd lens 1st RS 1.00 250.34 Air 45.27 element 2nd RS 4.75 -2087.25 Ge 44.87 3rd lens 1st RS 1.00 38.68 Air 41.09 element 2nd RS 14.70 27.57 Ge 27.80 Objective System Low Magnification System 3rd lens 1st RS 44.53 Flat Air 35.06 element 2nd RS 3.50 594.41 Ge 35.53 2nd lens 1st RS 5.25 -61.88 Air 37.08 element 2nd RS 4.80 -41.62 Ge 38.92 1st lens 1st RS 71.31 179.52 Air 42.94 element 2nd RS 4.75 121.31 Ge 42.58 Objective System High Magnification System Secondary 1st RS 22.09 -1108.90 Air 60.57 lens 2nd RS 7.50 -2232.29 Ge 62.49 element Primary 1st RS 48.79 -269.04 Air 107.01 lens 2nd RS 11.35 -184.85 Ge 111.46 element ______________________________________