Afocal refractor telescopes

Claim 2

An afocal refractor telescope formed by an achromatic telephoto 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 a primary lens element and a secondary lens element and said eyepiece being formed by at least two lens elements, each of the lens elements of the telescope being made of a material having a useful spectral bandpass in the infrared wavelength region and having refractive surfaces intercepting said optical axis, at least one of the refractive surfaces of said primary objective lens elements being aspheric and each of the refractive surfaces of the other lens elements of the telescope being substantially spherical,the aspheric surface possessing only a small degree of asphericity with a maximum aspheric depth of the same order as the wavelengths of said spectral bandpass, the secondary objective lens element being negatively powered and having a refractive index equal to or lower than the refractive index of the primary objective lens element which is positively powered, the telescope having an internal f-number in the airspace between the primary and secondary objective lens elements of the order of 1.0.

Claim 3

An afocal refractor telescope as claimed in claim 1, wherein each of the refractive surfaces of said primary objective lens element is aspheric.

Claim 4

An afocal refractor telescope as claimed in claim 1, wherein each aspheric refractive surface conforms to the equation ZC=+1-√1-C(CH.sup.2 +BH.sup.4 +GH.sup.6) where Z=distance along the optical axis C=inverse of the radius of curvature of a spherical datum surface H=radial distance perpendicular to optical axis B=first order aspheric coefficient G=second order aspheric coefficient.

Claim 5

An afocal refractor telescope as claimed in claim 1, wherein the secondary objective system lens element has a refractive index equal to or less than the refractive index of each of the other lens element of the telescope.

Claim 6

An afocal refractor telescope as claimed in claim 1, wherein the dispersive V-value of the secondary objective system lens element is less than the dispersive V-value of the primary objective system lens element.

Claim 7

An afocal refractor telescope as claimed in claim 5, wherein the secondary objective system lens element is mounted for movement along the optical axis.

Claim 8

An afocal refractor telescope as claimed in claim 1, wherein the lens elements of said telescope have refractive surface curvatures, separation distances between refractive surfaces (RS) as measured successively from the refractive surface of the eyepiece system furthest from said objective system, and materials as set forth in the following table: ______________________________________ Sep- aration Radius of Aperture Distance Curvature Diameter Item Surface (mm) (mm) Material (mm) ______________________________________ Eyepiece System 1st lens 1st RS 25.34 -199.20 Air 37.61 element 2nd RS 4.25 -66.25 Ge 38.41 2nd lens 1st RS 0.50 29.36 Air 34.42 element 2nd RS 11.94 22.88 Ge 23.29 Objective System Second- 1st RS 82.32 -300.77 Air 72.09 ary lens 2nd RS 7.50 -1799.70 As/Se/ 76.40 element Ge(BS1) Primary 1st RS 59.07 Aspheric Air 141.37 lens 2nd RS 15.71 -141.40 Ge 148.11 element ______________________________________ said 1st refractive surface of said primary objective system lens element having an aspheric profile in accordance with the following table: ______________________________________ Radial Distance Perpendicular Depth of Aspheric to Optical Axis (mm) Profile (microns) ______________________________________ 0.00 0.00 2.83 -0.03 5.65 -0.10 8.48 -0.22 11.31 -0.39 14.14 -0.59 16.96 -0.82 19.79 -1.07 22.62 -1.33 25.45 -1.58 28.27 -1.83 31.10 -2.05 33.93 -2.23 36.76 -2.36 39.58 -2.44 42.41 -2.45 45.24 -2.39 48.06 -2.26 50.89 -2.06 53.72 -1.79 56.55 -1.47 59.37 -1.11 62.20 -0.75 65.03 -0.41 67.86 -0.14 70.68 0.00 ______________________________________ where the depth of aspheric profile is the separation distance between points on the aspheric profile and the best fit sphere (which has a radius of curvature of -204.7094 mm) at equal radial distances perpendicular to the optical axis.

Claim 9

An afocal refractor telescope as claimed in claim 1, wherein the lens elements of said telescope have refractive surface curvatures, separation distances between refractive surfaces (RS) as measured from the refractive surface of the eyepiece system furthest from said objective system, and materials as set forth in the following table: ______________________________________ Sep- aration Radius of Aperture Distance Curvature Diameter Item Surface (mm) (mm) Material (mm) ______________________________________ Eyepiece System 1st lens 1st RS 19.52 -199.20 Air 32.80 element 2nd RS 4.18 -100.08 Ge 33.92 2nd lens 1st RS 8.50 -476.45 Air 36.54 element 2nd RS 3.75 -110.89 Ge 36.87 3rd lens 1st RS 0.50 30.79 Air 33.16 element 2nd RS 12.77 23.01 Ge 21.71 Objective System Second- 1st RS 74.06 -286.13 Air 66.69 ary lens 2nd RS 7.50 -7621.95 As/Se/ 70.97 element Ge(BS1) Primary 1st RS 61.95 Aspheric Air 137.91 lens 2nd RS 15.71 -135.64 Ge 144.82 element ______________________________________ said 1st refractive surface of said primary objective system lens element having an aspheric profile in accordance with the following table: ______________________________________ Radial Distance Perpendicular to Optical Axis Depth of Aspheric Profile (mm) (microns) ______________________________________ 0.00 0.00 2.76 -0.01 5.52 -0.05 8.27 -0.12 11.03 -0.20 13.79 -0.31 16.55 -0.42 19.31 -0.54 22.07 -0.65 24.82 -0.76 27.58 -0.85 30.34 -0.91 33.10 -0.95 35.86 -0.95 38.61 -0.91 41.37 -0.83 44.13 -0.72 46.89 -0.58 49.65 -0.42 52.41 -0.27 55.16 -0.13 57.92 -0.05 60.68 -0.07 63.44 -0.24 66.20 -0.61 68.95 -1.26 ______________________________________ where the depth of aspheric profile is the separation distance between points on the aspheric profile and the best fit sphere (which has a radius of curvature of -193.84 mm) at equal radial distances perpendicular to the optical axis.