Afocal zoom refractor telescopes

Claim 2

An afocal zoom refractor telescope formed by a variable magnification 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 a primary lens element and three other lens elements and said eyepiece system being formed by two lens elements, each of the six 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 other objective system lens elements which is proximal the primary lens element being colour corrective, with a V-value of not less than 120, negatively powered, having a lower refractive index than the remaining objective system lens elements, meniscus in shape and convex towards the real image and is fixedly coupled to the adjacent other objective system lens element which is meniscus in shape and concave towards the real image, said coupled lens elements having substantially zero separation between their adjoining refractive surfaces on said common optical axis, said separation increasing as distance off-axis increases and being mounted for movement in a first locus along the optical axis, the objective system lens element which is proximal the eyepiece system is mounted for movement in a second locus along the optical axis, and means are provided for simultaneously moving said lens elements non-linearly through said first and second loci whereby the magnification of said afocal zoom refractor telescope can be varied between minimum and maximum values.

Claim 3

A telescope as claimed in claim 1, wherein said one other objective system lens element is made of Chalcogenide glass, the remaining lens elements of the telescope being made of Germanium.

Claim 4

A telescope as claimed in claim 2, wherein said Chalcogenide glass is BS1.

Claim 5

A telescope as claimed in claim 1, wherein said one other objective system lens element has a refractive index at a wavelength of 10 microns and a temperature of 20° C. of not less than 2.45, each of the remaining lens elements of the telescope being made of a material having a refractive index at a wavelength of 10 microns and a temperature of 20° C. of not less than 4.0.

Claim 6

A telescope as claimed in claim 1, wherein the f/number at the internal real image is 2.51 throughout the magnification range between maximum and minimum values.

Claim 7

A telescope as claimed in claim 5, wherein said second locus is limited in extent to prevent said objective system lens element which is proximal the eyepiece system moving through the internal real image.

Claim 8

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 for the magnification value concerned, together with the nature of the material relevant to each such separation interval, in the following table: __________________________________________________________________________ Magnification Radius of Aperture Item Surface Separation Factor Curvature Material Diameter __________________________________________________________________________ Exit .0. 0 any Flat Air 15.30 Pupil Eyepiece 1st 33.29 any -64.39 Air 43.00 system refractive surface 1st lens 2nd 5.00 any -52.00 Ge 44.60 element refractive surface Eyepiece 1st 17.50 any 66.72 Air 48.30 system refractive surface 2nd lens 2nd 33.32 any 45.97 Ge 31.50 element refractive surface Objective 1st 66.00 X3.1 system refractive 35.42 X7.0 -117.96 Air 73.50 1st lens surface 16.93 X9.7 element 2nd 10.02 any -79.60 Ge 77.00 refractive surface Objective 1st 240.46 X3.1 system refractive 193.53 X7.0 -72.30 Air 90.00 2nd lens surface 210.27 X9.7 element 2nd 8.00 any -82.25 Ge 98.00 refractive surface Objective 1st 0.50 any 1301.07 Air 106.50 system refractive surface 3rd lens 2nd 15.30 any 843.67 BS1 109.00 element refractive surface Objective 1st 30.60 X3.1 system refractive 108.12 X7.0 -334.75 Air 168.00 Primary surface 109.86 X9.7 (4th) lens 2nd 16.00 any -249.09 Ge 174.00 element refractive surface __________________________________________________________________________ wherein numerical data is given in millimeters and negative radius of curvature denotes that the centre of curvature is on the exit pupil side of the surface whereas positive radius of curvature denotes that the centre of curvature is on the side of the surface remote from the exit pupil.