Dual band/dual FOV infrared telescope

Abstract: A dual field of view telescope for operating in both the medium infrared spectral band from 3.8 to 5 micrometers and the long infrared spectral band from 8 to 11.5 micrometers. The telescope includes an objective lens group A and an eyepiece lens group B. All of the lenses of each group are made of zinc sulfide, germanium or nonoxide chalcogenide glass. One of the lenses in the objective lens group is a negative germanium lens A3 which serves to correct for residual axial chromatic aberrations in both IR bands due to its different dispersion characteristics in these two bands.

Description: BACKGROUND 1. Technical Field This invention relates to optical devices, and more particularly, to telescopes capable of operating in the infrared frequency spectrum. 2. Discussion There is a need for telescopes capable of operating over a wide band of infrared radiation. Two of the bands of particular interest are the medium infrared (IR) band having wavelengths of from 3.8 to 5.0 micrometers and the long IR band having wavelengths of from 8 to 11.5 micrometers. These two bands correspond to regions of high transmittance within the atmosphere. Such telescopes can be used with a forward-looking infrared (FLIR) scanner and imager systems. Other applications of such a telescope would include infrared search-track (IRST) sensors and dual-band, target-recognition systems. Previously known dual-band or wide-band infrared telescope designs typically used toxic materials such as KRS-5 and AS₂S₃ or exotic materials such as potassium bromide, CdTe, GaAs or CsI. It was heretofore believed that it was necessary to employ these hard-to-use materials in order to correct chromatic aberrations in both spectral bands. From the EP-A-171 903 a telescope is known operating in the medium and the long infrared spectral band. The telescope described consists of an objective lens group and an eyepiece lens group being disposed along a common optical axis. The materials of the lenses for each group are selected from zinc sulfide, zinc selenide, and germanium. According to this prior art approach, the objective lens group consists of a front triplet and a rear triplet, wherein one of the lenses of the rear triplet is a negative germanium lens. In the view of the above it is the object of the present invention to provide for a telescope operating in both, the medium infrared spectral band and the long infrared spectral band which is able to correct the axial chromatic aberrations in said both IR-bands to very small residual values, the optical elements of which are easy to fabricate. This object is solved by a telescope in accordance with claim 1. In more detail, a telescope is provided operating in both the medium infrared spectral band having radiation with wavelengths from 3.8 to 5 micrometers and the long infrared spectral band having radiation wavelengths from 8 to 11.5 micrometers, said telescope including an eyepiece lens group of chalcogenide lenses disposed along an optical axis. Further, an objective lens group is provided consisting of a front doublet and a rear doublet which are disposed along said optical axis, wherein one of the lenses of the rear doublet include a negative Ge lens. The lenses of each group being selected from ZnS, Ge and nonoxide chalcogenide glass. The term "nonoxide chalcogenide glass" as used in the present invention has to be understood as being an infrared transmitting glass formed from a first constituent from group IV-A, a second constituent from group V-A, and a third constituent form group VI. The above lens materials are much more readily coated and easily fabricated into optical elements than the other materials noted above. Germanium has unusual dispersion characteristics in the two different spectral bands. It is dispersive in the middle IR band but not dispersive in the long IR band. As a result, it serves to correct the axial chromatic aberrations in both IR bands to very small residual values. In the preferred embodiment, a dual field of view (FOV) telescope design is used to provide both narrow field of view (NFOV) and wide field of view (WFOV) modes of operation. The lenses in the field switch group are also preferably selected from the three previously identified materials. BRIEF DESCRIPTION OF THE DRAWINGS The various advantages of the present invention will become apparent to one skilled in the art after studying the following specification and by reviewing the drawings in which: FIG. 1 is an optical diagram of a telescope made in accordance with the teachings of the present invention; and FIG. 2 is an optical diagram illustrating lenses of a field switch group being inserted into the optical path of the telescope of FIG. 1 to provide it with a wider field of view. DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, an afocal telescope capable of operating in the middle and long infrared bands is disclosed. As noted above, the middle or medium infrared band has radiation with wavelengths of from 3.8 to 5.0 micrometers. The long infrared band has radiation with wavelengths from 8-11.5 micrometers. The telescope of FIG. 1 includes two coacting groups of lenses located on the optical axis O of the telescope; an objective lens group A and an eyepiece lens group B. The objective lens group A serves to generate an intermediate image in front of the eyepiece group B which recollimates the infrared radiation. All of the lenses in both groups are made of materials which are easy to fabricate and use when compared with the types of materials previously employed in wide IR band telescopes. The lenses are either nonoxide chalcogenide glass, zinc sulfide or germanium. Although a variety of optical designs for the objective lens group A may be used, it appears necessary to employ a negative germanium lens in a rear doublet. The preferred design, as illustrated, provides excellent performance, with virtually no axial chromatic aberration in both spectral bands. In addition, it can be folded to occupy a small volume since it is in a refractive design form. In the preferred embodiment, objective lens group A consists of four elements: Al, A2, A3 and A4. Lenses A1 and A2 comprise a front doublet while lenses A3 and A4 comprise a rear doublet. Lens A1 is a positive lens made of nonoxide chalcogenide glass. For purposes of this invention, the term nonoxide chalcogenide glass" means an infrared transmitting glass formed from three element systems; one constituent from group IV-A (Si or Ge), a second constituent from group V-A (P, As or Sb) and third element, a chalcogenide from group VI (S, Se or Te) of the periodic table of elements. Examples of suitable nonoxide chalcogenide glasses are those commercially available from Amorphous Materials, Inc. of Garland, Texas under the trademark AMTIR-1 which is a Ge-As-Se infrared transmitting glass. The general properties of AMTIR-1 are set forth in the following Table I. Other suitable nonoxide chalcogenide glasses are TI20 (Ge-As-Se glass) and TI1173 (Ge-Sb-Se glass) available from Texas Instruments of Dallas, Texas and AMTIR-2 (Ge-Sb-Se glass) available from Amorphous Materials of Garland, Texas. Lens A2 is a negative lens made of zinc sulfide. The purpose of lens A2 is to act as a dispersive element which partially corrects the chromatic aberrations introduced by the positive lens A1. In the rear doublet, lens A3 is a negative element made of germanium while the positive lens A4 is made of AMTIR-1. The general purpose of the rear doublet (lenses A3, A4) is to correct the residual axial chromatic aberrations introduced by the lenses A1, A2 and lenses B1 and B2. Germanium has a higher index of refraction value in the medium band than in the long infrared band and germanium has unusual dispersion characteristics in the medium and long infrared bands. It is dispersive in the medium band. Accordingly, lens A3 acts as a negative element with strong dispersion and high negative power in the medium band. However, it is not very dispersive in the long band and, accordingly, lens A3 acts as a negative element with weak dispersion and low negative power. Thus, germanium lens A3 serves to correct all of the residual axial chromatic aberrations to less than 30 microradians in object space in both bands in the narrow field of view mode of operation illustrated in FIG. 1. This results in diffraction-limited optical performance for a relatively large diameter pupil in both IR bands over the central portion of the field of view without refocusing. The telescope of FIG. 1 is fabricated in accordance with the optical prescription given in the following Table II. The surface numbers in this table are those shown in FIG. 1. The figures for the radii and all other magnitudes are given in inches. In order to obtain the figures in cm, each figure has to be multiplied with 2,54. The radius (RD) refers to the radius of curvature of the particular surface specified in accordance with conventional practice. The thickness (TH) refers to the distance along the optical axis O from the surface for which the thickness is stated to the next highest numbered surface. Thus, the thickness from surface 2 to surface 3 of lens A1 is 0.60 inch. All of the lens surfaces are spherical except for surface 2 of lens A1 which is a conic surface as defined in the table. The section labeled "Clear Apertures and Obstructions", according to conventional practice, refers to minimum clear apertures which is a parameter defining the diameter of the aperture which permits all radiation within the specified field of view to pass through the surface without vignetting. The other headings also refer to conventional design parameters and are well known to those skilled in the art. The telescope of FIG. 1 can be modified by inserting a field switch group C of lenses in the optical path O. The exact construction of the field switch lens group C is not critical to the practice of the broader teachings of this invention and can vary somewhat from that illustrated in FIG. 2. Whatever design is chosen, it should cooperate with the objective lens group A so that the focal point of the combination is less than that provided by the objective lens group A standing alone. A suitable field switch group positioner mechanism 20 serves to place lenses C1, C2 and C3 into the optical path O as shown in FIG. 2 during a wide field of view mode of operation. The positioner 20 is used to remove the field switch lens group C to obtain the narrow field of view mode of operation of FIG. 1. The field switch lens group C of the preferred embodiment includes a front doublet consisting of lenses C1 and C2. Lens C1 is a negative AMTIR-1 lens while lens C2 is a negative zinc sulfide lens. This doublet is disposed adjacent to the front doublet of lenses A1, A2 of the objective lens group A. A positive AMTIR-1 lens C3 with a conic rear surface is disposed in front of the doublet A3, A4. The combination of the field switch lens group C and objective lens group A defines a focal length of about 20,50 cm (8.07 inches) as compared to a focal length of about 62,87 cm (24.75) inches in the narrow field of view embodiment of FIG. 1. The optical characteristics of the thus described dual-band/dual-FOV telescope are listed in the following Table III. Those skilled in the art should now realize that the present invention provides significant advantages over the wide band IR telescopes known in the art. It should be further understood that while this invention was described in connection with a particular example, that no limitation is intended thereby except as defined in the following claims.

Claims: A telescope for operating in both the medium infrared spectral band having radiation with wavelengths from 3.8 to 5 micrometers and in the long infrared spectral band having radiation wavelengths from 8 to 11.5 micrometers, said telescope including an eyepiece lens group (B) disposed along an optical axis (O); an objective lens group (A) consisting of a front doublet (A1, A2) and a rear doublet (A3, A4) disposed along said optical axis (O), wherein one of the lenses of said rear doublet (A3, A4) include a negative Ge lens (A3), the rear doublet (A3, A4) correcting for residual axial chromatic aberrations in the telescope, said lenses (A1, A2, A3, A4, B1, B2) of each group (A, B) being selected from ZnS, Ge and nonoxide chalcogenide glass; and said nonoxide chalcogenide glass being an infrared transmitting glass formed from a first constituent from Group IV-A, a second constituent from Group V-A and a third constituent from Group VI. The telescope of Claim 1 wherein said negative Ge lens (A3) is adjacent to a positive chalcogenide lens (A4) to form said rear doublet (A3, A4) in said objective lens group (A). The telescope of Claim 1 or 2 wherein said front doublet (A1, A2) includes a zinc sulfide lens (A2) and a chalcogenide lens (A1). The telescope of anyone of the preceding Claims wherein said front (A1, A2) and rear (A3, A4) doublets cooperate to generate an intermediate image at a given focal length in front of said eyepiece lens group (B). The telescope of anyone of the preceding Claims which further comprises: means (20) for positioning a field switch lens group (C) on the optical axis (O) of the telescope, said field switch lens group (C) cooperating with the objective lens group (A) to reduce the focal length of the combined groups (A, C) thereby providing a telescope with a wider field of view. The telescope of Claim 5 wherein said field switch lens group (C) is disposed between the front (A1, A2) and rear (A3, A4) doublets of the objective lens group (A). The telescope of Claim 5 or 6 wherein said field switch lens group (C) comprises at least one negative lens (C1, C2) and a positive lens (C3). The telescope of anyone of the preceding Claims wherein said field switch lens group (C) comprises: a doublet (C1, C2) adjacent to said front doublet (A1, A2) of the objective lens group (A) and a positive lens (C3) adjacent to said rear doublet (A3, A4) of the objective lens group (A). The telescope of Claim 8 wherein said doublet (C1, C2) of the field switch lens group (C) comprises a chalcogenide lens (C1) and a zinc sulfide lens (C2). The telescope of Claim 8 or 9 wherein the positive lens (C3) in the field switch lens group (C) comprises a chalcogenide lens (C3). The telescope of Claim 1, said telescope being a dual field of view telescope, wherein said front doublet (A1, A2) is defined by a positive chalcogenide lens (A1) and a negative ZnS lens (A2), the rear doublet (A3, A4) being defined by said negative Ge lens (A3) and said positive chalcogenide lens (A4); said eyepiece lens group (B) being defined by a doublet (B1, B2) including two positive chalcogenide lenses; further including a field switch lens group (C) having a plurality of lenses (C1, C2, C3) selected from the group of nonoxide chalcogenide glass and zinc sulfide, said field switch group cooperating with the lenses (A1, A2, A3, A4) of the objective lens group (A) to shorten the focal length of the combined lens groups (A, C) thereby providing a wider field of view for the telescope when the field switch lens group (C) is placed on the optical axis (O). The telescope of Claim 11 wherein said field switch lens group (C) comprises: a doublet (C1, C2) disposed adjacent the front doublet (A1, A2) of the objective lens group (A), the lenses of the doublet (C1, C2) being made of nonoxide chalcogenide glass and zinc sulfide; and a positive lens (C3) with a conic rear surface disposed adjacent the rear doublet (A3, A4) of the objective lens group (A).

Inventors: KEBO REYNOLD S, KEBO,REYNOLD S.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1989
Main classification: G02B23
IPC codes: G02B23, G02B1, G02B13, G02B15