Objective lens assembly for a portable missile launcher

Abstract: A portable missile launcher has an optical system with a single lens (22) for receiving visible and near infrared light energy from a target directing it to a prism (24) where the light is split into a part transmitted to an eyepiece (28) and a further part (30) to infrared tracking equipment (18). The objective lens (22) is a doublet having a front lens and a rear lens which cooperate to provide sufficiently small chromatic aberration to enable image quality correction for both visual examination and near infrared tracking.

Description: The present invention relates generally to a portable missile launcher, and, more particularly to an objective lens assembly for use in such a launcher for providing both a high powered visual telescope and for receiving electromagnetic energy from a missile beacon all through a single common aperture. In a known form of a missile launcher, a missile to be tracked includes an arc lamp beacon located at the rear of the missile which emits electromagnetic energy in the near infrared region for being picked up by the missile tracking system, and, by conventional processing techniques, the location of the missile with respect to an optically/thermally viewed target is determined. The tracking system can then either be manually or automatically operated to make any changes necessary in the missile flight path to ensure contact with the target. Portable missile launchers in the past have been constructed to include separate optical sighting apparatus and infrared sensing and tracking equipment requiring separate optical elements which must be manipulated. This, of course, results in higher production costs, complex handling procedures for the operator, decreased reliability, and increased weight and size. US-A-3 974 383 discloses a missile guidance system according to the preamble of claim 1 which utilizes one optical system for visually tracking a target and directing a missile thereto by means of infrared energy emitted from the missile. The optical system includes a single objective lens for receiving both the visible and infrared energy and directing those energies on a first prism. The first prism separates the visible energy from the infrared energy and directs the visible energy to an eyepiece for a viewer. First and second infrared images are derived from the infrared beam and processed electronicly to generate command signals for steering the missile to the target. Furthermore, from EP-A-0 124 682 a coaxial wideband refractive optical system has become known which includes a front of objective lens of a wideband transmitting material followed by a beam splitter and two doublet lenses for a simultaneous optical correction of electromagnetic energy wavelength in the visible and infrared spectrum. The front lens is a single, positive lens element which can have large color dispersion characteristics not normally associated with an objective lens element. It is an object of the invention to provide an improved optical system for a portable missile launcher comprising a single optical system for handling both the visual and infrared electromagnetic energy coming from the target and missile, preferably leading to savings in production costs, simpler handling procedures for the operator, improved reliability, and decreased weight and size. This object is achieved by an optical system according to claim 1. In accordance with the practice of the present invention, a single optical system is provided for a portable missile launcher having the capability of acting as a visual telescope via which the target can be monitored and also as a near infrared tracker for following the missile beacon. A common aperture objective lens includes an achromatized air-spaced doublet made from an anomalous dispersion crown optical glass in conjunction with a standard dispersion flint glass which coact to reduce chromatic aberration over the wide spectral band of operation required. Electromagnetic energy coming from both the target and the missile are collected by the common aperture objective lens and passed through a beam splitting Porro prism which provides good image quality to the reticle plane of the optical system eye-piece for the visual telescope. That is, the operator will not only be able to see a sharp magnified image of the target, but also a clear image of the reticle pattern. Moreover, use of the anomalous dispersion glass ensures that chromatic aberration is satisfactorily small at the tracker infrared energy detectors. Image quality correction for both modes of operation (i.e., visual and near IR tracker) and the use of the special anomalous dispersion glass enable the desired collecting of electromagnetic information both visual and infrared from the target as well as the missile through a single objective lens. A preferred embodiment of the invention is described in the following with reference to the accompanying drawings, in which FIG. 1 is an elevational, partially schematic view of a known portable missile launcher; FIG. 2 is a schematic optical layout of the present invention; and FIG. 3 is an enlarged elevational view of the doublet objective for use in this invention. With reference now to the drawing and particularly FIG. 1, a prior art portable missile launcher enumerated generally as 10 primarily operates to track a launched missile 12 during its flight toward a target 14 and has the further capability for sending control signals to change the course of the missile in order to home it in onto the target. More particularly, the missile 12 includes a xenon arc light 16 (sometimes referred to as a beacon) which emits a beam of electromagnetic energy in the near infrared region that is tracked by equipment identified generally as 18 in the missile launcher. On the other hand, the target and surroundings can be visually monitored by a telescope system 20 that has to be separately manipulated from the IR tracking system. In the present invention as is particularly shown in FIG. 2, a single objective lens 22 receives the visual electromagnetic energy from the target 14 and as well the infrared energy emanating from the arc light 16 carried by the missile 12. The electromagnetic energy received by the objective lens is then passed to a Porro prism 24 which consists generally of two prisms arranged to one another in a known manner so as to pass the visual light along one path directly through a reticle 26 to an optical eye-piece 28. Also, the infrared energy received by the Porro prism assembly is redirected along a second path 30 generally 90 degrees to that taken by the visual light into the tracker assembly 18 where it is subsequently processed for producing a display on a CRT 31, for example, and for accomplishing the other functions already alluded to. The objective lens 22 is a doublet arrangement, the front lens 32 being constructed of an anomalous dispersion glass which has as its main desirable result here of keeping the chromatic aberration sufficiently small at the tracker detectors 34 to ensure satisfactory tracker operation. More particularly, in a practical construction of the invention the front lens 32 is constructed of an anomalous crown glass referred to in the optics industry as LgSK2 having a front surface circular radius of curvature 36 of 8.52429 inches, a back radius of curvature 38 of -5.22041 inches, and thickness along optical axis 40 of 0.600000 inches. The rear lens 41 of the doublet is constructed of standard flint glass commonly referred to as BaSF50 having a front radius of curvature 42 of -5.22467 inches, a back surface radius of curvature 44 of -20.2942, and a thickness along the optical axis 0.250000 inches. The two lenses are separated by a constant air space 46 of 0.031788 inches. In the practice of the present invention, the image of the IR beam from the missile by which the missile is being tracked and the visual image of the target and missile enter the portable launcher via a single aperture, rather than through two separate forward looking apertures as in the prior art apparatus. This is advantageous, first of all, in eliminating the need for separate manipulation of apparatus by the operator during visual and IR tracking. Considering the anxious circumstances under which the described equipment may be operated, the benefit derived from increased simplicity of handling is manifest. Also, although in one known prior art launcher the optical system required six separate optical elements, in contrast there are only three needed in the present invention. This latter feature results in reduced size and weight, along with a corresponding reduction in manufacturing cost, and with a plus of increased reliability.

Claims: An optical system for a portable missile launcher (10) including a dichroic beam splitter prism (24) for receiving light having near infrared and visible light components from a target (14) and transmitting the same along both a first path to an eyepiece (28) and along a second path to means for processing and displaying the near infrared light, comprising a single objective lens (22) located in light receiving position between the dichroic beam splitter prism (24) and the target (14) providing chromatic aberration between incident infrared and visible light components sufficiently small to enable image quality correction for both visual examination via the eyepiece (28) and for processing and displaying the near infrared components; characterized in that said dichroic beam splitter prism is configured as a Porro prism (24); and in that said objective lens (22) is a doublet including a front lens (32) constructed of an anomalous dispersion glass and a rear lens (41) constructed of a flint glass, said front and rear lenses being separated by an air space (46). An optical system according to claim 1, characterized in that the front lens (32) has a front surface radius of curvature (36) of 21,65170 centimeters (8,52429 inches), a back radius of curvature (38) of -13,25984 centimeters (-5,22041 inches), and thickness along a central axis (40) of 1,52400 centimeters (0,600000 inches); and the rear lens (41) has a front radius of curvature (42) of -13,27066 centimeters (-5,22467 inches), back surface radius of curvature (44) of -51,54727 centimeters (-20,2942 inches), and thickness along a central axis (40) of 0,63500 centimeters (0,250000 inches).

Inventors: CARLSON JAMES J, NOYES GARY R, CARLSON, JAMES J., NOYES, GARY R.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1993
Main classification: F41G7
IPC codes: F41G3, F41F3, G02B23, G02B13, F41G7