Abstract:
The present optical system provides an all-reflective zoom optical system (10, 12, 14,16 and 60, 62, 64). An imaging mechanism including one or more movable mirrors (12, 14, 16 and 60, 62) is utilized to effect the change in the magnification, field of view or both of the system.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates to reflective telescope systems and, more particularly, to an all-reflective zoom optical system. 
     2. Discussion 
     The performance of conventional high quality telescopes when used on the earth for celestial viewing is principally limited to the earth&#39;s atmosphere rather than by the construction of the telescopes. Atmospheric effects not only limit the resolution of a telescope, but also absorb large portions of the electromagnetic spectral range. While in the past, little could be done about this limitation, today, with the help of earth satellites and other space vehicles, it is possible to place a telescope above the atmosphere and perform extraterrestrial observations without interference from it. As a result, there has arisen a need for a telescope which can more fully take advantage of this new environment. Also, it should be understood that the present invention may be utilized for other airborne or ground based applications that require viewing distant objects. This invention may also be used to transmit radiation. 
     The advantages of zoom optical systems are well known to those skilled in the art. However, virtually all of the known zoom optical systems utilize refractive optical elements in whole or in part. Refractive optical elements generally have one or more of the following disadvantages. Refractive systems generally have spectral limitations and chromatic aberrations. Refractive systems have size limitations, lens material limitations and a lack of radiation hardness. Refractive systems prohibit simultaneous multi-spectral operations in widely separated portions of the electromagnetic spectral range. Further, the refractive systems are sensitive to thermal changes and are exceptionally heavy when used in large aperture designs. 
     SUMMARY OF THE INVENTION 
     According to the teachings of the present invention, a system is provided which retains the versatility and benefits of zoom optics while eliminating the disadvantages of refractive optical systems. The present invention provides a fixed mirror and an all-reflective zoom system comprised of one or more movable mirrors which effect a change in magnification, field of view, or both. The lower magnification, coarser resolution, wider field of view extreme of the zoom range enables search and acquisition functions during operation of the system. The higher magnification, finer resolution and smaller field of view enables tracking and detailed imaging during operation of the system. 
     The present invention provides the art with an all-reflective afocal telescope which exhibits substantially unobscured capabilities. The present invention provides multi-spectral operation to enable simultaneous viewing of visual, television, laser and infrared spectra. The present invention enables beam splitting of collimated light among the various spectral bands at a viewing exit pupil. Imaging optics are generally placed behind the exit pupil to provide a final image plane. 
     In the preferred embodiment, the reflective zoom optical system is comprised of the following. A mirror is fixably positioned about a central axis. Also, the system includes a mechanism for reflecting an image of the object being viewed through an exit pupil for viewing. The reflecting mechanism is positioned to receive or reflect the light from the fixed stationary mirror and is movable through a plurality of positions to effect magnification, field of view, or both of the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to those skilled in the art after a study of the following specification and by reference to the drawings in which: 
     FIG. 1 is a schematic diagram of an along-scan view of an apparatus in accordance with the teaching of the present invention; 
     FIGS. 2a-2c are schematic diagrams of three zoom positions of the along-scan view of FIG. 1; 
     FIG. 3 is a schematic diagram of an along-scan view of another apparatus in accordance with the teaching of the present invention; and 
     FIGS. 4a-4d are schematic diagrams of four zoom positions of the along-scan view of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2a-2c, an afocal optical system is shown including primary 10, secondary 12, tertiary 14 and quaternary 16 mirrors. The system also includes a decentered entrance pupil 18 and a decentered exit pupil 20. 
     The primary mirror 10 includes a central axis 22 defining the system optical axis. The primary mirror 10 is fixably or stationarily positioned with respect to the optical axis. The primary mirror 10 is a positive power mirror and may be a paraboloid conic or higher order aspheric mirror. 
     The secondary mirror 12 is a negative power mirror and is movably positioned such that it is in a cassegrain configuration with the primary mirror 10. The vertex of the secondary mirror 12 would be positioned along the optical axis of the system. The secondary mirror 12 may be a hyperboloid conic or higher order aspheric mirror. 
     The tertiary mirror 14 is a positive power mirror. The tertiary mirror 14 is movably positioned such that its vertex would be positioned along the optical axis of the system. The tertiary mirror 14 may be of spherical shape, conic or higher order aspheric mirror. 
     The quaternary mirror 16 is a positive power mirror. The quaternary mirror 16 is movably positioned such that its vertex would be along the optical axis of the system. The quaternary mirror 16 may be of spherical shape, conic or higher order aspheric mirror. 
     The combination of movement of the secondary 12, tertiary 14 and quaternary 16 mirrors preserves the line of sight, focus collimation and exit pupil position during the zoom operation. All mirrors are rotationally symmetric and centered along a common optical axis and the movable mirror&#39;s movement is along the optical axis. The aperture entrance pupil is decentered with respect to the optical axis. The field of view is centered on the optical axis. Also, the size of the field of view in the object space varies at different magnifications. The exit pupil&#39;s 20 position is fixed during the zoom operation. 
     In operation, an object to be viewed is reflected by the primary mirror 10. The light beams 30 from the object being viewed are received and reflected from the primary mirror 10 to the secondary mirror 12. The light beams 30 are received by and reflected from the secondary mirror 12 to the tertiary mirror 14. The beams 30 form an intermediate image of the object being viewed between the secondary 12 and tertiary 14 mirrors at 31. The intermediate image, formed by beams 30, is reflected from the tertiary mirror 14 to the quaternary mirror 16 and through the remainder of the system and is ultimately reimaged at infinity after passing through the exit pupil 20. 
     In FIG. 1, the movable secondary 12, tertiary 14 and quaternary 16 mirrors are illustrated (in phantom) in varying positions along the travel paths of the mirrors. FIGS. 2a-2c illustrate the system at the three different phantom positions of FIG. 1 along the mirrors travel paths. The three positions are the 3.6×, 2.5× and 1.7× magnification positions. The field of view in the object space varies from 0.95° at the 3.6× position to 2° at the 1.7× magnification position. 
     At the 3.6× magnification position, the on-axis 80% geometrical blur diameter in the object space is less than 0.09 milliradians. At the 1.7× magnification position, the on-axis 80% blur diameter in the object space is less than 0.36 milliradians. At the 2.5× magnification position, the on-axis 80% blur diameter is approximately 4.6 milliradians. Thus, the system provides a room range which is appropriate for multi-field of view applications such as for search and acquisition functions as well as for tracking and detail imaging functions. 
     A specific prescription for the system in accordance with the present invention as illustrated in FIG. 1 is as follows: 
     
                       TABLE 1______________________________________OPTICAL PRESCRIPTION OF A SPECIFICEMBODIMENT OF OPTICAL SYSTEM OFTHE PRESENT INVENTION______________________________________                        Conic#    Description   Radius    Constant                               Thickness______________________________________18   Entrance Pupil              Infinite         19.010   Primary Mirror              -20.000   -1.0000                               -6.9899812   Secondary Mirror               -8.10139 -2.8103                               13.111014   Tertiary Mirror               -32.1935 0      -10.251816   Quaternary Mirror               23.5583  0      10.218220   Exit Pupil    Infinite[(+) Thickness are to the right;(+) Radii have centers to the right]TILT AND DEC DATA:        TYPE      YD          XD______________________________________18 Entrance Pupil        Dec       +12.0       020 Exit Pupil        Dec       -3.24927    0Clear Apertures and Obstructions     TYPE    CAY    CAX  YDEC   XDEC  TILT______________________________________10 Primary     Rect    5.80   2.70 9.00   0Mirror12 Secondary     Rect    2.20   1.00 2.70   0Mirror14 Tertiary     Rect    1.35   0.90 -1.10  0Mirror16 Quarternary     Rect    1.25   1.05 -3.25  0Mirror          REF       OBJ     REF   IMGRef OBJ Y-HT   AP Y-HT   SURF    SURF  SURF______________________________________-0.872687E + 18(0.5 DG)          2.5       0       1     15ER       EPR          A-MAG     LENGTH______________________________________10.559827    -.6909927    -3.6179832                           6.1210153No Aperture stopEvaluation Mode is AfocalAlternate EmbodimentsParameter                  Current Value______________________________________CFG 2:SAY FANG   Semi Field Angle                      .75SAY        18 Entrance Pupil                      1.875YD         18 Entrance Pupil                       8.9327766TH         10 Primary Mirror                      -7.932274TH         12 Secondary Mirror                       8.855943TH         14 Tertiary Mirror                      -7.7615949TH         16 Quaternary Mirror                      12.925344CFG 3:SAY FANG   Semi Field Angle                      1.0SAY        18 Entrance Pupil                      1.250YD         18 Entrance Pupil                       5.9577749TH         10 Primary Mirror                      -8.6313326TH         12 Secondary Mirror                       6.038513TH         14 Tertiary Mirror                      -6.5720958TH         16 Quaternary Mirror                      15.252334______________________________________ 
    
     Moving to FIGS. 3 and 4a-4c, another embodiment of the present invention is illustrated. In this embodiment, two movable mirrors are utilized to provide a 4 to 1 zoom range of operation. 
     Referring to FIGS. 3 and 4a-4c, an afocal optical system is shown including primary 60, secondary 62 and tertiary 64 mirrors. The system also includes an aperture entrance pupil 66 and a variable position exit pupil 68. 
     The primary mirror 60 is a positive power mirror and is movably positioned such that the vertex of the primary mirror would be along the optical axis of the system 70. The primary mirror 60 may be an ellipsoidal conic or higher order aspheric mirror. 
     The secondary mirror 62 is a negative power mirror and is movably positioned such that it is in a cassegrain configuration with the primary mirror 60. The secondary mirror 62 includes a central axis 70 defining the system optical axis. The vertex of the secondary mirror 62 would be positioned along the optical axis of the system. The secondary mirror 62 may be an ellipsoidal conic or higher order aspheric mirror. 
     The tertiary mirror 64 is a positive power mirror. The tertiary mirror 64 is fixably or stationarily positioned with respect to the optical axis. The tertiary mirror 64 may be a hyperboloidal conic or higher order aspheric mirror. All of the above mirrors may be point testable conics. 
     The combination of movement of the primary and secondary mirrors preserves the line of sight, and focus collimation during the zoom operation. The position of the exit pupil varies during the zoom operation. All mirrors are centered along a common optical axis 70 and the movable mirror&#39;s movement is along the optical axis. The aperture entrance pupil is decentered with respect to the optical axis. The field of view is centered on the optical axis. Also, the size of the field of view in image space varies at different magnifications. 
     In operation, an object to be viewed is reflected by the movable primary 60 mirror. The light beams 72 from the object being viewed are received and reflected from the primary mirror 60 to the movable secondary mirror 62. The light beams 72 are received by and reflected from the secondary mirror 62 to the tertiary mirror 64. The beams 72 form an intermediate image 74 of the object being viewed approximately at the secondary mirror 62. The intermediate image formed by the beams 72 is reflected from the tertiary mirror 64 and is ultimately reimaged at infinity after passing through the variable position exit pupil 68. 
     In FIG. 3, the movable primary 60 and secondary 62 mirrors are illustrated in phantom at varying positions along the travel path of the mirrors. FIGS. 4a-4c illustrate the system at four different or distinct positions along the mirrors&#39; travel paths. The four zoom positions are the 0.5×, 0.25×, 0.167× and 0.125× magnification positions. The field of view in image space varies from 0.25° at the 0.125× to 1.0° at the 0.5× magnification position. The field of view in object space is 2°. 
     At the 0.5× magnification position, the on-axis geometrical blur diameter in the image space is less than 0.15 milliradians. At the 0.25× magnification position, the on-axis blur diameter in the image space is less than 0.06 milliradians. At the 0.167× magnification position, the on-axis blur diameter in the image space is less than 0.05 milliradians. And finally, at the 0.125× magnification position, the on-axis blur diameter in the image space is less than 0.084 milliradians. Thus, the system provides a zoom range from 0.125× to 0.5×. When this telescope is used in reverse, the zoom range of 2× to 8× is appropriate for multi-field of view applications such as for search and acquisition functions as well as for tracking and detailing functions. The system as described may be used as a zoom laser transmitter/receiver. 
     A specific prescription for the system in accordance with the present invention as illustrated in FIG. 3 is as follows: 
     
                                           TABLE 2__________________________________________________________________________OPTICAL PRESCRIPTION OF A SPECIFICEMBODIMENT OF OPTICAL SYSTEM OFTHE PRESENT INVENTION__________________________________________________________________________                    Conic#     Description           Radius   Constant                         Thickness__________________________________________________________________________66    Entrance Pupil           Infinite Maximum Distance        8.68885 Between 66 and 6060    Primary Mirror           -5.00000 -0.76006                         -1.6574562    Secondary Mirror           -2.00000 -0.88995                         5.10442 Maximum Distance        30.8642 Between 62 and 6464    Tertiary Mirror           -61.7284 -1.0245                         -161.20768    Exit Pupil           Infinite__________________________________________________________________________[(+) Thickness are to the right;(+) Radii have centers to the right]TILT AND DEC DATA:    TYPE YD    XD   ALPHA BETA                              GAMMA__________________________________________________________________________66 Entrance Pupil    Dec  +1.125000               0.00000068 Exit Pupil    Dec  -1.107600               0.000000Clear Apertures and Obstructions      TYPE          CAY  CAX YDEC  XDEC   TILT__________________________________________________________________________60 Primary Mirror      Circ          0.55000  1.13  0.000E + 0062 Secondary Mirror      Circ          0.30000   0.320                         0.000E + 0064 Tertiary Mirror      Rect          5.6000               2.2500                   -5.70 0.000E + 00REF OBJ Y-HT  REF AP Y-HT                 OBJ SURF                       REF SURF                              IMG SURF__________________________________________________________________________-0.872753E + 18(1.00 DG)         0.25000 0     1      7ER        EPR          A-MAG LENGTH__________________________________________________________________________-119.26008     -.48741464   0.49757604                        34.311168No Aperture StopEvaluation Mode is AfocalAlternate EmbodimentsParameter                     Current Value__________________________________________________________________________CFG 2:TH      Maximum Distance between 66 and 60                         11.857000TH      60 Primary Mirror     -1.8253996TH      62 Secondary Mirror    2.1042071YD      68 Exit Pupil         -2.2136500TH      Maximum Distance Between 68 and 64                         -191.5385CFG 3:TH      Maximum Distance between 66 and 60                         13.065200TH      60 Primary Mirror     -1.9859589TH      62 Secondary Mirror    1.0565768YD      68 Exit Pupil         -3.3125500TH      Maximum Distance between 68 and 64                         -124.8636CFG 4:TH      Maximum Distance Between 66 and 60                         13.777950TH      60 Primary Mirror     -2.1498787TH      62 Secondary Mirror     0.50772223YD      68 Exit Pupil         -4.4726850TH      Maximum Distance Between 68 and 64                         +56.7823__________________________________________________________________________ 
    
     While the above describes preferred embodiments of the present invention, it will be understood that the prescriptions may be modified or changed without deviating from the scope of the invention. 
     The present invention has several advantages over conventional zoom type lenses. The present invention uses an all-reflective system to provide a zoom optical system. The present invention provides a multi-spectral instrument which may be utilized as an earth resources sensor with zoom capabilities. The present invention may also be utilized as a laser transmitter/receiver, target designator or FLIR in a ground based or space based application. Also, it is possible to use the present invention as a spectrometer telescope for spectral analysis of gases to determine their chemical composition. 
     It should be understood that while this invention has been described in connection with the particular examples hereof, that various modifications, alterations and variations of the disclosed preferred embodiments can be made after having the benefit of a study of the specification, drawings and the subjoined claims.