Abstract:
An optical system resembling a modified Gregorian astronomical telescope is disclosed in the specification and drawings. Lenses are used in the inter-focal region to modify the optical characteristics of the system. The use of such inter-focal lenses, can, for example, flatten the image plane of the field thereby increasing the field of view of the telescope.

Description:
RIGHTS OF THE GOVERNMENT 
   The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of telescopes. 
   2. Description of the Related Art 
   The Gregorian telescope includes a parabolic primary mirror, which reflects incoming light rays through a focal point to an elliptical secondary mirror, which then refocuses the light to the final focal plane. These telescopes are expensive to construct because of the geometries required of the primary and secondary mirrors. In addition, these telescopes have a narrow field of view due to the curved focal surface, limiting the operation to fields on the order of about five minutes of arc. 
   There is a need in the art to increase the field of view. In addition, because of the high cost of constructing the telescope due to the large primary and secondary mirrors being aspheric (more difficult to manufacture conic sections), there is a need in the art to utilize aspheric primary and secondary mirrors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the features, objects, and advantages of embodiments of the present invention can be more readily ascertained with reference to the following description, in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a simplified sectional view of one embodiment of the invention; 
       FIG. 2  is an enlarged view of the inter-focal region of  FIG. 1 ; 
       FIG. 3  is a simplified sectional view of an alternative embodiment of the invention; 
       FIG. 4  is an enlarged view of the inter-focal region of  FIG. 3 ; 
       FIG. 5  is a ray-traced spot diagram of the embodiment shown in  FIG. 3 ; 
       FIG. 6  is a simplified sectional view of another alternative embodiment of the invention; 
       FIG. 7  is an enlarged view of the inter-focal region of  FIG. 6 ; and 
       FIG. 8  is a ray-traced spot diagram of the embodiment shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   Referring to the drawings,  FIG. 1  depicts a modified Gregorian telescope  10  according to one embodiment. Light or other electromagnetic radiation rays  12  entering the telescope are directed to a primary mirror  14 , which reflects the light through an internal focus to a secondary mirror  16 , which then reflects the light to an image point and, in this embodiment, through a lens  18  positioned just before (adjacent to) the focal plane. 
   This particular embodiment includes three lenses in the inter-focal region  20 , shown in detail in  FIG. 2 . This inter-focal region  20 , located between the primary mirror  14  and the secondary mirror  16 , encompasses the focal point of the primary mirror  14  and the surrounding areas before and after the focal point of the primary mirror  14 . Referring to  FIG. 2 , the inter-focal region  20  is shown with three spherical lenses  22 ,  24  and  26 . In the illustrated embodiment, the surface of lens  24  is positioned at the primary focus. 
   In this embodiment, the primary mirror  14 , secondary mirror  16  and inter focal lenses  22 ,  24  and  26  are positioned along the same axis. However, any suitable arrangement of mirrors and lenses may be configured in alternative embodiments of the invention. 
   Various combinations of lenses may be selected for the inter-focal region  20 , positioned before, at and after the primary focus. By utilizing an appropriate selection and placement of these inter-focal lenses, a Gregorian-type telescope capable of imaging a field the width of a few minutes of arc can be expanded by a factor of 10 to 20, thereby allowing the imaging of fields of view in excess of 1.5 degrees across. This represents an imaging area performance gain of factors ranging from 100 to 400 better than previous Gregorian-type designs. 
   Use of these inter-focal lenses allows flexibility in specifying the shape of the primary mirror  14  and secondary mirror  16 . These lenses may act as partial null lenses to compensate for the inherent aberrations of the primary and secondary mirrors and, if properly designed, are achromatic and may also compensate for the chromatic aberration of field flattening lenses near the focal plane. 
   The inter-focal lenses may be sized and positioned as desired. For example, the inter-focal lenses may be sized and positioned such that the light reflected from the primary mirror will pass through the inter-focal lenses and then, after reflecting off the secondary mirror, will pass back through these inter-focal lenses. Alternatively, the inter-focal lenses may be sized and positioned such that only the light reflecting off the primary mirror will pass through these lenses and light reflected off the secondary mirror will not pass through them but will instead be reflected directly to the focal plane. 
   Functional designs have been developed utilizing elliptical, parabolic and spherical secondary mirrors. In one embodiment, four inter-focal lenses—in which only one is aspherical—is used with a spherical secondary mirror. In this example, the telescope may be constructed less expensively as the cost of producing a parabolic or elliptical secondary mirror is higher than a smaller parabolic or elliptical inter-focal lens. 
     FIG. 3  shows the layout of one embodiment of a four-meter telescope  30  having a parabolic primary mirror  32 , an elliptical secondary mirror  34 , and a field flattening lens  36 . This embodiment, as shown in  FIG. 4 , utilizes three inter-focal lenses  40 ,  42  and  44 . This embodiment may be used to produce diffraction limited images over a ¼ th  degree circular field of view. Referring to  FIG. 5 , ray-traced spot diagrams show the performance of this design. The circles represent the diffraction limit while the dots represent the performance at individual wavelengths. The design was optimized for light in the wavelengths of 400-1,100 nanometers (nm). 
   The optical prescription for this embodiment is presented in the table below. The nomenclature used is from the ZEMAX® commercial optical design package. All dimensions are in millimeters (mm). In this embodiment, as shown in the table, fused silica was used for the inter-focal lenses. However, any material or materials having the desired optical properties may be used. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Optical Prescription for Telescope 30. 
             
           
        
         
             
                 
                 
               Radius of 
                 
                 
               Semi 
                 
             
             
               No. 
               Comment 
               Curvature 
               Thickness 
               Glass 
               Diameter 
               Conic 
             
             
                 
             
           
        
         
             
               0 
                 
               Infinity 
               0.0000 
                 
               0.000 
               0.00000 
             
             
               1 
                 
               Infinity 
               10.0000 
                 
               0.000 
               0.00000 
             
             
               2 
                 
               Infinity 
               5564.8210 
                 
               0.000 
               0.00000 
             
             
               3 
               Primary (32) 
               −10000.0000 
               −4921.7957 
               Mirror 
               2000.262 
               −1.00000 
             
             
               4 
               Lens 1 (40) 
               2096.7496 
               −24.4257 
               Fused Silica 
               49.989 
               0.00000 
             
             
               5 
                 
               −1662.8108 
               −14.7492 
                 
               43.377 
               0.00000 
             
             
               6 
               Lens 2 (42) 
               −296.8339 
               −18.6272 
               Fused Silica 
               37.254 
               0.00000 
             
             
               7 
                 
               −1161.2286 
               −17.3780 
                 
               32.169 
               0.00000 
             
             
               8 
                 
               Infinity 
               −56.7209 
                 
               0.000 
               0.00000 
             
             
               9 
               Lens 3 (44) 
               94.9794 
               −16.1870 
               Fused Silica 
               32.894 
               0.00000 
             
             
               10 
                 
               64.6474 
               −10.9678 
                 
               37.608 
               0.00000 
             
             
               11 
                 
               Infinity 
               −12.0377 
                 
               0.000 
               0.00000 
             
             
               12 
                 
               Infinity 
               −471.9319 
                 
               0.000 
               0.00000 
             
             
               13 
               Secondary (34) 
               1035.6149 
               5564.8210 
               Mirror 
               200.000 
               −0.61611 
             
             
               14 
                 
               Infinity 
               210.9982 
                 
               119.986 
               0.00000 
             
             
               15 
               Lens (36) 
               −2685.6131 
               39.0018 
               Fused Silica 
               117.005 
               0.00000 
             
             
               16 
                 
               −1233.8027 
               750.0000 
                 
               117.117 
               0.00000 
             
             
               17 
               Focal Plane 
               Infinity 
               0.0000 
                 
               79.538 
               0.00000 
             
             
                 
             
           
        
       
     
   
   The layout for another embodiment of a four-meter telescope  50  is shown at  FIG. 6 . This embodiment includes a parabolic primary mirror  52 , a spherical secondary mirror  54 , and a field flattening lens  56 . This embodiment, as shown in  FIG. 7 , utilizes four inter-focal lenses  60 ,  62 ,  64 , and  66 . This embodiment may be used to produce diffraction limited images over a ⅙ th  degree circular field of view. In this embodiment, a larger aspheric mirror is traded for a much smaller aspheric lens in the inter-focal region. Referring to  FIG. 8 , ray-traced spot diagrams show the performance of this design. The circles represent the diffraction limit while the dots represent the performance at individual wavelengths. The design was optimized for light in the wavelengths of 400-1,100 nanometers (nm). 
   The optical prescription for this embodiment is presented in the table below. The nomenclature used is from the ZEMAX® commercial optical design package. All dimensions are in millimeters (mm). In this embodiment, as shown in the table, fused silica was used for the lenses. However, any material or materials having the desired optical properties may be used. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Optical Prescription for Telescope 50. 
             
           
        
         
             
                 
                 
               Radius of 
                 
                 
               Semi 
                 
             
             
               No. 
               Comment 
               Curvature 
               Thickness 
               Glass 
               Diameter 
               Conic 
             
             
                 
             
           
        
         
             
               0 
                 
               Infinity 
               0.0000 
                 
               0.000 
               0.00000 
             
             
               1 
                 
               Infinity 
               10.0000 
                 
               0.000 
               0.00000 
             
             
               2 
                 
               Infinity 
               5724.3994 
                 
               0.000 
               0.00000 
             
             
               3 
               Primary (52) 
               −10000.0000 
               −4916.4575 
               Mirror 
               2000.175 
               −1.00000 
             
             
               4 
               Lens 1 (60) 
               910.3374 
               −16.6664 
               Fused Silica 
               50.000 
               0.00000 
             
             
               5 
                 
               −87.2203 
               −27.1209 
                 
               43.814 
               0.00000 
             
             
               6 
               Lens 2 (62) 
               −103.9242 
               −19.7158 
               Fused Silica 
               39.415 
               0.00000 
             
             
               7 
                 
               338.5031 
               −53.3428 
                 
               36.299 
               0.00000 
             
             
               8 
                 
               Infinity 
               −84.0260 
                 
               0.000 
               0.00000 
             
             
               9 
               Lens 3 (64) 
               457.3144 
               −22.6180 
               Fused Silica 
               45.235 
               29.08592 
             
             
               10 
                 
               125.1166 
               −10.9678 
                 
               50.000 
               0.00000 
             
             
               11 
               Lens 4 (66) 
               −116.7997 
               −12.0377 
               Fused Silica 
               56.658 
               0.00000 
             
             
               12 
                 
               −116.4897 
               −561.4465 
                 
               56.905 
               0.00000 
             
             
               13 
               Secondary (54) 
               1304.2073 
               5724.3994 
               Mirror 
               200.000 
               0.00000 
             
             
               14 
                 
               Infinity 
               100.0139 
                 
               101.117 
               0.00000 
             
             
               15 
               Lens (56) 
               948.0596 
               66.2366 
               Fused Silica 
               99.355 
               0.00000 
             
             
               16 
                 
               2925.1418 
               833.7495 
                 
               96.713 
               0.00000 
             
             
               17 
               Focal Plane 
               Infinity 
               0.0000 
                 
               47.259 
               0.00000 
             
             
                 
             
           
        
       
     
   
   Characteristics of the described and illustrated embodiments are intended for illustrative purposes and are not to be considered limiting or restrictive. It is to be understood that various adaptations and modifications may be made to the embodiments presented herein by those skilled in the art without departing from the spirit and scope of the invention, as defined by the following claims and equivalents thereof.