Abstract:
Ultra wide-angle eyepieces with relatively long focal lengths and minimal spherical aberration of the exit pupil and field curvature. The eyepieces use six elements in a positive power lens group and two elements in a negative power lens group.

Description:
FIELD OF THE INVENTION 
     The invention relates to eyepieces suited especially for use with astronomical telescopes, and more particularly to such eyepieces having a wide field of view. 
     BACKGROUND OF THE INVENTION 
     Ultra wide-angle eyepieces employing positive power lens elements on one side of the internal focal plane and a negative power lens on the other side of the focal plane, i.e., between the positive lens and the telescope objective, have been disclosed heretofore as, for example, in my earlier U.S. Pat. No. 4,286,844. The eyepieces there disclosed were shown to be effective in achieving highly corrected flat fields of view. The improved field curvature is obtained by compensating the field curvature of the positive lens group by that of the negative lens. However, the use of the negative lens with strongly divergent rays requires a large aperture positive lens with attendant comparatively high manufacturing costs and weight. Consequently, the focal lengths of such wide angle eyepieces were kept relatively short, e.g., 13 mm., to minimize their weight. 
     Moreover, ultra wide-angle eyepieces, i.e., those having an apparent field of view exceeding 50 degrees, give rise to spherical aberration of the exit pupil. Rays with large exit angles will usually intersect the optical axis nearer to the last eyepiece lens element than those rays with small or moderate exit angle. This results in the so-called &#34;kidney bean&#34; effect when the circle of least confusion of the exit pupil of the eyepiece exceeds in size the pupil of the observer&#39;s eye. Thus, eyepieces as disclosed in my aforementioned patent were, for all practical purposes, limited to 13 mm. focal lengths for night observations and to 9 mm. focal lengths for daytime use. 
     GENERAL DESCRIPTION OF THE INVENTION 
     The object of the invention is to provide improved ultra wide-angle eyepieces. 
     Another object of the invention is to provide relatively smaller, less costly ultra wide-angle eyepieces with minimal field curvature. 
     Still another object of the invention is to provide ultra wide-angle eyepieces that are suitable for manufacture in longer focal lengths. 
     Yet another object of the invention is to provide longer focal length ultra wide-angle eyepieces with minimal spherical aberration of the exit pupil and field curvature. 
     In carrying out the invention, there is provided an ultra wide-angle eyepiece having six elements in the positive power lens group and two elements in the negative power lens group. The positive group elements may be 2-1-2-1, or 2-1-1-2, in form, but the surface closest to the internal focal plane must be convex or plano to minimize spherical aberration of the exit pupil, it cannot be concave. 
     Features and advantages of the invention may be gained from the foregoing and from the description of preferred embodiments thereof which follow. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic longitudinal sectional view showing one embodiment of the invention; 
     FIG. 2 is a similar view of another embodiment of the invention; 
     FIG. 3 is a similar view of still another embodiment of the invention; and 
     FIG. 4 is a similar view of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The eyepieces shown in FIGS. 1 to 4 with the lens specifications set forth in Tables I to IV, respectively, show excellent aberrational correction over fields in the range of 80 to 90 degrees diameter. The smaller eyepieces, i.e., those shown in FIGS. 2 and 4 and specified in Tables II and IV permit scaling up to longer focal lengths while retaining reasonable size and weight, and eye relief. 
     A measure of the improvement in spherical aberration of the exit pupil is the axial focal differences for telecentric rays entering the eyepiece corresponding to 30 to 40 degree semi-field angles. In a 10 mm. focal length eyepiece according to the disclosure in my aforementioned patent the focal difference is 3.14 mm. In a 10 mm. focal length eyepiece according to the present invention, the focal differences are: 1.93 mm. for the FIG. 1, Table I eyepiece; 0.94 mm. for the FIG. 2, Table II eyepiece; 1.9 mm. for the FIG. 3, Table III eyepiece; and 0.47 mm. for the FIG. 4, Table IV eyepiece. 
     Each eyepiece has an internal focal plane located between lens 6 and lens 7. A stop will generally be provided at the focal plane. 
     The lens data set forth in the following tables are, in all cases, for eyepieces having a 10 mm. focal length and an 80 to 90 degree field of view. All are well corrected for telecentric input beams as fast as f/4. Lens radii, lens thicknesses, apertures, and lens separations are given in millimeters. N is the refractive index of the glass, and V is the Abbe number for the glass. 
     The eyepiece illustrated in FIG. 1 and specified in Table I has a Petzval radius of -147.48, while that illustrated in FIG. 2 and specified in Table II has a Petzval radius of -66.24. The eyepiece illustrated in FIG. 3 and specified in Table III has a Petzval radius of -147.48, while the eyepiece illustrated in FIG. 4 and specified in Table IV has a Petzval radius of -144.09. 
     
                       TABLE I______________________________________                      Ap-                      er-Lens Radius      Thickness ture Glass N     V______________________________________r1 = -18.56L1               t1 = 1.65 8.24 SF5   1.6727                                       32.2r2 = +44.30L2               t2 = 7.83 12.36                           SK16  1.6204                                       60.3r3 = -18.56            s1 = 0.41r4 = planoL3               t3 = 4.53 15.66                           SK16  1.6204                                       60.3r5 = -44.30            s2 = 0.41r6 = +82.40L4               t4 = 14.01                      17.30                           SK16  1.6204                                       60.3r7 = -20.59L5               t5 = 2.47 19.77                           SF1   1.7174                                       29.5r8 = -82.40            s3 = 0.41r9 = +40.14L6               t6 = 9.06 19.77                           SK16  1.6204                                       60.3r10 = -82.40            s4 = 36.26r11 = -20.59L7               t7 = 2.06 9.89 LAK8  1.7130                                       53.8r12 = +15.57L8               t8 = 4.94 10.71                           SF1   1.7174                                       29.5r13 = +168.59______________________________________ 
    
     
                       TABLE II______________________________________                      Ap-                      er-Lens Radius      Thickness ture Glass N     V______________________________________r1 = -12.94L1               t1 = 0.94 5.46 SF5   1.6727                                       32.2r2 = +17.67L2               t2 = 6.38 9.10 SK16  1.6204                                       60.3r3 = -12.94            s1 = 0.44r4 = -58.39L3               t3 = 3.63 11.38                           SK16  1.6204                                       60.3r5 = -22.15            s2 = 0.44r6 = +73.57L4               t4 = 9.57 13.65                           SK16  1.6204                                       60.3r7 = -17.67L5               t5 = 1.38 13.65                           SF1   1.7174                                       29.5r8 = -58.39            s3 = 0.44r9 = +27.95L6               t6 = 6.82 15.02                           SK16  1.6204                                       60.3r10 = -73.57            s4 = 22.84r11 = -14.09L7               t7 = 1.88 6.83 LAF21 1.7883                                       47.4r12 = +33.63L8               t8 = 6.88 9.10 SF4   1.7552                                       27.6r13 = -46.46______________________________________ 
    
     
                       TABLE III______________________________________                      Ap-                      er-Lens Radius      Thickness ture Glass N     V______________________________________r1 = -24.25L1               t1 = 1.89 9.33 SF1   1.7174                                       29.5r2 = +44.45L2               t2 = 9.25 13.33                           SK16  1.6204                                       60.3r3 = -19.99            s1 = 0.16r4 = +130.93L3               t3 = 5.43 20.00                           SK16  1.6204                                       60.3r5 = -69.51            s2 = 0.16r6 = +48.50L4               t4 = 8.15 20.00                           SK16  1.6204                                       60.3r7 = -146.71            s3 = 0.16r8 = +88.33L5               t5 = 11.28                      20.00                           SK16  1.6204                                       60.3r9 = -32.88L6               t6 = 2.35 20.00                           SF1   1.7174                                       29.5r10 = -103.48            s4 = 39.83r11 = -20.64L7               t7 = 1.59 9.33 LAK8  1.7130                                       53.8r12 = +17.52L8               t8 = 4.25 9.33 SF1   1.7174                                       29.5r13 = +1144.93______________________________________ 
    
     
                       TABLE IV______________________________________                      Ap-                      er-Lens Radius      Thickness ture Glass N     V______________________________________r1 = -10.23L1               t1 = 0.79 4.75 SF5   1.6727                                       32.2r2 = +14.02L2               t2 = 5.94 7.53 SK16  1.6204                                       60.3r3 = -10.23            s1 = 0.28r4 = -46.05L3               t3 = 2.77 9.51 SK16  1.6204                                       60.3r5 = -17.62            s2 = 0.28r6 = +59.09L4               t4 = 7.13 10.89                           SK16  1.6204                                       60.3r7 = -14.02L5               t5 = 1.19 10.89                           SF1   1.7174                                       29.5r8 = -46.05            s3 = 0.28r9 = +22.31L6               t6 = 5.94 12.28                           SK16  1.6204                                       60.3r10 = -59.09            s4 = 19.03r11 = -7.61L7               t7 = 1.98 5.15 LAF2  1.7440                                       44.9r12 = +56.61            s5 = 6.93r13 = -82.46L8               t8 = 4.36 9.31 FK5   1.4875                                       70.4r14 = -12.63______________________________________ 
    
     Having thus described the invention, it is to be understood that the drawing and the specification are to be interpreted as illustrative rather than in a limiting sense.