Plossl type eyepiece for use in astronomical instruments
FIELD OF THE INVENTION 
     The invention relates to an eyepiece for astronomical instruments, and more particularly to a Plossl type eyepiece having improved coma and astigmatism correction. 
     BACKGROUND OF THE INVENTION 
     As is well known in the art, a Plossl type eyepiece is a relatively wide field eyepiece comprising two achromatic doublets in which the crown elements usually face each other. Such eyepieces are capable of good performance, i.e., acceptable degrees of aberrations, to about a 50° field. Generally, in order to minimize aberrations at the exit pupil and distortion, all air glass surfaces of the eyepiece are made convex. However, thre have been Plossl type eyepieces used commercially in astronomical instruments in which the external flint surfaces are plano. 
     GENERAL DESCRIPTION OF THE INVENTION 
     For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field. 
     Accordingly, it is an object of the present invention to provide an improved Plossl type eyepiece for use with astronomical instruments. 
     It is another object of the invention to provide a Plossl type eyepiece having improved coma and astigmatism aberrations at the edge of the field of view. 
     In carrying out the invention, there is provided a symmetrical eyepiece comprising two achromatic doublets in which the external surfaces of the flint elements are concave. Such a lens configuration provides a significant improvement in the correction of astigmatism and coma at the edge of the field. This results in a sharper image for large field angles with a relatively small undercorrected field curvature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing schematically shows a Plossl type eyepiece according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a 26 mm. focal length Plossl type eyepiece as shown in the drawing, according to one embodiment of the invention, the external concave lens elements are flint glass having a refractive index N d  equal to 1.667 and an Abbe number ν of 33, and the convex elements are heavy crown glass having a refractive index N d  equal to 1.658 and an Abbe number ν of 51. The lenses LI and LII are separated by an air space of one millimeter. Further lens data for the particular embodiment are shown in the accompanying table wherein lens radii and lens thicknesses at the optical axis are given in millimeters. 
     
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Lens      Radii         Axial Thickness                                   
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                R1 = -169.87                                              
                            d1 = 2                                        
LI              R2 = +21.598                                              
                            d2 = 9                                        
                R3 = -29.44                                               
                R4 = +29.44 d3 = 9                                        
LII             R5 = -21.598                                              
                            d4 = 2                                        
                R6 = +169.87                                              
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     An eyepiece constructed according to the foregoing data will be well corrected for spherical aberration, and axial and lateral color. Moreover, the astigmatism at a 25° semi field will have the following values: tangential focus position +0.31 mm. from paraxial; sagittal focus position -0.84 mm. from paraxial, which results in an astigmatism of 1.15 mm. Coma at a 25° semi field at f/5 relative aperture is 0.34 mm. 
     Having thus described the invention, it is to be understood that the specification and the drawing are to be interpreted as illustrative and not in a limiting sense.