| 摘要 |
A telescope eyepiece comprising a series of single-element lenses in a combination that cancels chromatism. A negative field lens produces chromatism that is partly cancelled by an intermediate positive lens. The resulting red and blue focal points behind the intermediate lens are exactly matched by the same focal points for parallel rays assumed to originate from behind the eye lens passing forward, thus creating an achromatic system that cancels the remaining chromatism. An additional one or two intermediate positive lenses may be added to increase the field of view and reduce the curvature of field. The eyepiece designer can set the distortion of the system to zero, or can adjust it in a range from minor pincushion to minor barrel distortion as desired. Spherical aberration is corrected by the selection of curves for the field lens. Astigmatism is corrected by the spacing of the field lens from the next intermediate lens. Different optical glasses are utilized to maximize the apparent field and minimize other aberrations. However, an acceptable 50 degree field-of-view eyepiece can be made using four elements of the same type of optical glass, such as fused silica. In this case, the ultraviolet, visible, and infrared rays are all in focus at once, providing perfect achromatism, as with mirrors.
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