Abstract: 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.

Abstract: The eyepiece of a telescope is provided with a plano-convex eye lens that adjustably pivots about the geometric center of its convex surface, inducing prism while maintaining the optical axis and focal plane of the eyepiece. The prism effect is adjusted to offset atmospheric optical dispersion when viewing celestial bodies near the horizon. Simple and inexpensive mechanical means for tilt adjustment are disclosed.