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The Achromatized Ramsden, or Kellner Eyepiece
In the Kellner a cemented doublet that provides a considerable improvement in performance replaces the first lens of the Ramsden, sometimes called the "eyelens" since it is next to the eye (Fig. 11.5). This allows the usable semi-field angle to be increased from 15 deg for the simple Ramsden to 20 deg for the Kellner (Fig. 11.6). In this particular example, the eye relief is only
This eyepiece still has poorly corrected astigmatism, but it is better than the Ramsden eyepiece. The spherical aberration and the axial color and lateral color are all at acceptable levels, although the semi-field of 20 deg is perhaps a little large for this type of eyepiece.
There is significant high-order coma at the edge of the field of view. If this were third-order (Seidel), it would be proportional to the first power of the field angle, so that the coma at 20 deg would be about 50% more than at 14 deg. It is clearly worse than this, suggesting a higher-order dependence on field angle.