The Kellner eyepiece was introduced by Carl Kellner in 1849 and is also referred to as an "achromatized Ramsden".
This type of eyepiece has an achromatic doublet which is used in place of the simple plano convex eye lens found in the Ramsden design and is there to correct the residual transverse chromatic aberration.
The one shown here to the right is a Meade 1.25 9mm
These eyepieces are a 3-lens design which are easy on the pocket and produce a moderately good image from low to medium power, consequently they are significantly better than both the Huygenian or Ramsden design.
The eye relief is superior to the Huygenian although not as good as the Ramsden eyepiece.
The main issue with Kellner eyepieces was internal reflections. However, with the advance in today's anti-reflection coatings it makes eyepieces very practical and cost effective options for small to medium aperture telescopes with a focal ratio f/6 or longer.
This design reduced almost all of the aberrations in Huygens and Ramsden eyepieces coupled with relatively good color correction and edge sharpness. The typical field of view is between 40 to 50 degrees.
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