Kellner Eyepiece
The modern Kellner eyepiece was first designed in 1849 by a German Mechanic named Carl Kellner. This eyepiece was an improved version of Ramsden’s Plano-convex lens. This is why the Kellner eyepiece is sometimes referred to as achromatic Ramsden or simply as Achromatic lens or Achromat.
Kellner eyepieces are designed to use a combination of concave and convex lenses which when put together produce eyepieces that are able to correct and reduce chromatic aberration. The eyepieces produce much clearer and sharper images than the simple lens by designed Ramsden.
Chromatic Aberration
Chromatic aberration or colour distortion happens when the glass lens used does not bring all the colours to the same focal point. The resulting effect is blurry images similar to the rainbow.
This is because there are different wavelengths of light on any given lens. For example the wavelength of a blue light does not focus to the same flat surface or plane as the wavelength of a red light. The longer the wavelength, the lesser the relative speed of light (Refractive index).
The work of achromatic lenses such as the Kellner eyepiece is to focus the different wavelengths of light on to the same flat surface in order to improve colour.
The technique of attaching together two lenses with varying refractive index results in what is called a doublet lens. The Kellner eyepiece is the first known achromatic doublet. The Kellner eyepiece design uses three elements grouped in to two as follows;
- achromatic doublet eye lens
- plano convex field lens.
The resulting output is sharper and brighter images at low to medium power. Their main drawback has been internal reflections which can be easily corrected with anti- reflection coating.
Achromatic Doublet eye lens
The doublet consist of a concave and a convex glass lens being cemented together to become an image magnifier. The two lenses when attached together become the doublet. One lens acts to eliminate distortions from the other lens and vice versa.
Features of an achromatic doublet
- A Concave Lens made of flint glass
Flint glass has high refractive index. Meaning that the speed of light travelling through is comparatively higher than other types of glass lenses.
Flint glass also combines a low Abbe Number. Generally an Abbe number is the measure of distortion of a material in relation to the speed of light (refractive Index). Abbe Number, is also known as the V-number or Constrigence.
- A Convex Lens made from Crown Glass
Crown glass has a low refractive index and a low Abbe Number. This means that light travels relatively slowly and the rate of distortion is low.
- Plano Convex lens
Located near the image field, the plano convex lens has one flat lens (plano) and the other is curved (convex).
Plano Convex lens Located near the image field, the plano convex lens has one flat lens (plano) and the other is curved (convex).
Kellner eyepieces provide quality images at a comparatively cheap price. They have an apparent field of view that ranges between 40 – 50 degrees.
At 17 degrees angular field and 15mm field stop, the lenses are considered useful on small and medium telescopes.