Telescope Eyepiece
Definition
An eyepiece is an optical system. Also called ocular, it is a type of lens that is placed closest to the eye when it is attached to an optical device. Not all eyepieces are designed the same. In an eyepiece, the lens closest to the eye is the eye lens and the one closest to the object is the field lens.
Function
The entrance pupil of the eyepiece is always located outside the optic system. The main function of an eyepiece is to magnify objects and to correct or eliminate lens aberrations and chromatic difference. The eyepiece as a magnifier forms a larger virtual image of the distant object projected in the lens.
Each eyepiece is designed to perform for a specific distance and specific objects. The goal of the eyepiece design is to distortion for the given distance. An eyepiece with high magnification can show a small degree (part) of the entire object. On the other hand low magnification eyepieces show higher degree of field of view.
Historical Evolution of the Eyepiece
HuyGen
The earliest known eyepiece was designed by Christian Huygens and is known as the Huygen eyepiece. It has both plano convex lenses facing the object lens. The Huygen has its aperture between two lenses.
The Huygen’s drawback was extreme short eye relief and small apparent field of view. This led to highly distorted images or aberrations. However, its importance cannot be downplayed because it inspired the design of the next eyepiece which was the Ramsden.
Ramsden
The eyepiece that came after the Huygen is the Ramsden. Named after its designer Jesse Ramsden, it combines two Plano convex lenses with the same focal length. The Ramsden eyepiece has the aperture in front of one lens.
The optical quality of a Ramsden eyepiece is much better than that of a Huygen although distortion and chromatic aberration is still quite high.
Kellner
Designed by Carl Kellner in 1849, the Kellner eyepiece is an improved version of the Ramsden. It is the first known doublet designed achromatic eyepiece. Kellner eyepieces have a 3-element (lens) design that has an achromatic doublet. The images are comparatively sharper and brighter with low to medium power. To improve its performance, the known problem of internal reflection has been corrected with anti-reflection coating. Today, all modern eyepieces are variations of the Kellner design. These Include;
Plossl
Plossl eyepiece incorporates 4 optical elements grouped into two sets of doublets. Designed by Georg S. Plossl it provides a large degree apparent field of view ample eye relief. It is ideal for viewing lunar and deep sky objects.
orthoscopic
Invented by Ernst Abbe, the orthoscopic eyepiece consists of a cemented triplet coordinated to a single plano-convex eye lens. Orthoscopic eyepieces are close to have good eye relief, and minimum chromatic aberration. With little internal reflection and a wide field of view at 40 to 50 degrees, it is one of the best eyepieces available.
Erfle
Erfle eyepieces have either five or six elements, It can either combine two achromats with a double convex lens in the middle or it can have two achromats three achromats with a double convex lens. Image distortion is encountered when high magnification is used. They are most useful for observing deep sky objects.
Nagler
The Nagler eyepiece is designed to give an extra wide field of view at 82 degrees. The number of elements found in a Nagler makes it comparatively heavy. Nagler designs combine positive and negative groups of elements in order to have long focal lengths