The Qualities Of A Telescope Eyepiece

Eyepiece Attributes
The eyepiece must produce an image with minimal or no aberrations within the range of objective focal ratios it is designed to magnify; display a wide field without perceptible curvature or distortion, transmit almost all light from the image without ghosts, glare or scatter, provide enough eye relief for comfortable viewing (preferably with an adjustable eye rest), withstand years of normal use, environmental exposure and the occasional accident without affecting any of the previous qualities, and be offered new at an affordable price…a tall order to satisfy.
The following are the key attributes defined related to the above description of a quality eyepiece:
Sharpness
All elements of the image appear in perfect focus, without any visible distortion or aberration. As all ocular errors increase with field height (distance from the central optical axis), sharpness is typically evaluated as any degradation of the image of a star as it is moved from the field center to the edge.
Brightness
This is the overall throughput or transmission efficiency of the eyepiece. Ideally, 100% of the luminance of the image will be transmitted to the eye; the eyepiece will absorb or deflect no light. In practice, brightness is reduced by the number of elements and groups (air/glass or glass/glass boundaries) in an eyepiece, and by lenses that are uncoated. These all cause reflections and ghost images, which divert light.
Contrast
This is the luminance difference between the lightest and darkest areas of an image, and is primarily produced by high transmission oculars that do not scatter light. Light scattering is caused by destructive interference in lens coatings, lens edges or ocular mountings that are not blackened.
Flatness
This quality is to display a wide field without perceptible curvature or distortion
Comfort
The comfort and convenience factors in a good eyepiece will vary with the individual, but typically include (1) comfortable eye relief, (2) eye guards or eye rests, especially in longer focal length eyepieces, (3) size and weight, (4) apparent field of view.
Durability
The eyepiece withstand years of normal use, environmental exposure and the occasional accident without affecting any of the previous qualities
Affordability
The vast array of designs and brands make eyepieces more affordable than ever before. It is not necessary to buy the most expensive eyepiece to get a quality image from your telescope. Telescopes with higher focal ratios are able to use less expensive, less precision made eyepieces. Actually the eyepieces such as Kellner that come with the inexpensive department store telescopes (or bought separately), give good views in in telescopes of longer focal lengths.

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