Telescope Eyepieces

In this article we will touch briefly on how you should go about choosing your eyepiece. Choosing the right eyepiece depends on what you plan to view, how particular you want to be about image quality and field of view, and how much you are willing to spend.

Consider Optical Correction Eyepieces

The main aim of telescope eyepieces design is to get all the light rays to form a sharp image that can be seen by the viewer. How well this occurs depends completely on the f-ratio of the telescope. Telescopes with low f-ratios require more highly corrected eyepieces because the cone of light entering the eyepiece is converging more sharply. With an f/10 telescope, any well-made eyepiece will give a sharp image. With an f/4 telescope, only the best modern eyepieces will yield completely sharp images from out to the edge of the field of view.

Eye relief in Eyepieces

The optical design of telescope eyepieces also determines the eye relief (this is the distance from the viewer’s eye to the eyepiece lens when the image is in focus). If you wear glasses, you probably need at least 15mm and preferably 20mm of eye relief to see the entire field of view. With insufficient eye relief the outer portion of viewing will be eliminated, which will result in a “keyhole effect” meaning that the viewer will only see a portion of the image.

Apparent Field Of View In Eyepieces

The optical design of telescope eyepieces also determines the size of the image that the viewer sees. An eyepiece's field of view is the angular diameter, expressed in degrees (°), of the circle of light that the eye sees. Most eyepieces have an apparent field of about 40° to 50°.

Barrel Size of Eyepieces

Telescope eyepieces come in several different barrel diameters - .965", 1.25", and 2". The smallest of these sizes is found mostly in low-end, cheap telescopes and tend to be of poor quality. Most amateur good quality telescopes are designed to accommodate the 1.25" eyepiece size. The large, 2" models are used mostly with higher-end telescopes aimed at serious hobbyists.

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