The distance between the eyepiece’s last lens and the spot where your eye must be to see the whole field. Glasses wearers need at least 15 to 20 mm. Short focal length eyepieces of older designs often have very short, uncomfortable eye relief.
ToolsEyepieces
Related terms
- Eyepiece
The small interchangeable magnifier you look through; it sets the magnification. Its focal length is engraved on it: the shorter it is, the higher the power. The standard barrels are 1.25 inch (31.75 mm) and 2 inch (50.8 mm) wide.
- Exit pupil
The small disc of light that leaves the eyepiece and enters the eye; its diameter is the aperture divided by the magnification. A 200 mm telescope at 40× gives a 5 mm exit pupil. Between 5 and 7 mm the image is at its brightest (nebulae); around 0.5 to 1 mm you reach the maximum useful magnification.
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