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.
ToolsEyepieces, Useful formulas
Related terms
- Magnification
How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.
- Minimum magnification
The lowest magnification that still sends all the light into the eye: aperture divided by 7 (a young eye’s dark-adapted pupil is about 7 mm wide). For a 200 mm telescope that is about 29×. Below it, the exit pupil is wider than the eye’s pupil and some light is wasted.
- Eye relief
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.
← The whole glossary