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.
ToolsEyepieces, Useful formulas
Related terms
- 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.
- 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.
- Dark adaptation
The time the eye takes to become sensitive in the dark: the pupil opens within seconds, but the retina gains most of its sensitivity over 20 to 30 minutes. A single bright white light undoes it all. That is why observers use a dim red light.
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