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.
Related terms
- Red light
A dim red torch for reading a chart without losing your dark adaptation: the retina’s rods are almost blind to red. Keep it as dim as possible. A phone screen should also be switched to red and minimum brightness.
- Averted vision
Looking slightly to the side of a faint object instead of straight at it. The centre of the retina has few rods, the cells that sense dim light; shifting your gaze by about ten degrees makes the object show up better. The gain is about one magnitude.
- 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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