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.

Related terms

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.
Sketching: Drawing what you see at the eyepiece, in pencil on white paper, under a red light. It forces you to look long and carefully: you notice far more detail. Galaxy and nebula sketches are often made in negative, then inverted.

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