What should be used on a light source to preserve night vision while observing?
Answer
A red filter
Human eyes require time to adjust to dark conditions to effectively perceive faint light from celestial objects. Using a standard white light source causes the pupils to constrict, effectively destroying night vision and requiring the eyes to re-adjust to the darkness. A red filter allows for illumination that is sufficient to read charts or operate equipment without causing the pupils to constrict, thereby preserving the observer's ability to see in the dark.
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