Red flashlights are used to read star charts without affecting your night vision. The subdued red light allows you to see but does not cause your pupils to constrict. Allowing your eyes to stay fully dark adapted greatly enhances your ability to see detail in faint objects. Red flashlights are a requirement at all organized star parties, but you should really use one any time you observe in order to improve your night vision. Don't be the one to turn on a white light at a star party - the other participants won't be too happy with you!