It only takes an instant of bright light and your night vision is gone for another 15-20 minutes. This includes looking at a bright moon or planet in the telescope. Red light and night vision. If you have been to a star party, you probably have heard that red light is all we use. Sometimes astronomers get down right nasty about white light.
Red light is very low in energy.
There is not enough energy in red light to activate the muscles around the iris or cause the cones to revert to bright light mode. It is also why red light can be used in a photo dark room. There is not enough energy in red light to spoil the photo paper or film.