Make your own red flashlight for night viewing in the planetarium or outdoors.
The nerve receptors in our eyes are of two types, rods and cones. Cones enable us to see colors, and are active in daylight. Rods are for night vision, and sense shades of grey. When you walk into a dark room like the Planetarium, a minimum of 20 minutes is required for one's rods fully to turn on; i.e., for your vision to become accommodated to the dark. This is why our Planetarium shows are visually rather dull and boring at first, to give everyone time to "get their night eyes."
In the Planetarium or under the starry sky outdoors we often need to check out a star chart, use a planisphere, or read lab notes. Yet white light blinds us to the night, turning off our rods and stimulating our cones. Fortunately, red light is different, and does not impair night vision. If you have a red flashlight, you can check out your star chart and still be able to look up and see the stars.
Next: Using the Planisphere