Dark Adaptation

The human eye takes time to adjust to the dark. Your eyes' pupils expand to nearly their full nighttime size within seconds whenever you step out into the dark. But the most important part of dark adaptation involves chemical changes in the retina, and these require many minutes.

After spending 15 minutes in darkness you might think your night vision is fully developed. But in fact your eyes gain as much as another two magnitudes of sensitivity — a factor of six — during the next 15 minutes. Thereafter, dark adaptation improves very slightly for 90 minutes more. So don't expect to see faint objects at their best until a half hour or more into an observing session.

In practice, complete darkness is unattainable. Light pollution aside, you need some light to see what you're doing. Astronomers have long used dim red flashlights because red light harms night vision least. In near-darkness you see with your retina's rod cells, and these are nearly blind to the far red end of the visible spectrum. When you see red light your cone cells are at work; they are responsible for normal daytime color vision. (You have three types of cones — red, green, and blue — but only one type of rod, which is fairly insensitive to red.) You want to use your red cones for reading charts and working with hardware, while protecting your rods for delicate work at the eyepiece.

You can get a dim, diffuse glow by rubber-banding red paper over the front of a flashlight. You also can dim and somewhat redden a two-battery flashlight by installing a bulb rated for three or four batteries. Much better than these traditional approaches, however, is a red LED (light-emitting diode) flashlight. Its purer, deeper red light more sharply discriminates between rod and cone vision. LEDs also use very little current, so batteries last for years. Many LED flashlights are available for astronomers.

Another trick for preserving dark adaptation is to observe with one eye and read charts with the other. Keep the observing eye closed or covered with an eye patch when not in use.