You've probably noticed that when you first go outside from indoors on a starry night, you can see relatively few stars. But then, as your eyes adjust to the darkness, many more stars come into view. This phenomenon is called dark adaptation, and it is crucial for visual astronomy, especially for observation of faint objects, such as galaxies and nebulas.
There are many ways you can improve and maximize your night vision. But first we must understand how night vision works.
Dark Adaptation: A Complex Process
Dark adaptation begins as soon as you enter a dark environment, and it happens in several steps:
- The iris of your eye opens the pupil, the "black hole" in the center, to its maximum width, usually 5 to 7 millimeters. Though this is the most visible aspect of dark adaptation, it is only the first step.
- Next, a pair of chemicals in the eye, rhodopsin and iodopsin, begin to take effect. These two chemicals are always present in the eye, but they break down in the presence of light, so when the eye is exposed to bright light these chemicals have no effect. But when in the dark the concentration of these chemicals begins to grow and the rod cells and cone cells in the retina become more and more light sensitive.
- At first, most of the increase in night vision comes from the cones, which are densely concentrated in the center of the retina. They are highly sensitive to color and are important for distinguishing fine detail. After 7 minutes or so the cones have reached their maximum sensitivity, while the rods, which are insensitive to color but are more sensitive than cones to low levels of light, keep gaining in sensitivity for another 20-30 minutes.
At the end of a half-hour or so the eye has achieved almost all of its dark sensitivity, with a small increase continuing until about 1 hour or so.
The Rods Have It
When your eye is fully dark adapted, most of your night vision comes from the rod cells in the retina. But the rods are not color sensitive, which is why in the dark you can see only shades of gray. The bright colors you see in pictures of nebulas and galaxies are typically only visible in photographs (film being much more sensitive to colors than a dark-adapted eye).
Most people are aware that night vision is not in color, but few realize that you see less fine detail at night as well. This is because the rods are not as tightly packed as the cones, so they cannot distinguish detail nearly as well. To prove this to yourself you need only look at a tree about 50 feet away: In the daytime you can clearly make out the leaves in the tree; at night you can make out the outline of the tree, but not the individual leaves.
Ten Tips For Improving Your Night Vision
So now that you know how night vision works, here's how to maximize your ability to see in the dark.
1) Observe from a dark site. Any amount of light will reduce your dark adaptation, so get away from street lamps, porch lights, car headlights, and urban skyglow.
2) Avoid bright sunlight as much as possible during the day prior to an evening's observing session, especially later in the afternoon. Exposure to intense light can hamper your dark-adaptation for a long time! Wear sunglasses when you have to go outside.
3) If you are in a light-polluted location consider wearing dark glasses or special red night-vision goggles at all times except when looking through the eyepiece. It may seem odd to wear dark glasses at night (and certainly don't do that when you're driving), but it can be a real help.
4) When you need some light to see what you're doing, use a dim red flashlight, the dimmer the better. A red light with adjustable brightness is very handy because it allows you to dial down the brightness to the bare minimum required. (Red light works best because it is less efficient than white light at breaking down the iodopsin and rhodopsin that allow your eye to see in the dark.)
5) Your eyes adapt to darkness independent of one another, so if you have to look at something bright do so with one eye, saving the dark adaptation of your other eye.
6) In light-polluted areas, do whatever you can to block ambient light from your eyes. For instance, consider using a dark shroud over your head to block out distracting light when at the eyepiece. Cupping your hand around your eye and the eyepiece helps, too.
7) When you take a break during a night of observing, say to go inside to warm up or grab a bite to eat, put on a pair of red goggles. If you don't need to see what you're doing, cover your eyes with a dark cloth and relax. Even though your eyes may seem fully dark adapted, after a half hour with your eyes completely sealed from light you may find that you gain a bit more acuity.
8) Use averted vision. The rod receptors, which are most sensitive to dim light, are more highly concentrated around the periphery of the retina than in the center. This means that you can see faint objects better by looking slightly off to the side of them rather than straight at them. Try it.
9) Breathe deeply. Avoid the tendency to slow your breathing rate or hold your breath when concentrating intently on a dim object. Reduced oxygen diminishes your night vision. Many experienced astronomers use the trick of "oxygen loading" before observing a particularly faint object, to enhance their visual acuity. Breathe deeply for 15 to 30 seconds just before looking into the eyepiece, and continue doing so as you observe. Don't go overboard, though. If you start feeling dizzy, breathe normally!
10) Avoid drinking alcoholic beverages before or during an observing session. Alcohol is a depressant and will decrease your visual acuity. Wait until after you're finished to crack open that cold one!
In astronomy the name of the game is seeing as much as you can possibly see. For that reason it pays to take a few extra steps to achieve and maintain your maximum dark-adapted night vision, particularly because it is so easy to do.