Averted Vision

One of the best things you can do for your eyes is to take short breaks. Simple one-minute exercises done every 20 minutes will dramatically reduce eye fatigue. Change the focus by glancing at someone's nearby telescope. Then, lightly cup your eyes with your palms, and relax for 60 seconds. Or simply look away from the eyepiece, and roll your eyes up and down, around and side to side for 20 seconds. Then relax, eyes closed, for 30 more seconds.

Deep-sky objects often disappoint beginners not only by lacking obvious detail, but also by lacking the brilliant colors recorded in photographs.

In order to show us color, a deep-sky object must have a high enough surface brightness to stimulate the retina's cone cells - and the list of deep-sky objects this bright is short. The brightest parts of Great Orion Nebula (M42) qualify, as do some small but high-surface-brightness planetary nebulae. The ability to see color in dim objects varies greatly from person to person, and surprises may occur.

Averted vision is not the way to look for color. The cones are thickest in the fovea, so stare right at your object. In this case, the lowest useful power should work best. A large telescope aperture is especially advantageous for those who seek to see color in deep-sky objects.

It's also worth talking about comfort at the eyepiece. There are many observers who use various contortions and gyrations to look through the eyepiece. But when we are seated comfortably at the eyepiece, we see a great deal more than while standing.

Dark adaptation is the process by which the eyes increase their sensitivity to low levels of illumination. The human eye takes time to adjust to the dark. Your eyes' pupils expand to nearly their full nighttime size (7 mm) 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 total darkness you might think your night vision is fully developed. But in fact your eyes gain 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 the first 30 minutes the eye's sensitivity increases 10,000 fold, with little gain after that. Dark adaptation approaches it's maximum level in approximately 30 to 45 minutes under minimal light conditions. If the eyes are then exposed to a bright light, their sensitivity is temporarily impaired, the degree of which depends on the intensity and duration of the exposure. Brief flashes from high-intensity strobe lights have been shown to have little effect on night vision. This is because the pulses of energy are of such short duration (milliseconds). Durations of bright light lasting one second or longer, however, can seriously impair night vision.

Rhodopsin (sometimes called visual purple) is the substance in the rod cells responsible for light sensitivity. The degree of dark adaptation increases as the amount of visual purple in the rod cells increases through biochemical reactions. Each person adapts to darkness in varying degrees and at different rates. In a darkened theater, the eye adapts quickly to the prevailing level of illumination. Compared to the light level of a moonless night, this level is high.

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 lights do not significantly impair night vision if proper techniques are used. Rod cells are much more sensitive to blue light and are least affected by the wavelength of a dim red light. In near-darkness you see with your retina's rod cells, and these are 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 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. To minimize the effect of red light on night vision, the intensity should be adjusted to the lowest useable level and the illuminated object should only be only viewed for a short time.

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.

We have decreased resolution at night, and little color vision as well. Resolution diminishes at night for multiple reasons: reduced number of retinal cells firing; the color shift in sensitivity vs. the focusing ability of the eye; chromatic aberration of the eye; variable transparency of the lens and humors of the eye; etc. In dim light, the spectral sensitivity of the rods peaks at about 505 nm, and in bright light the peak of the cones is about 560 nm. Cones outnumber rods only in the center of the retina, which is the area of the greatest density. It is also the area most heavily used by daytime, direct, vision. But rods do fire during the day or we would have no peripheral vision at all.

Make your "pushing the telescope's limit" observations no earlier than midnight, when your eyes will be at their greatest sensitivity.

For those with new telescopes, or equipments that have been added to your telescopes, perform a setup at home first. Any problems revealed in the daylight will be the ones you won't have later. As a second step, set it up at night out in your yard and observe as if you were at your remote dark sky-site. It's surprising what a test-run like this will teach you about your setup and what it requires.

The single most important factor in deep-sky observing is light pollution. Its worst effect is on dim, extended objects.
This means that even if you live in a badly light-polluted area you can take pleasure in what can be seen through the sky-glow. Just remember not to blame yourself or your telescope for what may seem like mediocre results. Rather, make a note to bring your telescope along on country getaways.

NOTE: A dark sky matters even more than telescope size; a small instrument in the country will show faint nebulae and galaxies better than a large telescope in a city.

Conventional wisdom holds that low magnification (low power) works best for deep-sky viewing. After all, low power concentrates an extended object's light into a small area, increasing its apparent surface brightness (the amount of light hitting any square millimeter of your retina). This assumption is usually false. High powers should do better on many faint deep-sky objects. The reason is subtle but important, so we'll go into some detail.

Unlike a camera or other purely mechanical lens system, the eye loses resolution in dim light. This is why you can't read a newspaper at night, even through you can see the newspaper, and even though your large nighttime pupil should theoretically resolve the letters even more sharply than in daylight. Studies show that the eye can resolve detail almost as fine as 1 arc minute (1/60 of a degree) in bright light but can't make out features smaller than about 20 or 30 arc minutes (1/3 to 1/2 of a degree) wide when the illumination is about as dim as the night sky. This is almost the size of the Moon as seen with the naked eye. So, details in a very faint object can be resolved only if they are magnified until they appear tens of arc minutes across. In many cases, this can require using extremely high power!

Why does the eye work this way? The explanation lies in how the visual system has adapted to cope with night. Photographic film records light passively, but the retinal nerve system contains a great deal of computing power. In dim light, the retina compares signals from adjacent areas. A faint source covering only a small area - such as a small galaxy in the eyepiece - may be completely invisible at the conscious level. But it is being recorded in the retina, as evidenced by the fact that a larger galaxy with the same low surface brightness is visible easily. In effect, when rod cells see a doubtful trace of light they ask other rods nearby if they're seeing it too. If the answer is yes, the signal is passed on up the optic nerve to the brain. If it's no, the signal is disregarded.

When an image is magnified by high power, its surface brightness does indeed grow weaker. But the total number of photons of light entering the eye remains the same. (A photon is the fundamental particle of light. Experiments show that most people can detect as few as 50 to 150 photons per second.) It doesn't really matter that these photons are spread over a wider area; the retinal image-processing system will cope with them. At least within certain limits. A trade-off is needed to reach the optimum power for low-light perception: enough angular size but not too drastic a reduction in surface brightness.

What does all this mean for deep-sky observers? Simply that it's wise to try a wide range of powers on any object. (A judiciously chosen, high-quality zoom eyepiece makes this a breeze.) You may be surprised by how much more you'll see with one than another.

One more point: There is a folk belief among observers that a telescope of long focal length (high f/ratio) gives a cleaner, higher-contrast view of dim objects than a short focal-length scope. But f/ratio is not the issue. A long-focus telescope is simply more likely to be used at high power! (It's also more likely to have high-quality optics, because "slow" mirrors and lenses are easier to manufacture well than "fast" ones.)