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Tips for observing

Home > Tips for observing
Deep-sky observing involves its own techniques. All are aimed at helping the eye to see in near-total darkness. Here are many detailed important pointers every observer should know.

Averted Vision

Avoiding eye fatigue

Capturing color

Comfort

Dark adaptation

Eye miscellany

Eye patch

Know your telescope

Sky brightness

Using high powers

Viewing faint objects

Other tips

1) Averted Vision

When you look directly at something, its image falls on your retina's fovea centralis. This spot is packed with bright-light-optimized cone cells and provides sharp resolution under strong illumination. But the fovea is fairly blind in dim light. So to see something faint, you have to look slightly away from it. Doing so moves the image of your target off the fovea and onto parts of the retina that have more rod cells, which see only in black and white but are more light-sensitive than the cones.

To see this effect at work, stare straight at a moderately faint star. It will disappear. Avert your gaze just a bit; there it is again.

Practice concentrating your attention on things a little off to one side of where your eye is aimed. This technique is called averted vision. You'll be using it almost all the time when deep-sky observing.

Avoid placing the object very far on the "ear side" of your center of vision; it may fall on the retina's blind spot there and vanish altogether. In practice, finding how far to avert your vision is a matter of trial and error. Not enough and you don't get the full benefit; too much and you lose the ability to resolve details.

Your peripheral vision is highly sensitive to motion. Under certain conditions, wiggling the telescope makes a big, dim ghost of a galaxy or nebula pop into view. When the wiggling stops, the object disappears again into the vague uncertainty of the sky background.

But under other conditions, just the opposite technique may work, especially with objects that are both faint and tiny. According to Colorado astronomer Roger N. Clark's 1990 book Visual Astronomy of the Deep Sky, some studies indicate that the eye can actually build up an image over time almost like photographic film - if the image is held perfectly still. In bright light the eye's integration time, or "exposure time," is only about 1/10 second. But in the dark, claims Clark, it's a different story. A faint image may build up toward visibility for as long as six seconds if you can keep it at the same spot on your retina for that long. Doing so is quite contrary to instinct, because in bright light fixating on something tends to make it less visible with time.

Long exposure times might possibly be one reason why an experienced observer sees deep-sky objects that a beginner misses. Perhaps the veteran has learned, unconsciously, when to keep the eye still. It also may help to explain why bodily comfort is so essential for seeing faint objects. Fatigue and muscle strain increase eye motion.


2) Avoiding Eye Fatigue

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.


3) Capturing Color

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.


4) Comfort

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.


5) Dark Adaptation

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.


6) Eye Miscellany

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.


7) Eye Patch

To ease dark adaptation, wear an eye patch over your observing eye while setting up. Put it on as long before the start of your observing session as possible and you will be awarded with a fully dark-adapted eye right at the beginning of your session. Move the patch to your non-observing eye while at the eyepiece so that you may keep open while observing. This relieves the strain on your eye muscles and improves observing.


8) Know your telescope

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.


9) Sky Brightness

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.


10) Using High Powers

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.)


11) Viewing faint objects

This is pretty widely known and used, but mostly with the Dobsonian-mounted telescopes. That is, MOVE THE TELESCOPE while observing really faint stuff. Just tap the telescope tube or grab the font of the telescope and give it a little jerk, to bring out faint objects into view.


12) Other Tips

  1. Every deep-sky observer, even those with computer-pointed telescopes, will appreciate highly detailed star charts such as those in Uranometria 2000.0 or the Millennium Star Atlas. If you know exactly where a faint deep-sky object is supposed to be located in your telescopic field of view, you will be able to detect objects about a magnitude fainter than you could otherwise see with certainty. That's about like increasing your telescope's aperture by 60 percent.

    When you pour all your concentration into examining a deep-sky object at the very limit of vision, does it get even harder to see after 10 or 15 seconds? Does the sky background brighten into a murky gray? Diagnosis: you're holding your breath without realizing it. Low oxygen kills night vision fast. An old variable-star observer's trick is to breathe heavily for 15 seconds or so before trying for the very dimmest targets. And keep breathing steadily while you're looking.

    Night vision also is impaired by alcohol, nicotine, and low blood sugar, so don't drink, smoke, or go hungry while deep-sky observing. Bring a snack. A shortage of vitamin A impairs night vision, but if you've already got enough of it, taking more won't do any good. Almost no one in the developed world manages to get vitamin-A deficiency any more. So don't expect carrot juice to improve your eyesight.
     

  2. Prolonged exposure to bright sunlight reduces your ability to dark-adapt for a couple of days, so wear dark glasses when spending much time outdoors. Make sure the label on the dark glasses says they block ultraviolet light (both UVA and UVB); some cheap ones don't. Over the years ultraviolet daylight (and maybe even bright visible daylight) ages both your eye lens and retina, reducing sensitivity and increasing the likelihood of degenerative diseases such as cataracts and macular degeneration. So if you wear ordinary eyeglasses outdoors, ask your optometrist to have an ultraviolet-filter coating applied to them. This option is so cheap and easy, and will reduce your lifetime UV exposure so much, that every eyeglass wearer ought to get it regardless of any immediate medical need.
     
  3. Most of all, be patient. If at first you don't see anything where a star cluster, nebula, or galaxy is supposed to be, keep looking. Then look some more. You'll be surprised at how much more of the scene glimmers into view with prolonged scrutiny - another faint little star here and there, and just possibly the object of your desire. After you glimpse your quarry once or twice, you'll glimpse it more and more often. After a few minutes you may be able to see it nearly continuously where at first you thought there was nothing but blank sky.
     
  4. You can be confident that your observing skills will improve with practice. Pushing your vision to its limit is a talent that can only be learned with time. "You must not expect to see at sight," wrote the 18th-century observer William Herschel, often considered the founder of modern astronomy. "Seeing is in some respects an art which must be learned. Many a night have I been practicing to see, and it would be strange if one did not acquire a certain dexterity by such constant practice."