18Jun2017

Secrets of Deep-Sky Observing

Night Sky-2

You’re sure that you’ve finally got your telescope aimed at the position of the object of your desire. You’re keen as mustard to get started.

The crosshairs of your finder are on its exact location according to your star map. Now what can you hope to see? If it’s a bright star it will be obvious and beautiful but contain no detail. A star as seen in a telescope is a tiny blaze of brilliant light looking about the same as a star does to the naked eye, only brighter.

More interesting but generally more difficult are “deep-sky objects.” This term covers the vast variety of nebulae, star clusters, galaxies, and anything else beyond the solar system that appears extended: having a visible size, rather than just being a starlike point. Many hundreds of these ghostly glows and subtle spatterings are within reach of a modest telescope.

Once you’re precisely aimed you may see, with luck, a very dim, shapeless, glowing smudge floating among the stars. While finding it may bring a thrill of accomplishment, many novices are let down by the sight. “Is that all there is…to galaxies? It’s nothing like the pictures in the books!”

You’ve just come up against the fact that the human eye cannot perform as well as a camera does at very low light levels. We are daytime animals that evolved in the skirts of a blazing sun; our eyes are not well designed for the dark of night and space. Your real-life view of a galaxy will never match the spectacular photos so common in books and magazines. But here lies the challenge. Many deep-sky objects do show a surprising wealth of detail when studied long and well even with the eyes nature gave you.

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A telescope’s ability to collect light depends on the size of the objective lens, which is used to gather and focus light from a narrow region of sky.

A telescope serves a different function on deep-sky objects than on the Moon, planets, or scenes on Earth. In those cases, its main purpose is to magnify distant detail. With deep-sky objects, on the other hand, a telescope’s main purpose is to collect a lot of light for your less-than-sensitive eye. The issue is not that the objects are too small to see without optical aid. It’s that they’re too dim.

Accordingly, deep-sky observing involves its own techniques. All are aimed at helping the eye to see in near-total darkness. Here are some pointers.

Sky brightness

The single most important factor in deep-sky observing is light pollution. Its worst effect is on dim, extended objects of just the sort we’re considering. 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. If you live in a badly light-polluted area, take pleasure in what you can see through the skyglow–but don’t blame yourself or your telescope for mediocre results. Plan to bring the telescope on getaways to the country.

Dark adaptation

The eye takes time to adjust to the dark. Your eyes’ pupils expand to nearly their full nighttime size within seconds of when you step out into the dark, but the most important part of dark adaptation involves chemical changes in the retina that require many minutes.

After the first 15 minutes in total darkness you might think you’re night vision is fully developed, but no. Tests show that your eyes gain about another two magnitudes of sensitivity — in other words, a factor of six in how faint you can see — 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 a dim red flashlight because red light has less effect on night vision. The reason is that in near-darkness you see with the “rod” cells in your retina, and these are blind to the far red end of the spectrum. When you see red light your “cone” cells are at work; these are the receptors 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.) The idea is to use the red cones for reading charts and swapping eyepieces, while protecting the rods for the most delicate work at the eyepiece.

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Red paper rubber-banded over the front of a flashlight provides a dim, diffuse glow. In a two-battery flashlight, install a bulb rated for three or four batteries. Its light will be dim and somewhat reddened, and the batteries will last longer.

Much better than the traditional flashlight and red filter, however, is a red LED (light-emitting diode) flashlight. Its red is purer and deeper, so the division between rod and cone vision is more sharply drawn. LEDs also use much less current, so the batteries last for years. Many LED flashlights for astronomers are now available. Or see the article “Make Your Own Red LED Light.” FRED

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.

Averted vision. When you look directly at something, its image falls on the fovea centralis of your retina. This spot is packed with bright-light receptors, the cone cells, and gives 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 off the fovea and onto parts of the retina that have more rod cells.

To see dramatically how this works, stare right at a star. It will disappear. Look away a little; there it is again.

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

Your eye is most sensitive to a faint object when it lies 8° to 16° from the center of vision in the direction of your nose. Almost as good a position is 6° to 12° above your center of view. Avoid placing the object very far on the ear side of your center of vision. There it may to fall on the retina’s blind spot 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 resolving power, the ability to see details.

Wiggling the scope

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 by averted vision. When the wiggling stops it disappears again into the vague uncertainty of the sky background.

But under other conditions, especially involving faint objects that appear tiny, just the opposite technique may work. According to Colorado astronomer Roger N. Clark in his 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 0.1 second. But in the dark, claims Clark, it’s a different story.

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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; 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.

Using high powers. Conventional wisdom holds that low power works best for deep-sky viewing. After all, low power concentrates an extended object’s light into a small area and thus increases its apparent surface brightness (the illumination of a given area on the retina). But as Clark proved after digging through laboratory vision studies, this assumption is usually false. High powers should do better on many faint deep-sky objects. The reason is subtle but key to understanding how low-light vision works, so we’ll go into some detail.

The essential point is that the eye, unlike a camera or other purely mechanical lens system, loses resolution in dim light. This is why you can’t read a newspaper at night — even through you can see the newspaper and your eye lens theoretically resolves all the letters just as sharply as in daylight.

Studies show that the eye can resolve detail as fine as 1 arc minute in bright light but can’t make out features smaller than about 20 or 30 minutes wide when the illumination is about as dim as the dark-sky background in a telescope. 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 to this large an apparent size–which can require using extremely high power!

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The explanation lies in how nature has adapted the visual system to cope with night. Photographic film records light passively, but the nerve system in the retina 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 grow weaker. But the total number of photons of light entering the eye remains the same. (A photon is the fundamental particle of light. Most people can detect as few as 50 to 150 photons per second entering the eye.) 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. 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 they’re easier to manufacture.)

Colour

Deep-sky objects sometimes disappoint beginners not only by their frequent lack of obvious detail, but also by the absence of the brilliant colours recorded in photographs. In order to see colour, we must view something with a surface brightness great enough to stimulate the retina’s cone cells, and the list of deep-sky objects this bright is short.

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The great Orion Nebula M42 qualifies (some people can make out the pastel yellow or orange in parts of its brightest region), 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.

Heavy breathing

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 while the sky background brightens a little 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.

Other Tips

Night vision 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. Virtually no one in the developed world manages to get vitamin-A deficiency any more. So don’t expect eating carrots to improve your eyesight.

Prolonged exposure to bright sunlight reduces your ability to dark-adapt for a couple of days, so wear dark glasses at the beach. Make sure the label on the dark glasses says they block ultraviolet light (UVA and UVB); some cheap ones don’t. Over the years ultraviolet daylight ages both your eye lens and retina, reducing sensitivity and increasing the likelihood of degenerative diseases. So if you wear eyeglasses outdoors, ask your optometrist to have an ultraviolet-filter coating applied to your glasses. This option is so cheap and easy that everyone buying glasses ought to get it regardless of any immediate medical need.

Take your time. Most of all, be patient. If at first you don’t see anything at the correct spot, keep looking. Then look some more. You’ll be surprised at how much more 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 — what astronomers call “steadily holding” an object. This where you thought at first there was nothing but blank sky.

You can be sure 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.” Source: Alan MacRobert via Astronomy Education

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