Contents
1. Introduction
2. Equipment
3. Planning
4. Techniques
5. Visual art
6. References

5. The Art of Visual Observing

WHEN YOU HAVE successfully star-hopped to your target, don't expect to see right away everything it has to offer. The first look always shows less than comes out with continued inspection. The great observer Sir William Herschel wrote:

"You must not expect to see at sight . . . 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."

Acquiring such "dextrous vision" is one of the most valuable skills a deepsky observer learns. Deepsky observing has its own techniques, most of which are aimed at helping your eye see in near-total darkness. When viewing the Moon or planets, the telescope's main purpose is to magnify distant detail. Deepsky objects, on the other hand, depend on a telescope's light-collecting ability. They are not too small to be seen without optical aid, they are too dim.

5.1. The Eye and Dark Adaptation

TO MAXIMISE YOUR SENSITIVITY to dim light you should allow sufficient time for your eyes to get used to the dark. You must also allow for continued dark adaptation during the observing period. During the first minute, the eye's sensitivity increases ten fold. In 20 minutes it increases 6000 fold and forty minutes of dark adaptation increases sensitivity 25,000 times!
This means that a faint object which was overlooked in an early part of your observing run may be readily seen later, so don't expect to see faint objects at their best until at least a half hour into an observing session.

Note that light adaptation is much faster that dark adaptation, so even a brief exposure to light will destroy your night vision. It is also best to consistently avoid all light for as long as possible before observing. Prolonged exposure to bright sunlight reduces your ability to dark adapt for a couple of days; wear dark glasses at the beach. In the long run, ultraviolet light ages both the eye lens and retina, reducing sensitivity. So if you wear eyeglasses outdoors, ask your optometrist for UV-filtering lenses. Night vision is also impaired by alcohol, low oxygen and low blood sugar. So don't observe while drinking, smoking or fasting. In fact, nicotine retards dark adaptation to such an extent that even a single cigarette smoked half an hour before your eyes start dark adapting will slow down this process.

5.2. Sky Brightness

LIGHT POLLUTION is the most serious hazard the deepsky observer faces. Deepsky objects are extended sources of light, and their visibility is influenced by the contrast between the object and the background sky. Light pollution increases the brightness of the night sky and thus decreases the contrast between object and sky. A dark sky is even more important than a large telescope: a small instrument in the country will show faint clusters and galaxies better than a very large telescope in a city. If you have to live with light pollution, take pleasure in what can be seen.

The degree of light pollution is sometimes rated by determining the faintest star visible with the naked eye. With no light pollution, the limiting magnitude is usually assumed to be 6.5, though some people with exceptional vision can see fainter. Under such conditions, the sky is packed with stars, the Milky Way is a mass of swirling, jumbled detail and any clouds appear blacker than the sky itself. At a limiting magnitude of 5.5, clouds are brighter than the sky because they are lit from below. The Milky Way is still easily visible but far less detailed. At limiting magnitude 4.5, the Milky Way is barely detectable as a faint, nearly featureless band. City dwellers typically face a limit of 3.5. The Milky Way is completely invisible, and Epsilon Crucis, the fifth brightest star of Crux, lying just inside the cross, is invisible. Two of the four stars that make up the rectangle of Musca (south of Crux) are also gone. At magnitude 2.5, stars are very few and far between. Only three stars of Crux can be seen, and the whole of Musca disappears.

The good news is that you can see through light pollution. A pair of binoculars in the heart of Johannesburg show fainter stars than can be seen with the naked eye from Sutherland. A dark spot in the backyard of a suburban home is a convenient site. You'll find that if you observe late at night, the sky is slightly darker since more lights get turned out. Any nearby light, say a bothersome street light, can be shielded off by some sort of screen around the telescope. In pays to minimise the light from the bright sky that gets into your telescope. Extend the front of the tube with a long dewcap made from stiff blackened cardboard. On a reflector, place a ring of black paper around the main mirror, so that light from the ground doesn't come in around the mirror's edge. Stray light can be blocked off at the eyepiece by pulling a black T-shirt over the eyepiece-end of the telescope and then inserting your head in the neck of the garment. This "observing capsule" can be worn around the neck as a scarf when not at the eyepiece. Using the T-shirt in this fashion warms it slightly, which helps to prevent dewing of the eyepiece during use. Another basic strategy to combat light pollution is to penetrate it with high powers. For more details, see the section on magnification below.

"As stargazers we should practice what Lee Cains calls 'the serene art of visual observing.' We must learn to see with the mind as well as the eye. This means really examining and contemplating the varied scenes before us in the eyepiece. All deep-sky objects deserve at least 15 minutes of your time. Glancing at an object once it's found and then rushing to another and another is like reading only the Cliff's Notes of the world's great novels." - James Mullaney

5.3. Seeing and Transparency

AS MENTIONED ABOVE, there is more to a deepsky object than meets the eye at the first glance. This is to some extent caused by the atmosphere and also by the nature of human perception.

The quality of the telescopic image, as far as this depends upon the condition of the atmosphere, is known as astronomical seeing. When you view an object at high powers under average seeing conditions, the image shimmers and boils. The degree of disturbance can be estimated using the Antoniadi scale. The severity of this shimmering changes from night to night and sometimes from minute to minute. When the seeing is poor, a small telescope will show twinkling stars which jump about playfully, but with a large aperture telescope, the lateral motions will be averaged out resulting in a steady blob.

When a deep blue, breezy afternoon turns to a dark and clear night, we have a night of high transparency. The dark sky and high contrast afford ideal conditions for viewing faint stars and extended objects such as galaxies and nebulae. High transparency and good seeing usually avoid one another. The hazy summer doldrums often produce the best seeing and are excellent for revealing double stars and planetary details.

5.4. Prolonged Observing

THE NATURE OF HUMAN perception plays a significant role in deepsky observing. In our day-to-day experience of the world we are used to seeing things easily. If something can't quite be made out, our natural reaction is to move closer. But this is impossible in astronomy. Instead, we have to get everything we can out of very distant views. This means learning new visual skills that involve active, concentrated effort. As you watch an object quiver and churn in the eyepiece, unsuspected detail will flicker into view during brief moments of stability, only to fade out for a while before being glimpsed again. The image of a difficult object builds up rather slowly. First one detail is notice and fixed, and you think there's nothing more to be seen. But after a few minutes another detail becomes evident, then another. The skilled observer learns to remember these good moments and ignore the rest, building up a gradual, integrated picture of the object.

Related to this is the fact that the eye, like a camera, can build up an image over time -- according to skilled observer Roger N Clarke. It has been found experimentally that a faint image will build up towards visibility for as long as six seconds. This may seem counter- intuitive, but bear in mind that most of your visual experiences have been in bright light; under these conditions the eye's "exposure time" is only about 1/10th of a second. Furthermore, fixating on an object in daylight tends to make it less visible. In fact, if the eye is held completely stationary, it becomes completely unable to see anything! In the dark, however, things are different.

To make use of the eye's extended viewing capacity, you will need to keep the image at the same spot on your retina; this helps explain why bodily comfort is so essential for viewing faint objects. Fatigue and muscle strain increase random eye movement. This does not, however, mean you have to stare at the object. It is the physically non- tense but mentally alert approach that succeeds on faint objects. If you use your right eye to observe, don't close your left eye tightly. This places unnecessary strain upon the eye. Keep it open and wear a eye-patch or cover your eye with a cupped hand.

5.5. Averted Vision

WHILE KEEPING THE IMAGE on the same spot on the retina helps the image to build up, looking directly at it will probably cause it to disappear! This is so because the light then falls on the fovea centralis, a region packed with bright-light receptors (cone cells) but fairly poor in dim-light receptors (rod cells). The rods are concentrated around the edges of the retina. By looking slightly away from a faint object, its light falls onto the edge of the retina where it is picked up by the sensitive rod cells. This very important technique is known as averted vision. Your eye is most sensitive to a faint object when its image lies 8 to 16 degrees from the centre of your vision in the direction of your nose. Almost as good a position is 6 to 12 degrees above your centre of view. Never place the object to the right of centre in your right eye, or left in your left eye — the image is likely to fall on the retina's blind spot and vanish altogether. See Inset 1 for a striking demonstration. Incidentally, averted vision is not the way to look for colour in deepsky objects. The rod cells do not respond to colour, whereas the cone cells do. You should thus look directly at the object when examining it for colour.

5.6. Sketching

AN EXCELLENT WAY to train yourself to see better is to make sketches. These don't have to be works of art; the idea is to record details more conveniently than through words. An open cluster requires no artistic talent whatsoever. To give you some indoor practice, try making sketch copies of photos of open clusters. You may want to enlarge the photo with a photocopier and then sketching it from a distance.

When you sketch at the telescope, remember to note down the date and time, instrument details, sky condition and the size of the field of view. Also indicate on the sketch where north and east are. It is strongly recommended that you sketch as much as possible. While you are making the sketch, you are continually examining the object, paying close attention to certain smaller details. This close scrutiny often results in the discovery of hitherto unseen features. A sketch also serves as an excellent record of the object you are studying. Detailed objects require lengthy descriptions that may become confusing when read later. We all know the saying that a picture is worth a thousand words.

5.7. Choice of Magnification

WHEN CHOOSING the "best" magnification for an object, you must bear in mind that the eye has very poor resolution in dim light. In bright light, the eye can resolve detail finer than 1 arc minute, but can't make out features smaller than 20 arc minutes when the illumination is about as dim as the dark-sky background in a telescope. This means that details in a very faint object can be seen only if they are magnified sufficiently. While a low-power eyepiece concentrates a faint extended object's light and increases its apparent surface brightness (the illumination of a given area on the retina), it does not enlarge it sufficiently for clear resolution. Unlike a star, an extended source such as a galaxy or nebula will grow dimmer as the magnification is increased. Such an object's surface brightness is proportional to the area of the exit pupil. Thus, an object viewed with an exit pupil 1mm in diameter has only 2 percent of the surface brightness is has with a 7mm exit pupil.

As magnification is increased, the sky background grows dimmer at the same rate that the object does, so the contrast remains the same. But with higher magnifications, delicate structure is larger and hence more visible. Faint stars are best seen at high magnification since the star's image remains constant while the background grows dimmer, improving contrast. What all this means is that it is 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.

"Most of all,
practice. There's
no other way
to master
deep-sky
observing.
And don't
quit on any
object, no
matter how
vague it may
look, until
you've given
it a good,
long, thorough
scrutiny."
--MacRobert

5.8. Practice Makes Perfect

PRACTICE IS THE ONLY sure-fire way to improve your skills as a deepsky observer. Don't give up on an object, no matter how vague it may look. Have another go. Consider this passage from The Amateur Astronomer's Handbook by James Muirden:

"No opportunity should be lost to train the eye to work with the telescope; to observe the same object with different powers so as to see the effect of magnification; to try to see faint stars; and to draw planetary markings. In the beginning, to be sure, this may all seem to be wasted effort; the observing book will fill up with valueless sketches and brief notes of failure. But this apparently empty labour is absolutely essential; for, as the weeks pass, a steady change will be taking place.
Objects considered difficult or impossible to see will now be discerned at first glance, and fainter specters will have taken their place. Indeed, these former features will now be so glaringly obvious that the observer may suppose that some radical improvement has occurred in the observing conditions. But the credit belongs entirely to the eye . ."

"The secret of
success is
constancy to
purpose."
-- Disraeli