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Deepsky Observer's Companion tutorial
Part 5: The art of visual observing

In this section: dark adaptation, sky brightness, seeing and transparency, prolonged observing, averted vision, sketching, magnification, practice makes perfect!

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."

Keep observing. You will get better with practice. Your brain learns to see more as you do more observing.

Alan MacRobert

Acquiring such "dextrous vision" is one of the most valuable skills a deep sky observer learns.

Deep sky 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. Deep sky 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.

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 6,000 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.

(The next section, Astronomy and vision, discusses dark adaptation and other physiological factors in greater detail).

Sky brightness

Light pollution is the most serious hazard the deep sky observer faces. Deep sky 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.

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

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 a big city shows 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.

Seeing and transparency

As mentioned above, there is more to a deep sky 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.

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.

Alan MacRobert

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.

Prolonged observing

The nature of human perception plays a significant role in deep sky observing. In our day-to-day experience of the world we are accustomed 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 noticed 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. Clark. 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 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.

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° from the centre of your vision in the direction of your nose. Almost as good a position is 6 to 12° 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.

Incidentally, averted vision is not the way to look for colour in deep sky 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.

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 try sketching it from a distance.

As with most endeavours, skill and hard work produce quality results ... Good drawings do not require special artistic talent or experience, but they do demand close attention, much time at the telescope, much time redrawing… and honesty in not recording details remembered from photographs but not positively seen.

Alan MacRobert

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 (see Appendix 9). 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.

A deep sky drawing conveys precisely what the mind sees, and is an expression of the observer's experience. A sketch is the result of the eye's amazing range of sensitivity, and the mind's powerful ability to integrate and interpret. It depicts what is seen through the eyepiece – something that photos and CCDs can never do.

Why sketch?
A drawing of a deep sky object makes a personal record of what was seen by the observer, and so is satisfying as an end in itself. However, drawing also forces the observer to look for more detail and, in time, this will develop a trained eye that will be useful in all types of observing (Macdonald, 1993).

Tom Polakis says:

"Drawing what you see through a telescope is a good way to document subtle details. By comparing renditions made on different nights, you can look for changes due to sky conditions or your growing ability as an observer."

Roger Clark writes:

"It's often said that a picture is worth a thousand words. Composing a thousand words takes much time and thought and still may leave the reader with the wrong mental image. So when it comes to accurate recording of scientific data, there's often no substitute for a picture.

In past centuries a scientist was necessarily a draftsman. Nowadays scientists in almost all fields rely on photography to record images, and the pencil and sketchpad no longer rank as essential scientific tools. Visual astronomy, however, remains an exception."

Michael Sweetman of Tucson, Arizona, wrote in Astronomy's Letters column:

"I've been drawing objects since I began observing, and I draw every time I go to the telescope. I approach my drawings as scientific illustrations. When you're preparing for a drawing, proper eyepiece and magnification selection are important… Too little power and the delineation of details can't be made out, too much power, and contrast and form in extended objects like nebulae are lost. A realistic drawing also requires proper paper selection and drawing technique, refined from hours of experience at the telescope and practice away from the telescope… As with most endeavours, skill and hard work produce quality results."

Sketching equipment
If good observations are to be made, it is essential that the observer is comfortable and relaxed. An observing chair of some sort should be used at the eyepiece. A clipboard is handy for holding the rest of your drawing equipment, including your red observing light - I use a red mini-reading-lamp, with the bulb replaced by two super-bright LED's, to throw an even light over the sketching area.

The drawing paper should be illuminated by a dim red light, since a white light will destroy too much of the observer's night vision which is so essential for visual deep sky observing. Macdonald (1993) suggests using an A5-sized hardcover sketch book, the paper of which should be high-quality cartridge paper. I use 80g bond laser-printer paper, cut in half into an A5 size. Before the page is divided, a checklist and circle is laserprinted. The circle on the sketch pad represents your field of view in the eyepiece. Macdonald suggests a 50mm diameter circle; Mitchell 75-100mm; I prefer 60mm.

You'll need a pen to plot stars. Mitchell (1995) is of the opinion that "a pencil works better than a pen because a pen cannot convey gradations of brightness." While a normal ball-point or fountain pen certainly won't work, a fine-point felt-tip pen (koki-pens, Sharpies (?)) is perfect. They make exquisitely small dots with their tiny fibre tips. For brighter stars, slightly more pressure on the pen produces a larger blob. For really bright stars, either use a thicker tipped pen, or colour in a little circle. Of course, bright stars remain point sources, so don't draw them as large disks; rather as bloated spots.

A soft-leaded pencil or charcoal stick (sold at art supply shops) and an eraser is also on your shopping list. Only one type of pencil is used at the eyepiece, says Macdonald; different grades of pencil may be used to enhance the drawings indoors later on. He writes that the "ideal sketching pencil is the Ebony pencil (#6325) made by Eberhard Faber. It is coarse and soft enough to let you use just about any sketching technique you want."

Finally, you'll need a "smudge stick" or cottonwool earbuds to smudge portions of the drawing. This creates the impression of haziness and testifies to the fact that there are few straight edges and neat boundaries to deep sky objects.

Drawing techniques
Before you put pencil to paper, study the object intently. Try different eyepieces to see the most detail; use various filters to enhance contrast. Use averted vision to pick out the fainter detail, letting the overall impression build up in your mind. David Coleman (1994), commenting on sketching Mars, notes "I began each drawing session by not drawing the planet! It was important for me to spend a quiet 15 or 20 minutes carefully observing. It takes practice and patience to train your eye to pick up faint detail, so try not to rush right into drawing."

Every time I sketch, I'm impressed by how much detail I would have missed had I just looked at the object for a short while and noted a description. Fainter stars and subtle detail is revealed through extended observing. Studying the object with intense concentration and averted vision, says Mitchell, reveals more and more detail. When you do start sketching, draw only what you see.

The first step in sketching is to plot the positions of the brightest stars in the field of view. These stars serve as markers that keep the drawing's proportion correct. Start by plotting a prominent star in accurate relation to the field of view circle. Now plot a second one at the appropriate distance, and angle. Work from the outside, inwards. Examine the field, and pay close attention to other stars that make distinctive triangles with the two already drawn. Select a star that makes a recognisable shape, and add it in. Continue in this way by making triangles, or extended lines, or even rectangles, with new stars. In this way, a framework is erected within which fainter stars may be filled in.

Drawing in the bright skeleton of stars should be done quickly. Spend more time imprinting the image in the mind than staring at your sketchpad. "While dividing attention between the eyepiece and sketchpad, preserve as much night vision as possible by keeping your red light subdued. Limit exposure to light by spending most of your time studying the object, and then draw bits of remembered detail in short bursts." Once you've selected a spot to position a star, see if there are other triangles in which it is also involved, that can confirm its position.

If your initial framework is not accurate, rather start again. If, as you get on, you plot a star in the wrong place, make sure you correct it. Because I use a pen to plot stars, it's not a simple matter of erasing. Instead, I place the tip of the pen on the offending star, and draw a short (<1mm) tick away from it. When the drawing is retouched indoors, there stars are removed. A note in the margin can also draw attention to any alterations as needed. Continue plotting the fainter stars, in relation to the brighter ones, until you've added all the stars you can see.

With the stars in place, sketch the major details of the object, capturing the general shape. Mitchell says: "This later serves as a template when it comes time to fill in any subtle detail in the object's shape." I advise that you do this very lightly; often, as you continue observing the object, this overall impression changes, especially on complex objects. This again emphasises that prolonged observing shows detail not seen in the initial scrutiny.

With the basics recorded, refine the sketch by adding details: the glittering of stars resolved in globulars, dark dust lanes in galaxies, and so on. Each type of object has a slightly different approach.

Open clusters are my favourite sketching target. Accurate placement of stars is vital, as is the faithful rendering of their brightnesses. Slowly build up the image, working from the outside inwards, using triangles and lines to position the stars. If there are an overwhelming number of stars, slightly defocus the eyepiece, which hides the fainter clutter. Then refocus to fill in the fainter members. Open clusters, by the way, respond well to moonlight. While the brightened night sky drowns out fainter deep sky objects, many star clusters can be seen reasonably well. Take advantage of a clear but moonlit light to prepare sketches of open clusters. Fainter stellar members can be added in on a dark night.

Globular clusters can be a real challenge, especially for larger telescopes. Start by drawing the core dark, and the outer regions in successively fainter layers of pencil - say two or three separate layers. This should give a zoned or tree-ring appearance to the sketch, but this is eliminated by careful smudging, either with a smudge stick or an earbud. This creates a realistic nebulous effect, if you make sure the edges fade naturally with no discernible edge. Be careful that you don't inadvertently increase the size of the object with too much smudging; rather start out slightly smaller and build up the correct size with repeated pencilling and smudging. The shading should as accurately as possible reflect the brightness profile of the object; does it brighten suddenly or gradually; is the brightening slight or marked?

To round off the drawing, add stars that are involved in, or very close to, the cluster. The resolved stars should be added in from the cluster edge, working inwards. Of course, in the case of a well resolved, rich globular cluster (say Omega Centauri in a 15-inch) it's not a good idea to accurately plot every star; simply create the general impression. Mitchell cautions, however, that you shouldn't "get carried away and resort to madly peppering the cluster with stars at random."

Galaxies are drawn in much the same way as globular clusters, starting with the darker central area (e.g. an elongated bar), working outward. Successive smudging defines the outer reaches of the galaxy, while an eraser is used to indicate obscuring dust lanes.

Planetary nebulae need a different approach. Many planetaries have well-defined disks that don't need smudging. Whether its small and bright, or large and faint, first sketch in the outline of the disk. Then fill in the centre so that the nebula becomes a smooth disk. Some planetaries, however, are diffuse, and their disks need to be slightly smudged.

Diffuse nebulae are probably the most difficult. They are often so faint that smudged pencil creates too strong an image. Macdonald suggests you rub your forefinger or cotton bud with the pencil until it is coated with a fine layer of lead. I prefer to rub the pencil a number of times on a scrap part of the paper. When it is well-coated with lead, I then load a cotton bud by drawing it over this "lead palette". Use this coated cotton bud to draw the shape of the nebula. Brighter portions may be enhanced by smudging with the finger.

Dark nebulae can be captured with the same approach, although some of them have well-defined borders and are thus more like planetary nebulae. Since some of these nebulae are extremely large, a rich-field telescope, or large binoculars, show them better. Such a wide field, however, often includes a great number of brighter stars, needing a longer time to sketch accurately. I prefer preparing the star-field beforehand, by printing out on an A4 sheet an unlabelled starmap, down to say 8th magnitude. At the eyepiece, the dark nebulae are then filled in on this framework; the idea, after all, is to sketch the nebula, not the background stars.

Finishing the drawing
As you study the field, notice at which edge of the eyepiece the stars appear to move out. Indicate this position on your sketch - this is west. East is on the opposite side, of course. To indicate north, turn the sketch so that east is pointing upward. If you are using a Newtonian, which has two mirrors, north is to the right. If you have a one-mirror system, like a refractor with a star-diagonal, north is to the left. Don't forget to also record the date (and time), instrument and eyepieces used, and the observing conditions which may influence the quality of the drawing. Also see the section on using an Observing Checklist for details.

Preparing the final version
Your field sketches are not supposed to be finished works of art, but rather rough drafts. For complex objects, you'll probably have made several drawings. When you've completed an evenings sketching, return indoors to prepare a better rendition of your work, under normal lighting conditions. Combine the rough sketches into a composite version and make any corrections that you noted.

When you next observe, take your sketch out to the telescope for a moment of truth. Compare it to the view in the eyepiece, looking for and noting any inaccuracies. In this way, you can ensure maximum fidelity in your final sketch. Your final sketch should readily show the casual viewer what the skilled observer was able to discern only with time and effort at the eyepiece.

Reproducing and displaying sketches
McDonald suggests redrawing your sketches if they are to be displayed or sent to other observers or observing sections. He redraws the sketch, enlarging it by representing the field of view with a 100mm circle. "This time," he notes, "different grades of pencil are used to highlight different features. For example, a 4B (very soft) pencil is used for the cores of galaxies or very bright planetary nebulae, and an HB for faint nebulosity. It should be remembered, however, that the relevant positions and brightnesses of the stars and nebulae must be the same as in the original drawing. Otherwise, the drawing will lose its accuracy." I much prefer to do the reproduction digitally, by scanning in the final drawing into a computer graphics file. Open clusters can be scanned in and retouched with minimum effort. Nebulous objects require a certain amount of knowledge of graphics editing software to deal with properly. This creates a permanent record with all the benefits of a digital document. If it is necessary to make a hand-drawn copy, photocopy your original, perhaps enlarging it as necessary. Nebulae, which almost always reproduce badly, can now be touched up as discussed above.

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 one 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 two 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 the other.

Practice makes perfect

Practice is the only sure-fire way to improve your skills as a deep sky 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.."

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