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Deepsky Observer's Companion tutorial
Part 2: Observing equipment

In this section: binoculars, telescopes, star atlases, miscellaneous equipment.


"Start with binoculars. Ease of use, cost and performance make binoculars the ideal 'first telescope'."

Alan MacRobert

It is a misconception that a large telescope under dark pristine skies is needed to observe the deep sky. Nothing is further from the truth: a pair of binoculars can keep you happily occupied for years. Binocular vision, i.e. using both eyes, has several advantages over monocular vision: faint objects are slightly more easily seen, fine detail is more quickly (if not better) seen, and it is less tiring on the eye. Compared to a telescope, the binocular offers a wider field of view. Beginners are strongly advised to spend a few months exploring the sky with binoculars, getting to know the constellations and generally scouting the terrain before graduating to the telescope.

In selecting a pair of binoculars for astronomical work, one of the most important factors is the magnifying power of the glasses. Low-power glasses are ideal for wide-field views of the Milky Way, but higher-power glasses are preferred for the Moon, Sun and planets. Bear in mind that as magnification power increases, so does the binoculars' weight. A pair of 7x glasses can be hand-held comfortably for an extended period, but 10x binoculars will need some kind of support during viewing.

The second binocular specification refers to its aperture, i.e. the diameter of the binocular lens. This can range from 30mm to 80mm or more. The larger the aperture, the more light the instrument can collect.

Incoming light is focused by the main lens and the image is then magnified by the eyepiece. The final image is presented to your eye as a round disk of light known as the exit pupil. The diameter of the exit pupil can be found by dividing the aperture by the magnification; a 7x35 glass has a 5mm exit pupil (35mm 7 = 5mm) across; a pair of 11x80's has an exit pupil with a diameter of 7.3mm. The human eye has an opening, or pupil size, of about 2.5mm in bright daylight and about 7mm in total darkness.

When you use a pair of binoculars for astronomy, the size of the exit pupil must match your own pupil size. If the exit pupil is larger, then all the light that is collected is not being passed into your eye and goes wasted. If the exit pupil is smaller than your eye's pupil, then you are wasting some of the eye's light-gathering capability.

In choosing the combination of magnification and aperture, you should consider the conditions at your observing site. If you observe under dark rural skies, your pupils will fully dilate to around 7mm, and the binoculars should match this; suitable examples include 7x50, 10x70 and 11x80 glasses. If your site suffers from light pollution, your pupils will dilate to only 5- or 6-mm, in which case 7x35 or 10x50 glasses would be better suited. Table 1 below, from Phil Harrington's excellent Touring the Universe Through Binoculars, compares some popular binocular configurations.

Table 1: Popular binocular configurations.

7x35Easily hand-held; excellent widefield views of Milky Way and deep sky objects.Smaller exit-pupil restricts dark sky effectiveness.
7x50Easily hand-held; light-gathering ability sufficient for hundreds of objects; best choice.Larger aperture may cause skyglow problems in urban and suburban areas.
10x50Good choice for urban and suburban users who want a little higher magnification.May require a tripod.
10x70Excellent for star clusters, nebulae and galaxies.Heavy; tripod usually needed.
11x80Excellent for faint objects.Tripod needed; heavy.


Telescopes are available in two basic types; mirror (reflector) or lens (refractor) designs. The size of a telescope is specified by the diameter of its largest optics (primary mirror or objective), traditionally expressed in inches.

Amateur reflectors typically have mirrors of 4- to 12-inches (10 to 30cm) in diameter. A popular line of telescopes in the United States sports a 17.5-inch (44cm) mirror, and even 25-inch (63cm) mirrors are available commercially. Refractors, on the other hand, typically have 2- to 5-inch (5 to 13cm) lenses. The main reason for the popularity of the reflector is its cost: a 6-inch refractor could sell for R 40,000 while a 6-inch reflector can be as inexpensive as R 2,500! Other considerations, such as size and transportability, also favour the reflector.

"Most objects within reach of any telescope, no matter how large or small it is, are barely within reach… If flashy visuals are what you're after, go watch TV."

Alan MacRobert

Bigger is better in deep sky observing. The larger the objective (primary mirror or lens), the more light can be collected. But a large telescope used in the city can never show the faint objects that can be seen with a smaller telescope used under dark skies. And a large telescope that is too cumbersome to use will remain in the garage or loft, collecting dust and not starlight. Buy what you can afford, but make sure it gets used. (It is, of course, entirely possible to grind your own mirror and assemble the telescope yourself. But that's another story.)

Telescopes can be mounted in two basic ways. The simplest mount is known as the alt-azimuth configuration. The telescope moves around two axes, up-down and left-right, with the whole mounting standing level with the ground. Binoculars mounted on a tripod is an example of an alt-azimuth mounting.

Nearly all professional telescopes are mounted in an equatorial configuration. The telescope still moves around two axes, but the whole mounting is now tilted upwards, pointing to the celestial pole of the sky. Mounted in this fashion, it now swings east-west and north-south. If a motor is attached to the mount, the telescope can be made to turn slowly towards the east at the same rate that the stars move westward. In this manner, a star remains in view, with the telescope tracking it continually. This type of mounting has the advantage that its axes can be calibrated in right ascension and declination, the co-ordinate system used to describe an object's position in the night sky. If the mounting is accurately set up, you can find an object by merely turning the telescope until the setting circles point to the object's co-ordinates. On the down side, aligning the telescope properly with the celestial pole can be tricky, and this type of mount is also more expensive than the alt-azimuth configuration. For the deep sky observer, setting circles are useful but not essential.

The important issue when considering a mounting is its stability. A stable mount does not leave the telescope wobbling after you bump it. It is most unpleasant if the image bounces around each time you touch the telescope to adjust the focus.

Star atlases

All navigators, no matter how experienced, rely on maps to guide them. As captain of your telescope, you will need a good star atlas to find your way about the night sky. It is one of the most important observing aids for the astronomer.

Many introductory astronomy books contain star maps for the beginner, some good, some not so good.

Available on this website, yours for the taking, are several free star chart collections: for the beginner, I'd recommend the Southern Star Wheel planisphere and the Discover! Workbook for learning the constellations. When you're ready for the next step, the set of ConCards are highly recommended. You may also find the Deep Sky Explorer Atlas (south and equatorial) useful.

"Fine maps bring the fascination of hunting out faint secrets in hidden sky realms."

Alan MacRobert

An excellent set of charts can be found in the Atlas of the Night Sky, published in 1984 by Crescent Books. These charts, drawn by master cartographer Wil Tirion, show stars as faint as 6th magnitude, which is the faintest you can see with the naked eye from a good suburban site. Many deep sky objects are plotted, and variable stars brighter than 6th magnitude at maximum are shown. Notable objects are listed beside each pair of charts. These charts are exceptional because the whole night sky is shown on only five A3-sized pages. They are excellent for use with binoculars, and their handy size make them easy to transport and use. For the beginner, these charts are arguably the best available, since they accurately summarize the night sky into clear, understandable sections.

The more experienced telescopic observer will need charts that show fainter stars and more deep sky objects. The Uranometria 2000.0 atlas, first published in 1987 by Willmann-Bell, is an excellent star atlas. A two-volume set, it contains 473 computer-generated charts, showing 330 000 stars down to magnitude 9.5. The complete Revised New General Catalogue (RNGC) and an additional 10 300 deep sky objects are shown. Each volume has a two-page hemisphere starmap which indexes the charts in that volume, making it easy to find the chart you need. The index maps are also very useful to help you orient yourself when you start your observing session.

The current 'Rolls Royce' of printed star atlases is the Millenium Star Atlas. Even though it is significantly larger (and more bulky) than the Uranometria, it shows much fainter stars (down to 11th visual magnitude) and covers the sky in 1 548 charts!

Miscellaneous equipment

Deep sky observing is done in the dark. Well, almost. You will need some light to read the star charts, make notes and find your mug of hot coffee. A normal torch, useful during a power cut, is unusable at the telescope. Astronomers traditionally use a dim red flashlight because red light has less effect on night vision.

It is easy to make your own night-vision torch. Simply place sufficient layers of dark red cellophane (used for gift wrapping) in front of the torch, thereby cutting down and filtering the light. I slip a red water balloon over the layers of cellophane to hold them in place. You can also paint the bulb with red nail polish for a more permanent arrangement. As a guideline, if you can see the torch light in daylight, then its still too bright.

"If the hours we spend under the stars are precious, an observing log helps us remember them. Relying on memory alone just isn't good enough; as years pass, details fade away until events might as well not have happened."

David H. Levy

Because of the notorious nature of human memory, you will need some way of recording your observations. At the telescope, a pencil and writing pad on a clipboard works well, as does a Dictaphone or tape recorder. I find using a small Dictaphone easier than writing descriptions by hand, partially because my handwriting is illegible but mainly because it is more convenient and allows greater freedom of expression. For sketching, a dark pencil, eraser and clipboard are useful, as well as sheets of paper with predrawn circles representing the field of view. Whichever record keeping method you choose, you should note the date and time of the observation, where the observation was made and with which instrument and the magnification used. Record the sky conditions (seeing and transparency) and also include comments on clouds, haze or the presence of moonlight. Many observers design a standard observing form, which can be filled in at the telescope (see Appendix 9 for an example). This ensures you don't forget some important detail. After the observing session, your rough observing notes should be edited and transferred to a standard record keeping system, which will map your progress as a deep sky observer.

Remember to dress appropriately for your observing session. Jeans and a T-shirt might be fine for warm summer nights, but cold winter nights demand a wiser choice. Jeans don't keep my legs warm enough, so I wear a pair of long-johns underneath. Two tracksuit pants (instead of the jeans) also work well. A sensible jacket, with several pockets, can be worn over a jersey and shirt to keep the upper part of your body warm. Wear two pairs of woolly socks and make sure your shoes or boots provide insulation, so that your feet don't leak heat to the ground. Most important of all, protect your extremities. Your nose, ears and fingers will loose heat more quickly. A balaclava and scarf will serve admirably, and a pair of warm, non-bulky mittens will keep your fingers agile and comfortable.

Observers often overlook the fact that they will be spending a considerable amount of time sitting at the eyepiece. Make sure your chair is comfortable and supports you in the right places. Bear in mind that the eyepiece will not be at a constant height throughout the night; some form of adjustable chair may be needed. When observing with my binoculars, I use a plastic milk crate (either on end or on its side) and various cushions for fine adjustment. Be careful not to strain your back or neck when observing - you grow tired much sooner and your overall performance is adversely affected.

Don't forget to pack in some snacks. Something hot to drink after you have just found that elusive galaxy adds to the reward. A sandwich, or some chocolate adds a little extra boost during your vigil at the eyepiece.

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