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 Choosing A Telescope - Part 1

By Adrian Ashford

This is an exciting time in which to become an amateur astronomer. Never before have novice stargazers been presented with such a vast array of telescopes and accessories to pursue their hobby. Naturally, this brings the burden of choice: a bewildering variety of instruments makes it difficult for the uninformed consumer to make the right decision.

Whether you're seriously considering buying that first telescope or just daydreaming about it, this on-line guide will help you narrow your options. First we'll explore the variety of telescopes available, and then we'll discuss some key features — the size of the primary lens or mirror, type of mount, portability, computerisation, and accessories. We'll also look at the tradeoffs, because every instrument has its advantages and disadvantages.

But before you buy anything, you must determine what's important to you. What do you want to look at, where will you observe from, how experienced an observer are you, and how much to you want to spend? Answer these key questions, familiarise yourself with what's currently on the market, and you'll be well on your way toward acquiring a scope that will satisfy you for years to come.

Before examining the different telescopes available, it's worth illustrating some basic principles of how they work. The most important aspect of any telescope is its aperture, or the diameter of its main optical component, which can be a lens or a mirror. A scope's aperture determines its light-gathering ability and its resolving power (the ability to see fine detail in an image). What does this mean in real terms? Resolution is related to the instrument's diameter. So, with a 15-centimetre telescope you can discern lunar craters as small as about a kilometre across — half the size of those visible in a 7.5-centimetre scope under similar conditions using identical magnifications. However, the same two instruments turned toward a faint galaxy on a moonless night would tell a different story. Since the surface area of a 15-centimetre mirror is four times that of a 7.5-centimetre mirror, it can collect four times as much light, meaning the galaxy would appear four times brighter in the larger instrument.

Power Isn't Everything

It may surprise you, but a telescope's aperture is not what determines its magnification ("power"). When seeing an astronomical telescope for the first time, a novice will invariably ask, "How much does it magnify?" The fact is that a telescope can provide an almost infinitely variable range of magnifications, depending on the eyepiece used with it. Two main factors limit the power we can use on any given instrument: aperture (again) and atmospheric conditions. A finite amount of detail is present in the image produced by a telescope's main mirror or lens, so we must find the optimum magnification to extract all of this detail without overly spreading out the target's precious light, making it too dim to see clearly. This is generally why low powers are employed to look at faint subjects like galaxies and nebulae. Just as enlarging a photograph too much will reveal the grain of the negative, so too will excess magnification make your target grainy, not sharp.

How much power is too much? Fortunately, there's a simple rule that we can apply to find top magnification: Twice its aperture in millimetres. This means a high-quality 100-mm scope should not be pushed beyond about 200x. To put this in perspective, even a small instrument that has good optics will show you Saturn’s rings or the principal cloud belts on Jupiter, since these can be seen at a magnification of 75x. On the other hand, if you see a 60-mm department-store scope labelled as capable of delivering 300x, you'll know it's advertising hype and you should wisely move on to the next instrument!

Calculating Magnification

Now we know the maximum practical power of any given instrument — but how do we calculate it? Every scope has a focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. This is not always the same as the length of the tube, since (as we will see later) some designs optically "fold" the light path internally. Focal length is a figure that you'll usually see printed or engraved near the eyepiece focuser and usually lies in the range 400- to 3000-mm, depending on the aperture and type of telescope. Eyepieces have focal lengths, too — 25- or 10-mm, for example. Simply divide the focal length of the scope by that of the eyepiece to determine the magnification. A 2000-mm focal length scope used with a 25-mm eyepiece will therefore deliver 2000/25 = 80 power (or 80x).

Why Does the Moon Look Fuzzy?

Even with the best telescope you'll notice that you can discern finer lunar or planetary detail on some nights than on others. If you look closely in the eyepiece you'll see that planets and stars appear to shimmer and waver on these less-than-perfect nights. The fault lies not with the observer or the instrument but with the Earth's turbulent atmosphere or local conditions such as heat radiating from a nearby sidewalk that had been in sunlight during the day. Astronomers refer to turbulent nights as having bad "seeing."

Large apertures allow observers to pick out faint objects and fine detail on the Moon and planets, but regardless of aperture, the better the seeing the more you can see. Since steady air is so important, large telescopes — those in the 25-cm-plus category — are often limited to 250 or 300x on all but the very steadiest nights. Moreover, any experienced observer will tell you that with constant practice you'll see more detail in an image.

Is Bigger Always Better?

So why go for a telescope larger than 25-cm aperture if the sky conditions will limit you? Large apertures are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulae, and star clusters. These so-called "deep-sky" objects are most often viewed at much lower powers than the Moon or planets, so seeing quality is less of an issue. Also, larger aperture generally leads to shorter exposure times for those interested in astrophotography, especially when combined with a short focal length.

Even if a large instrument is within your budget, there's the question of portability. A large scope in amateurs' hands requires either housing in a permanent observatory or the services of some willing buddies to move and assemble it for each observing session. Clearly, there's a trade-off between convenience and performance, and everyone will have his or her own definition of what is portable. It's easy for anyone to succumb to the all-too-prevalent malady of "aperture fever," where amateurs are seized by a compulsion to buy the largest telescope their budget allows. The sad fact is that the leviathan is all too often consigned to the basement or closet, having been judged too unwieldy or heavy for regular use.

Scopes of Every Size and Shape

Having gained an appreciation of a few important optical principles governing a telescope's performance, we can now explore the different types of scopes available. You'll be forgiven for thinking there's a seemingly infinite variety from a quick perusal of the advertisements in the astronomical press. Yet for all their varied shapes and sizes, telescopes can be divided into three distinct classes: refractors, reflectors, and catadioptrics.


A refractor is the instrument that conforms to the stereotypical image of a telescope — a long, gleaming tube with a large lens at one end and an eyepiece at the other. Light passes through the objective (the lens in front) and is bent (refracted) to a focus at the eyepiece. Refractors have been with us in their various forms for 400 years and are often sought out by lunar and planetary observers who value their crisp, high-contrast views that can take high magnifications. In fact, when well made they can provide the finest images attainable for a given aperture. The other advantage of the refractor is that it's generally more rugged than other types of scopes, because its lenses are less likely to come out of alignment. For this reason refractors are well suited to those who wish to have a "pick up and go" instrument or who have no desire to tinker with the optics.

But these convenient features come at a price. A fine objective lens is a work of art that requires special glass and exquisite optical crafting to perform well. For this reason, refractors are the most expensive instruments of any given aperture. In their commonly encountered forms the tube lengths can be unwieldy — sometimes 10 to 15 times the size of the diameter of the lens. This means a 10-cm refractor can be nearly 2 metres long! Since the eyepiece is at the lower end of the tube, a tall tripod is required if you expect to observe objects that are overhead. The tripod has to be solidly built to prevent vibration at high powers, so it may be heavy or unwieldy. For deep-sky observers a refractor may not be enough light grasp for viewing faint objects, and the fields of view may be quite narrow. Modern design has led to shorter, more manageable refractors, but at a correspondingly higher cost.

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Choosing a Telescope

Big scopes, little scopes, fat scopes, skinny scopes — how do you know which one is right for you? This on-line guide will help. Whatever kind of telescope you decide to buy, don't skimp on quality. A good instrument will serve you for decades. Australian Sky & Telescope / Craig Michael Utter.



Rupes Altai

Even the best telescope will not give of its best on nights of poor 'seeing'. This lunar image shows the Rupes Altai mountain range leading to the 89 km diameter crater Piccolomini (lower right). Courtesy Adrian Ashford




This shows a cross-section of a modern short tube refracting telescope. Most refractors possess tube lengths between 10 and 15 times their aperture, which can make them unwieldly. However, lunar and planetary observers favour the high-contrast images such instruments provide. Australian Sky & Telescope / Gregg Dinderman.