How to Choose a Telescope

Big scopes, little scopes, fat scopes, skinny scopes — which one is right for you?
Sky & Telescope / Craig Michael Utter.

Editor's Note:
This is an abridged version of an article that was originally published in 2002 in SkyWatch, our annual publication. Everything in the article is still valid, but the emphasis would probably be different if it were written today. In particular, Go To drives have become much cheaper, better, and easier to use over the last decade.

If you want a somewhat longer article that you can print out and read at your leisure, click here to download "What to Know Before You Buy" from SkyWatch 2010 as a 2-megabyte PDF file.


An Introduction to Telescopes

This is an exciting time to become an amateur astronomer. Never 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: the bewildering variety makes it hard for an uninformed consumer to make the right decision.

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

Before you buy anything, you must determine what's important to you. What do you most want to look at? How dark is your sky? How experienced an observer are you? How much to you want to spend? What storage space do you have, and how much weight do you want to carry? Answer these key questions, familiarize yourself with what's on the market, and you'll be well on your way to acquiring a scope that will satisfy you for many years to come.

Before examining the different telescopes available, it's worth knowing the basics of how they work.

Aperture: A Telescope's Most Important Specification

The most important aspect of any telescope is its aperture, the diameter of its main optical component, which can be either a lens or a mirror. A scope's aperture determines both its light-gathering ability (how bright the image appears) and its resolving power (how sharp the image appears).

What does this mean? The bigger the aperture the better. With a 6-inch telescope you can discern craters on the Moon as small as about a mile across — half the size of those visible in a 3-inch scope (under the same conditions using the same magnification). The same two instruments turned toward a faint galaxy on a moonless night would tell an even more dramatic story. Because the surface area of a 6-inch mirror is four times that of a 3-inch mirror, it collects four times as much light, meaning the galaxy would appear four times brighter. (Astronomically speaking, that's 1.5 magnitudes brighter.)

Magnification Isn't Everything

It may surprise you, but a telescope's aperture is not what determines its magnification ("power"). When seeing a telescope for the first time, a novice will usually ask, "How much does it magnify?" The answer is, "Any amount you want." Any telescope can provide an almost infinite range of magnifications, depending on the eyepiece you put into its eye end.

But don't get the idea that super-high powers will do you any good. Two main factors limit the power that shows a decent view with a given instrument: aperture (again) and the atmospheric conditions.

Only so much detail exists in the image created by a telescope's main mirror or lens, so you must find the optimum magnification to see this detail — without spreading out the target's precious light too much, making a dim object too dim to see or turning a bright object into just a big blur.

This is why observers generally use low powers for looking at faint things like galaxies and nebulae, and no more than medium-high powers for bright things like the Moon and planets. Just as enlarging a photograph too much will simply show you the grain in the film or the pixels on the chip, so too will excess magnification just make your target blurry.

How much power is too much? There's a simple rule to find the top useful magnification: 50 times your telescope's aperture in inches, or twice its aperture in millimeters. And that's if the scope has perfect optics and the night air happens to be unusually steady.

This means that a high-quality 4-inch (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 small, 60-mm department-store telescope scope labeled as delivering "300 power!!!", you'll know it's advertising hype and you should wisely look elsewhere.

Calculating Magnification

Now you know the maximum practical power for any given instrument. But how do you get it? What do those little numbers on the eyepieces tell you about the magnification they give?

Every scope has a focal length, which is effectively the distance from the primary lens or mirror to the image it forms. (This is not always the same as the length of the tube, since, as we'll see later, some telescopes optically "fold" the light path internally.) Focal length is the large number you'll often see printed or engraved on the front or back of the scope, usually between about 400 and 3,000 millimeters depending on the scope's aperture and type.

Eyepieces have focal lengths too — 25mm or 10mm, for example. Simply divide the focal length of the scope by that of the eyepiece; that's the magnification. For instance a 1,000-mm focal length scope, used with a 25-mm eyepiece, delivers 1,000 / 25 = 40 power (or 40x).

Here's a simple magnification calculator:

Telescope focal length =
Eyepiece focal length =
Magnification =


Why Does the Moon Look Fuzzy?

Even the best telescope will give fuzzy views on nights of poor atmospheric 'seeing'. This lunar image shows the Altai Scarp mountain range leading to the 90-km-diameter crater Piccolomini (lower right). It's a frame in a short GIF movie (102k) you can run by clicking the image, showing how turbulent air ripples and distorts the view, blurring fine detail.
Adrian Ashford
Even with the best telescope, you'll notice that you can discern finer lunar or planetary detail on some nights than on others. Sometimes the sharpness of the view even changes from one second to the next. At high power, you'll see that planets and stars shimmer and blur on most nights. The fault lies not with the scope but with Earth's turbulent atmosphere, and sometimes with very local conditions such as warm air rising from a nearby asphalt driveway that soaked up solar heat all 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 — even those in the 10-inch-plus category — are often limited to 250x or 300x on all but the very steadiest nights.

Any experienced observer will tell you that with practice, you'll see more detail in an image — not only because your eye gets better trained, but because the longer you look, the better your chance of catching a few moments of unusually steady atmospheric seeing.

Is Bigger Always Better?

So why go for a telescope larger than 10-inch 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 things: galaxies, nebulae, and star clusters. These so-called "deep-sky" objects are generally viewed at much lower powers than the Moon or planets, so the quality of the atmospheric seeing 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.

But even if a large instrument is within your budget, there's the question of portability. A really large amateur scope requires either a permanent observatory so you never have to move it, or willing buddies to help you lift and assemble it for each observing session, then take it down afterward. Clearly, there's a tradeoff between convenience and performance &$151; and everyone will have his or her own definition of what is "portable." It's easy to succumb to "aperture fever," in which you're seized by a compulsion to buy the largest telescope you can. The sad fact is that the leviathan is all too often consigned to the basement or closet, being too heavy and unwieldy for regular use. Remember, the telescope that you use most often is the one that will actually show you the most.

Pay close attention to the weight of the scope you're considering buying, usually listed in the small print. Get a barbell or a log that weighs this much on your bathroom scale. Carry the log around with you. Carry it back and forth from where you'll store the telescope to where you'll use it. Are there stairs along the way? How often will you want to do this at the end of a long day?

Telescopes of Every Size and Shape

Having gained an appreciation of a few important optical principles governing a telescope's performance, and the tradeoff between performance and portability, we can now explore the different types of scopes available.

You'll be forgiven for thinking there's an infinite variety from the ads in the astronomical press. Yet for all their varied shapes and sizes, telescopes can be divided into three classes: refractors, reflectors, and catadioptrics.

Refractors

A cross-section of a modern short-tube refracting telescope. Most refractors have tube lengths between 8 and 12 times their aperture, which can make them unwieldy. However, lunar and planetary observers favor the high-contrast images such instruments provide.
SkyWatch / Gregg Dinderman.
A refractor is the stereotype of how a telescope is supposed to look — a long, gleaming tube with a large lens in front and an eyepiece at the back. The front lens (the objective) focuses light to form an image in the back. The eyepiece is a little magnifying glass with which you look at the image.

High-quality refractors are often sought out by lunar and planetary observers who value their crisp, high-contrast images that can take high magnification. In fact, when well made a refractor can provide the finest images attainable with a given aperture.

Another 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 nice features come at a price. A really fine large objective lens is a work of art that requires special glass and individual hand-crafting. For this reason, refractors are the most expensive instruments of any given aperture.

Also, in their commonly encountered forms, refractor tube lengths can be unwieldy. A 4-inch refractor can be 4 feet or more long. And since the eyepiece is at the lower end of the tube, a tall tripod is required if you expect to observe objects overhead. Such a tripod has to be very solidly built to prevent wobbles at high powers, so it may be heavy or unwieldy, not to mention expensive. For deep-sky observers a refractor may not have enough light grasp for viewing faint objects, and the fields of view may be narrow. Modern optical design has led to shorter, more manageable refractors, but at a correspondingly higher cost.

It's Done With Mirrors

A cross-section of a typical Newtonian, the most common reflecting telescope. Dollar for dollar, a reflector is the best-value scope you can buy. Occasional cleaning and realignment of the optics may lessen its appeal to some users.
SkyWatch / Gregg Dinderman.
The second type of telescope, the reflector, uses a mirror to gather and focus light. Its most common form is the Newtonian reflector (invented by Isaac Newton), with a specially curved concave (dish-shaped) primary mirror in the bottom end of the telescope. Near the top a small, diagonal secondary mirror directs the light from the primary to the side of the tube, where it's met by a conveniently placed eyepiece.

If you want the most aperture for your money, the reflector is the scope for you. When well made and maintained, a reflector can provide sharp, contrasty images of all manner of celestial objects at a small fraction of the cost of an equal-aperture refractor.

The tube of a Newtonian is considerably more manageable, too. Its length is rarely more than eight times the diameter of the primary mirror, and frequently less. This means an 8-inch Newtonian can be housed in a tube hardly over 4 feet long, fitting in the back seat of a small car for transportation to dark, rural skies. Combine this with the Newtonian's generally low center of gravity well below the eyepiece, and you end up with an instrument on a compact, stable mounting that presents the eyepiece at a convenient height for just about any sky orientation.

And there's another benefit. A reflector is, by and large, the only type of telescope that shows you a "correct-reading" image rather than a mirror image. This is especially important when you're trying to compare what you see in the eyepiece to what's on a star map.

A variant of the Newtonian telescope known as the Dobsonian, shown here, has become extremely popular thanks to its low cost, ease of use, and portability.
SkyWatch / Craig Michael Utter
For the best value of all, much consideration should be given to a particular type of reflector known as the Dobsonian. This is a Newtonian on a very simple, very rugged mount. These extremely popular instruments are available in apertures from 4 inches to more than 30 inches and represent the ultimate in observer convenience for casual viewing.

Like all reflectors (there are other types, but we'll skip them because they're rarely encountered in amateur hands), a Newtonian will require occasional maintenance. Unlike a refractor's solidly mounted lens, a reflector's mirrors can get out of alignment and hence will need periodic collimation (adjustment) to ensure peak performance, particularly if the telescope is moved frequently. This is no big deal once you get the hang of it, and the mirrors of the average Newtonian may not require tweaking for months at a time. But for those not mechanically inclined, having to collimate a reflector even occasionally may be frustrating.

The reflector's open tube means that dust and dirt are more likely to accumulate on the optical surfaces even if you're careful to cover the tube in storage, and this will mean occasional cleaning. Also, the aluminized surfaces of a reflector's mirrors may need to be sent off for recoating every 10 or 20 years — more frequently if you live in a badly air-polluted urban area or by the sea.

he Best of Both Worlds

A cross-section of a Schmidt-Cassegrain telescope. Roughly midway in price between Newtonians and refractors, catadioptrics are both compact and easy to upgrade owing to a wide range of accessories. Click the image to see one popular variant, the Maksutov-Cassegrain.
SkyWatch / Gregg Dinderman.
Then there's the third category of instruments, the catadioptric or compound telescope. These were invented in the 1930s out of a desire to marry the best characteristics of refractors and reflectors: they employ both lenses and mirrors to form an image. The greatest appeal of these instruments is that, in their commonly encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain), they are very compact. Their tubes are just two to three times as long as wide, an arrangement allowed by "optical folding" of the light. The smaller tube can use a lighter and thus more manageable mounting. The upshot is that you can obtain a large-aperture, long-focus telescope that's very transportable.

But here too there are caveats. Like the Newtonian, the Schmidt-Cassegrain needs occasional optical collimation that lessens its appeal to those disinclined to tinker. Their fields of view can be rather narrow, too. In terms of cost, aperture for aperture, the catadioptric lies midway between the reflector and the refractor. Like a Newtonian, the popular forms of compound telescopes have a secondary mirror in the light path of the instrument, and this slightly degrades performance for critical lunar and planetary observations. Even so, when well made, a Schmidt-Cassegrain or Maksutov will deliver very fine images of a wide variety of celestial objects.

In common with refractors, the tubes of catadioptrics are sealed so that dirt and dust are largely excluded — a big plus for an instrument that you're going to take out into the country. But if you live in an area where dew occurs (which is almost everywhere), some sort of collar or extension to prevent misting of the exposed corrector plate at the front of the tube is a must.

In practice, many people seeking a highly versatile, very portable (for the aperture) scope that can be used for all sky subjects and for astrophotography will tend to opt for some form of compound instrument. Scopes of this type also tend to be the most highly "technologized," with many options such as computerized pointing and photographic adaptations. In short, they're excellent general-purpose scopes that can use a wide variety of accessories.

Telescope Mounts

The mount of a Dobsonian, as illustrated here, is a prime axample of an alt-azimuth, or 'alt-az' scope. The tube moves up-and-down (in altitude) and left-to-right (in azimuth). Other alt-az mounts may have slow-motion controls to permit easy tracking at high powers.
Sky & Telescope / Chuck Baker
The best telescope in the world is useless unless it's on a solid, stable, smoothly-working mount, one that permits it to be directed to the desired part of the sky and to follow a celestial object smoothly and precisely as the Earth turns beneath it.

In realistic terms, a "stable" mount is one that, when you're using a moderate to high power, will not vibrate for more than a second or so after you rap the tube. In particular, the view can't wiggle so much when you hold the focus knob that you can't see when you've found the sharpest focus. And when you let go, the aim must not jump to one side. This completely eliminates the typical "department-store" semi-toy telescope from consideration.

While there are variations on a theme, you'll encounter two types of mount: altitude-azimuth (or "alt-az") and equatorial.

An alt-az mount operates like a tripod's pan-and-tilt head, moving the scope up-down (in altitude) and left-right (in azimuth). Equatorial mounts also possess two axes, but they're tilted so that one can be aligned with the rotational axis of the Earth.

If you're intending to use a small telescope for casual sky viewing or daytime use (say, birdwatching), you'll find the alt-az mount preferable. Well engineered mounts of this type will have finely threaded slow-motion controls that enable the scope to be moved smoothly by tiny amounts, especially important when you're using high powers. The value of such refinements will be all too apparent when you are tracking a star or planet at high magnification.

The Dobsonian is a form of alt-az mount. Inexpensive materials such as particleboard and Teflon figure in its construction, resulting in a low-cost, low-center-of-gravity mount that (ideally) glides smoothly about both axes with fingertip control. A Newtonian reflector mounted in this fashion is not only extremely easy to set up and intuitive to use, but very good value, too.

An equatorial mount makes the tracking of celestial objects easier as the Earth turns. Once correctly set up, the scope need only be turned about one axis to follow an object across the sky — and a drive motor can do this automatically. An equatorial is mandatory for most astrophotography.
Sky & Telescope / Chuck Baker
For a telescope intended for astronomy, and for which photography is a future prospect, consideration should be given to some form of equatorial mount that automatically counteracts Earth's rotation. It's far easier to track a celestial object with a scope mounted this way, since you need only concern yourself with turning the scope about one axis — not two simultaneously, as in the alt-az. When an equatorial mount is properly set up, turning the slow-motion control of its polar axis is all that's required to keep an object in view.

More sophisticated mounts, including modern high-tech alt-az mounts, have built-in electric motor drives to do this, freeing you to concentrate on observing.

So is one type of mount better than the other? Not really, since each has its strengths. For the casual observer who wants a highly portable scope that can be quickly set up in a variety of locations, an alt-az is preferable — especially a Dobsonian. An equatorial, while virtually mandatory for most forms of astrophotography and critical observations of the Moon and planets at high power, needs to have its polar axis aligned with the rotational axis of the Earth. While polar alignment is not particularly difficult and becomes routine with practice, it can take a little time at the start of your observing session if you want to do it really precisely (necessary for photography but not for just looking).

"Go To" Telescopes

A current trend is the robotic 'Go To' telescope that has a built-in computer and database to make finding objects simple. . . in theory. The downside is that most designs require you to perform an alignment procedure each time you use it, so you still need to know your naked-eye stars!
Sky & Telescope / Craig Michael Utter
Currently in vogue are the computer-controlled robo-scopes appearing on the market in various guises. These have mounts that are controlled either by a built-in computer or remotely by an external PC. This allows you to direct the scope to any object in the computer's database.

At first glance these "Go To" units would appear to be the answer to a novice's dream, since they ostensibly take the hard work out of finding elusive objects like faint galaxies, star clusters, and asteroids. "Hey," you might think, "I don't have to learn the sky!"

But it's not quite like that.

There's no denying that when well engineered (read expensive), these robotic scopes are great fun to use, as they almost magically slew across the sky in search of whatever you've keyed in, zeroing in on the target to be presented in the eyepiece. But this technology is only beginning to mature to the point where these scopes will automatically orient themselves when you take them outside and switch them on. Almost all Go To systems will ask you to enter the geographical location of your viewing site (or the nearest city) and the date and time at the beginning of each observing session. This lets the onboard computer calculate the positions of any celestial objects you may care to look at. Often you'll also have to level the telescope's tube, point it north (or south in the Southern Hemisphere), and then launch into an alignment procedure that uses two bright stars (which you must know by name) to synchronize the telescope's coordinate system with that of the sky.

It's true that this setup routine is easily mastered with practice. But it does take time. And for someone completely unfamiliar with the sky, the vast majority of the current batch of robotic scopes have the potential to be very frustrating at first. Still, some help is on the way. The newest crop of Go To scopes include their own Global Positioning System devices to at least tell you (and the telescope) exactly where you are and what time it is, making setup a little easier.

Then there's the question of how accurately the mechanical parts actually point the telescope where the electronics think it's pointing. At astronomical magnifications, there is no room here for even very tiny errors — meaning any costcutting in the mechanical design and manufacturing. A cheaply made Go To scope won't work, no matter how fancy the electronics are.

Here's one last thing to keep in mind: the money spent on a Go To scope's electronic mount could be invested in a traditionally mounted scope of larger aperture.

Finders

Some form of low-power, wide-field-of-view finderscope needs to be mounted on your main telescope, to help you point it where you want. Look for optical finders (top) that have front lenses (objectives) larger than 1 inch (25 mm). Or, if you prefer, opt for a reflex sight that projects a red dot or bullseye onto the sky. A reflex sight limits you to naked-eye targets with no magnification, but you can still star-hop to fainter targets.
SkyWatch / Craig Michael Utter
When used at medium to high power, a telescope shows you only a tiny little bit of sky. This make pointing at a target a frustrating process unless the scope has a finder.

As the name suggests, a finder assists you in locating celestial objects. All but the smallest scopes need one. Most common is a miniature telescope attached by a bracket near the eyepiece of the main scope. It has a low magnification and hence a wide field of view, and is equipped with crosshairs like a gunsight. Once you align it correctly with the main scope, centering an object in the crosshairs gets it into the main telescope's view.

You need a reasonably big, high-quality finderscope. Look for one that has an aperture (front lens) larger than an inch (25mm) and appears well-made. Dinky, nearly worthless finders are all too common on cheap telescopes.

A popular alternative is the reflex sight, which projects a point or ring(s) of light on the background sky when you look from behind. Many people prefer this intuitively simple option, but you're limited to naked-eye objects because this type of finder has no magnification and no more light-gathering aperture than the pupil of your eyeball. You can, however, still "star-hop" from naked-eye targets to deep-sky objects using the main scope at its lowest power — if you have sufficiently detailed sky maps.

Can I Photograph What I See?

Recording what you see, either on film or digitally, can be a rewarding experience. A quality telescope on a driven equatorial mount, a guide scope, and a camera will get you started in astrophotography. And these days taking a photo is only half the fun. Image-processing software lets you enhance your picture to bring out things in it that may otherwise escape the eye.
Courtesy Alan Dyer
Assuming you've bought a new scope, it's almost inevitable that you'll wish to use it to capture the beauty of a planetary image on film or to emulate the marvelous gallery of deep-sky photographs that grace magazines such as Sky & Telescope. In principle there's no reason why you shouldn't be able to, given the necessary equipment, inclination, and time. But it's wise to get used to operating your new scope visually, and learning your way around the sky, before embarking on the astrophotographic adventure.

Photography of the heavens can be incredibly rewarding, but it's as much an art as a science. The learning curve can be steep, the equipment can get expensive, and getting it right can consume a lot of time. While any telescope will permit you to shoot the Moon, for just about everything else you'll need a scope on a very rigid, well-engineered, and precisely driven mount.

Everything Has Its Price

While it may be tempting, resist the urge to buy the cheapest telescope available. Most of these are poor quality optically, mechanically, or both, and will disappoint. If you've a budget of less than $200, consider good binoculars instead.

That said, quality instruments can sometimes be obtained secondhand that an experienced member of your local astronomy club may be willing to check out on your behalf. Or have you considered building one yourself? If you're gifted with your hands and enjoy working in wood, it's possible to buy the optics and make a top-quality Dobsonian reflector yourself. Again, members of your local club may help.

Even if you have lots of money to spend, don't buy the largest, most expensive telescope you can find just yet. Start smaller and more manageable. If you're just learning to identify the constellations, many of the advanced features of a really expensive instrument aren't likely to be any use to you. And remember not to get something too heavy to set up, take down, and store.

And, remember you need more than glass and metal. Be sure to save some of your budget for additional eyepieces to expand the scope's magnification range, a very detailed sky atlas (essential!) and good guidebooks, and any number of other accessories — particularly if you have astrophotography in mind.

Last Thoughts

Whoever you are and wherever you live, there's a telescope just right for you. One of the rewards of amateur astronomy is sharing the universe with others through your scope — and you're never too young to start!
Sky & Telescope / Craig Michael Utter
So, is there a perfect telescope out there waiting for you? Actually, there is — it's the one you'll use most often!

An optically superb but massive refractor will be effectively useless if you can't carry it outside, and the largest Dobsonian will not show you the faintest galaxies if the only place you can use it is a light-polluted parking lot in a city.

Consider carefully what you feel to be your primary observing interest, where you're likely to be able to observe, and what is "portable." Weightlifting is good for you, but not everyone enjoys it.

Contactyour local astronomy club, which may have observing nights when you can try various scopes and chat with their owners. Don't be shy. Your local club wouldn't have put itself in our database unless they wanted you to call.

A telescope is a big investment for most people, and the universe is not going away. So take your time. Use binoculars to get familiar with using star charts and guidebooks to ferret out faint, difficult wonders. Doing this will develop exactly the knowledge and skills that you'll need to use a telescope well. When you do buy, you'll then be more likely to make a decision you're really happy with, and you'll possess an effective key to unlocking a lifetime of cosmic wonders.

It's a clear night — what are you waiting for?

COMMENT