Buying the best telescope: We all want the best buy for our budget. Here's what you need to know to acquire a telescope that can last a lifetime.
But then there's the reality. Turn the pages and you're faced with a selection of hundreds of models! Which one is best? Which one to choose? Relax. Any one of these telescopes will work well. But somewhere in the Buyer's Guide there may be a model that's just right for you, and knowing a little about what makes telescope tick will help you find it.
The job of a telescope is to collect and focus light--much more light than your eyes can gather on their own. Looking through a telescope is like having a huge eye capable of seeing in the dark. Telescopes collect light in one of two ways: in refractors a lens does the job; in reflectors, the task falls to a dish-shaped mirror.
Refractors are the classic spyglass instruments. Galileo's pioneering telescopes of 400 years ago were refractors. If you see a telescope used as a prop in any TV show or movie, or caricatured in a cartoon, chances are it's a refractor.
Reflectors are another tried-and-true design. They've been around since the 1660s, when Isaac Newton first devised a telescope that uses only mirrors. Today, one of the most popular forms of telescope for backyard stargazers remains the Newtonian reflector, little changed from Newton's early design.
Several other optical designs have grown in popularity during the past decades, though all are forms of reflectors.
Aperture, Aperture, Aperture
In the real-estate business, the mantra is "location, location, location. "With telescopes, it's aperture. Scopes are ranked by the diameter of their main lens or mirror--it's a 90-mm refractor or a 6-inch reflector. No serious amateur astronomer ever speaks of a 200-power telescope! That's because any telescope can be made to magnify at any power if you insert a different eyepiece. But at high magnifications the image is likely to look dim and blurry. The maximum useful power for any telescope is 50 times its aperture in inches, or 2 times the aperture in millimeters. So a 4-inch (100-mm) telescope won't perform well if pushed above 200x.
While magnification does make things look closer, sheer power is not the important specification--aperture is. The more light gathered, the brighter the image. The challenge in seeing most celestial targets is not how far away they are but how dim they are. You can see the vast Andromeda Galaxy, some 2.5 million light-years away, with your unaided eyes, but much fainter Pluto, closer to home, requires an 8-inch telescope to sight.
An 8-inch telescope provides images four times brighter than a 4-inch telescope because doubling the aperture increases the collecting area by a factor of four. More aperture also produces a more detailed image. An 8-inch scope reveals features on the Moon and planets half as small as will a 4-inch--doubling the aperture improves resolution by a factor of 2. However, no telescope has enough resolution to show you objects as small as the Apollo flags on the Moon!
So if a big scope shows brighter images and more detail than a small one, should you be seriously considering a monster scope that's bigger than you are? Not if you're just starting out. Even an 80- or 90-mm telescope will show excellent planetary detail and, under dark skies, hundreds of the brightest nebulae and galaxies. Move up to a 6-inch scope and you'll begin to resolve globular star clusters into thousands of stars and see bright galaxies as more than smudges of light. Planets look brighter, with more obvious colors and finer etched-in detail. An 8-to 10-inch instrument shows spiral structure in some galaxies and reveals thousands of fainter galaxies beyond the reach of smaller scopes.
Fast Scope, Slow Scope
All telescopes have a focal length, the distance the light beam travels (usually stated in millimeters) from the main lens or mirror to the point where it reaches focus at the eyepiece. Telescopes with the same aperture can have different focal lengths. An easy way of describing this variation is to refer to a telescope's focal ratio, or f/ratio. This is the scope's focal length divided by its aperture. So while a 200-mm reflector with a focal length of 2,000 mm is an f/10 telescope, a 200-mm instrument with a focal length of 1,000 mm is an f/5.
Using terminology borrowed from camera lenses, the f/10 scope is considered "slow," while the f/5 scope is "fast." You might think this implies that the f/5 scope is better, but this is not necessarily so. As long as the aperture and magnification are the same, images will look identical in brightness in either instrument. After all, both scopes collect light with the same 200 mm of aperture.
The difference is that a fast scope (with a focal ratio of f/6 or lower) provides a wide field of view that's best for panoramic views of the Milky Way and observing large deep-sky objects (targets beyond the solar system). However, fast refractors will often show some false color around bright objects, and all but the most expensive fast scopes tend to suffer from soft planetary images when pushed to high powers.
On the other hand, a slow telescope (with an f/ratio higher than f/6) will usually support the high powers needed for seeing small details on the planets. But it's not as compact and isn't as capable of showing wide-field sky vistas.
Optics are only half the telescope. The other half is the mount. The finest optics in the world are of little use if they're mounted on a platform that's hard to move or that bounces wildly every time you touch it. Mounts come in two main types: models that can track the sky as it turns from east to west, and models that can't. It might seem obvious than the former is better. Why would anyone ever want a non-tracking mount?
Cost is one factor. Simple, inexpensive altazimuth mounts that move up and down in altitude and side to side in azimuth are popular on many entry-level refractors. They also have the attraction of ease of use--just set them outside and they're ready to go, with no fuss over alignment procedures. Fine slow-motion controls allow the owner to keep objects centered by manually tweaking each axis, Etch A Sketch fashion.
An altazimuth variation that's popular on Newtonian reflectors of all sizes (from $200 starter scopes to 36-inch monsters) is the Dobsonian mount. Named for its inventor, John Dobson, this mount consists of a wooden cradle that turns on pads of Teflon. The tube smoothly spins around and swings up and down, and keeping objects centered in the eyepiece just takes a gentle nudge of the tube. While that seems like an inconvenience, it nets the owner a high-quality, large-aperture reflector for less than the cost of a similar Newtonian on a tracking mount--and in a rock-solid package that is much lighter, more portable, and easier to set up.
Getting a mount to track the sky can be done a couple of ways. A popular low-tech method is to use a German equatorial mount, a T-shaped mount that has one axis of rotation aimed toward the celestial pole (near the North Star in the Northern Hemisphere). When the mount is properly aligned, slow movement of the scope's polar axis will keep an object in view. This motion can be accomplished by a manual slow-motion control or by an electric motor, often available as an option. The motor turns the scope at the same rate as the sky, keeping targets nicely centered in the eyepiece. This allows you to concentrate on observing rather than on operating the telescope. The drawbacks of an equatorial mount are the additional expense and weight, and the extra effort in aligning the mount properly.
To "Go To" or Not
The higher-tech method of tracking the sky uses a computer-controlled motor on each axis. The mount can be an altazimuth, which doesn't need to be polar aligned, but the computer does have to be synchronized to the sky by aiming the scope at one or more bright stars. Once that's done, the computer will drive the telescope to find objects on command, then pulse each motor at the correct rate to keep the object centered in the eyepiece. This is called a Go To telescope, and models with this capability can be purchased for as little as $300 to $400.
Keep in mind that a computer isn't needed to enable a telescope to track the sky. Any German equatorial mount with a low-cost motor can follow the stars and planets as they march east to west across the heavens. What the computer makes possible is automatic finding of celestial objects. You can use a Go To scope to zip to many more objects than you might otherwise find by hunting them down with the aid of star charts. Such mounts are especially helpful under urban skies, where it can be hard to "star-hop" to objects using star patterns as guides because you can't see enough stars.
This all sounds very seductive and, indeed, these scopes are immensely popular. The downside is the expense-versus-aperture tradeoff. For the same price as a 3.5-inch Go To scope you could get a 6-inch reflector on a standard equatorial mount or an 8- to 10-inch Dobsonian. Note that some Dobsonians are available with add-on computers that don't slew the telescope but do point the way to hundreds of objects in a "push-to" mode, a nice combination of high and low tech.
The Choice Is Yours
With all this advice in mind, it's natural to ask: "So what's the best scope?" In the end, it all comes down to you--your interests and the size of your wallet. Just remember: the best telescope is the one you'll use most often.
While there's no end of choices in complete telescope systems, another option is to select an optical tube assembly (OTA) from one manufacturer and mate it to a mount from the same or another supplier. In fact, some highly prized telescopes such as apochromatic refractors are available only as optical tubes, and similarly, a number of the best mounts can be had only as separate items. This "component" method of assembling a telescope is even employed by some manufacturers who primarily offer complete systems but will let you mix and match optics and mounts as you like. A list of OTAs and mounts begins on page 94.
If all the choices seem daunting, consider that the telescope you buy now, especially if it's your first scope, doesn't have to do everything. Most amateurs eventually find that a stable of 2 or 3 telescopes is ideal. Perhaps a small portable "grab-and-go" refractor or a Go To Maksutov-Cassegrain is needed to complement a big reflector. But no matter what scope you buy, you'll be surprised to discover that if you use it regularly, it'll show you far more than you ever would have imagined.
1 HOW THEY WORK
A refractor is what everyone thinks a telescope is supposed to look like: a skinny tube with a lens at one end that brings light to a focus in the eyepiece at the other.
* Require little maintenance (the main lens stays aligned even under mild abuse).
* In small sizes can be economical and portable, making them popular starter scopes for all ages.
* Inexpensive refractors (ones called achromats) are unable to focus all colors to the same point, which results in a halo of blue light around bright objects. Reducing this flaw requires turning to premium apochromatic refractors, which employ exotic and expensive glass.
2 HOW THEY WORK
A Newtonian has a concave primary mirror (like a shaving mirror but much more exacting in finish) that sits at the bottom of the telescope tube, collecting the light that rains down the tube, then bouncing it back up toward the top of the tube, where it reflects off a small flat mirror. This secondary mirror shoots the light beam out the side of the tube into the eyepiece. Despite what you might think, the presence of the secondary in the middle of the light path makes little difference in the image quality.
* Available in much larger sizes and, per inch of aperture, cost less than refractors. For this reason alone, Newtonian reflectors retain their popularity.
* Tube may be long and awkward to aim.
* Need to occasionally adjust the tilt of the mirrors to keep the optics aligned.
3 HOW THEY WORK
In a Maksutov-Cassegrain or Schmidt-Cassegrain the light passes first through a refractor-like corrector lens, then hits the primary mirror. This collects and bounces the light up toward a small secondary mirror, which reflects the light back down the tube, where it exits through a hole in the primary. The eyepiece sits at the back of the telescope, as it does in a refractor.
* Folded optical path allows for a very compact tube. This, in turn, makes for a more portable telescope than a Newtonian of similar aperture.
* Compound scopes are more expensive than classic Newtonians of equal aperture.
* Require periodic adjustment to keep the mirrors aligned.
A Features Checklist
1. Tripod Superb optics are useless without good support. When buying a scope it's sometimes possible to upgrade to a sturdier tripod, or a heavy-duty tripod might be available as an accessory worth purchasing at a later date.
2. Mount When comparing telescopes you'll sometimes find optically similar models offered on different mounts. The more substantial mount will likely provide the steadier view and is usually a worthwhile upgrade. Motor drives, once an optional extra, are now often available as standard.
3. Finder Telescopes usually come with an observing aid to help locate celestial objects. Most common are finderscopes--mini-telescopes mounted on the main tube. They have low magnification, a wide field of view, and are described as being 8 x 50 or 6 x 30; the first number is the magnification and the second is the diameter of the lens in millimeters. Finders with a 6x magnification (or more) are generally of decent quality; the best scopes often include a 50-mm finder with 7 to 9 power. If yours has a 5 x 24, seek an upgrade.
3a. Finder Some telescopes come with what's called a reflex or unit-power finder. The style varies, but most provide a small window that you look through to see a red dot or crosshair pattern superposed on a naked-eye view of the sky (they don't magnify anything). They're used on Go To scopes for finding bright stars during the initialization process, but you'll also see them on large scopes used in dark skies by experienced observers. If you live under a light-polluted sky, at least a 6 x 30 optical finder is a better choice on non-Go To scopes.
4. Focuser The majority of focusers accept standard 1.25-inch-diameter eyepieces. (Avoid scopes with focusers that need eyepieces with a barrel diameter of 0.965 inch, or 24.5 mm.) Some telescopes have focusers capable of accepting giant 2-inch eyepieces - great for big-gulp views of the sky and a definite plus on the features checklist.
5. Star Diagonal Every scope except a Newtonian should include a star diagonal, as it makes for more comfortable viewing.
6. Eyepieces On all astronomical telescopes the focuser (or star diagonal) accepts interchangeable eyepieces. Non-removable eyepieces are found only on spotting scopes, which are generally unsuitable for astronomy. A quality telescope will include a set of eyepieces that provides reasonable magnifications, such as 40x and 100x. The best eyepiece types often included as standard equipment are the Kellner, MA (modified achromat), and Plossl designs. Avoid the Ramsden (marked AR or SR on the barrel) and Huygens (H or AH) varieties. With eyepieces, you get what you pay for. Upgrading to a better eyepiece often results in a marked improvement in a scope's performance.
For Your Consideration ...
Type of Scope: 6-to 8-inch (150- to 200-mm) Dobsonian reflector
Bottom Line: The aperture is generous, the optics are usually good, and the mount rock-solid and easy to set up. Best of all, most are reasonably priced. However, there's no tracking ability, and the telescope may be physically too large for some situations (apartment dwellers, for example).
Type of Scope: 3.5-to 5-inch (90- to 125-mm) Maksutov-or Schmidt-Cassegrain, on either a classic German equatorial or a Go To mount
Bottom Line: Provides reasonable aperture in an ultra-compact package that can be easily carried outside in one or two pieces for spur-of-the-moment viewing. Go To versions are good for city skies.
Type of Scope: Small achromatic refractor on an altazimuth or German equatorial mount
Bottom Line: The slow f/ratio models provide sharp, contrasty images that make them a good choice for lunar and planetary viewing. Fast, short-tube achromats are very compact (so are nice travel scopes) and are best for wide-field deep-sky viewing. Apochromatic refractors perform superbly on all classes of objects, but you'll pay a premium price because they use exotic and expensive glass.
Type of Scope: 6-to 8-inch Schmidt-Cassegrain or Schmidt-Newtonian (another hybrid variation) on a high-quality German equatorial mount
Bottom Line: If the budget allows, then an equatorially mounted 8-inch Schmidt-Cassegrain is a superb all-purpose telescope for all types of celestial viewing as well as for astrophotography. These scopes remain among the most popular instruments for serious backyard stargazing, as they provide an ideal balance of aperture, portability, and flexibility of use.
Type of Scope: 10-to 16-inch reflector
Bottom Line: Avid deep-sky fans with dark skies will want to settle for nothing less. In this size class, a Ritchey-Chretien (another form of specialized reflector) or a large Schmidt-Cassegrain is a good choice--but only if you have a permanent observatory or storage facility. For portable field use, a large-aperture, custom-made Dobsonian with a break-apart truss tube provides the compactness needed for ease of transport and setup.
ALAN DYER is a Sky & Telescope contributing editor and coauthor, with Terence Dickinson, of The Backyard Astronomer's Guide (Firefly Books, 2002).
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|Date:||Jan 1, 2006|
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