ALL ABOUT BEGINNERS' TELESCOPES
Written By Phil Harrington
You may have had a passing interest in astronomy for years. Maybe you would thumb through different astronomy books and magazines at your local library or bookstore, just to keep up with goings-on in the rest of the universe. Whatever the reason, you started to think "gee, maybe I should buy a telescope." You've always wanted one ever since you were a kid, but just never decided that it was the right time . . . until now. Maybe it was a school assignment that your child brought home, or maybe even a story in the news. Whatever piqued your interest, you have decided that the universe is no longer a spectator sport. You want to participate!
But where to begin? Even a quick glance through this issue will show there are so many telescopes available that trying to determine which is best for you would appear to be impossible. Fortunately, that's not so at all. You just need a little help deciding on which telescope is right for you. That's where this article comes in.
Let's begin with the basics. As you may know, there are three broads categories of telescopes based on their optical design the refractor, the reflector, and the catadioptric. The refractors is easily identified by its long, thin tube that holds a large lens in front called the objective lens. Light passes through the objective, which refracts, or concentrates, the light to a focus toward the back end of the tube, where it passes through the eyepiece and out to the observer's eye. Although markedly improved over the past four centuries, this is the type of telescope that Galileo first used in 1609 to discover craters on the Moon, the phases of Venus, and the four major satellites in orbit about Jupiter.
Rather than using a single lens in front as Galileo's telescope did, most modern refractors actually have two lenses up front nested together to create what is called an achromatic objective. These combine to greatly reduce an optical imperfection called chromatic aberration, which causes bright objects to be encircled with fuzzy halos of vivid green, yellow, and purple.
Rather than rely on a lens in front, a reflector gathers and focuses light with a large primary mirror at the bottom of its tube. The primary, ever-so-slightly concave, reflects light back up to the front of the tube, where it draws to a focus. Over the years, several different reflecting telescopes have been devised to eliminate that problem, with the most popular coming from Isaac Newton in 1672. His Newtonian reflector inserts a small, flat mirror into the front of the telescope, diverting the light out through a hole in the side of the tube and into the eyepiece.
Finally, catadioptric telescopes are sometimes called compound telescopes since they combine some of the features of the refractor as well as the reflector. Light first passes through a large, clear lens, called the corrector plate, which tweaks it ever so slightly before reflecting off the primary mirror at the back of the tube. Bouncing off the primary, the light reflects toward the front of the tube, where a secondary mirror awaits to direct the light to the eyepiece.
Catadioptric telescopes can be further divided into two categories that differ by the curves of their corrector plates and mirrors: the Schmidt and the Maksutov. Of these, the most popular is the Schmidt-Cassegrain, sometimes abbreviated SCT. Maksutovs are also becoming very popular.
Regardless of their design, all telescopes share many common functions and terminology. For instance, we always refer to a telescope's size not by the length of its tube, but instead by its aperture, the diameter (usually expressed in inches, centimeters, or millimeters) of the instrument's main optic. The length of a telescope is determined by its focal length, the distance from the objective lens or primary mirror to the focal point, where the light rays come to a focus. As with aperture, focal length is commonly expressed in either inches, centimeters, or millimeters. Telescopes also have focal ratios, which is simply the number you get by dividing the focal length by the aperture. A 4-inch telescope with a focal length of 40 inches has a focal ratio, or f-number, of f/10, while a 6-inch telescope with a focal length of 48 inches has a focal ratio of f/8.
Optical quality should always be a top priority when selecting a telescope. After all, not all telescopes are created equally. Some are outfitted with flawless optics, while others use lenses and mirrors that are barely able to achieve focus. For a mirror or lens to perform properly, its curve(s) must be extremely accurate. Manufacturers often say that their telescope optics are diffraction limited. Diffraction limited means that the optics are so good that performance is limited only by the wave properties of light itself, and not by any flaws in optical accuracy. That is an important statement to look for, but you should also double check their return policy, just in case... All of the telescopes listed here are considered to have quality optics that will produce acceptable images.
Your wants and desires should also be given a high priority when selecting a telescope. Just like buying anything, be it a car, a home, or even something for tonight's dinner, we all have our own unique personal needs. So, too, is it with telescopes. A telescope that is right for me may not be for you, and vice versa.
First, consider what you want to do with the telescope. What type of observing interests you? Some telescopes are better at viewing, say, the planets than others. If you would like to wade into the world of astrophotography, then you may need a different telescope than if you simply want to look around. Let's consider a few options.
Realize that not all telescopes are created equally. Each of the three major types has its own strengths and weaknesses. For instance, if you are more interested in viewing the Moon and planets, you should be more concerned with image sharpness than with image brightness. After all, the Moon and naked-eye planets are among the brightest objects in the night sky. A telescope needn't make them brighter still. In this case, an achromatic refractor might be your best choice. Well-made refractors are famous for extremely sharp images and high image contrast, two critical ingredients in a high-quality planetary telescope.
When choosing an achromatic refractor for planet-watching, stick with one that has a focal ratio no less than f/8 or so. Small refractors with faster focal ratios tend not to magnify a planet's image sufficiently, unless paired with a very short focal length eyepiece. Except for some very expensive, exotic designs by companies such as Tele Vue, Meade, and Vixen, short focal length eyepieces are very difficult to focus clearly. Refractors with faster focal ratios also suffer from chromatic aberration more than those with higher f/ numbers. Chromatic aberration can cloud subtle planetary details.
If you long to view distant star clusters, nebulae, and galaxies, most beginner-level refractors will prove too small to show much beyond a few bright examples of each. In this case, you are most interested in light-gathering ability, making a reflector your best bet. Inch for inch, reflectors, which are less expensive to make, get you more aperture for your money than any other kind of telescope. But at the same time, don't buy such a large telescope that it proves too difficult to move! Most beginners will probably do best with a 6-inch reflector. If the telescope is for a young child, then a lighter weight 3- or 4-inch reflector would be a wiser choice, while some people may prefer a larger, though heavier 8-inch reflector. Your choice depends entirely on you.
What if you live in a high-rise apartment or condominium? Then, either of these telescopes might be too cumbersome to carry and set up. If so, then look at a small, "short-focus" refractor or reflector. Short-tubed telescopes are quite popular among urban astronomers because they can be carried outside with one hand, while toting books and charts in the other. One caveat about these, however. Some short-tubed reflectors boast an internal lens that stretches their focal ratios to longer proportions. While most work fine, these auxiliary lenses tend to introduce image distortions more readily that those without.
Stargazers short on storage space might also consider a catadioptric telescope. Unfortunately, the vast majority of these are above the price ceiling of $500 for this article, but one or two hang below that level. While certainly suitable, keep in mind that their prices do not include tripods, which are required for viewing. A sturdy tripod will tack on another $200, perhaps more.
Running neck-and-neck in terms of importance when selecting a telescope is the quality of its mounting. A good mount must be strong enough to support the telescope's weight while minimizing any vibrations. It must also provide smooth motions when moving the telescope from one object to the next and allow easy access to any part of the sky.
Telescope mounts come in two flavors: altitude-azimuth and equatorial. Altitude-azimuth mounts move both in azimuth (horizontally, left to right) and in altitude (vertically, up and down). Better altitude-azimuth mounts are outfitted with "slow-motion controls," one for each axis. By twisting one or both of the slow-motion control knobs, the observer can finely adjust the telescope's aim. While many beginner's telescopes come on altitude-azimuth mounts that are far too small and lightweight to carry their weight, those listed here are judged to be satisfactory.
In the past 25 years, a variation of the altitude-azimuth mount called the Dobsonian mount has become extremely popular for supporting 6-inch and larger Newtonian reflectors. The telescope tube moves up and down on a pair of circular altitude bearings that ride in a rocker box, which sits on the ground and pivots left and right. Dobsonian mounts are often made from either wood or wood by-products, such as laminated chip board. While many may think that a wooden telescope mount sounds very primitive, in reality, wood dampens vibrations far better than metal. So, despite their simplistic appearance, Dobsonian mounts are wonderfully stable.
Both traditional alt-az mounts as well as Dobsonian mounts share some drawbacks. Unless you are observing at the poles, celestial objects appear to trace long arcs across the sky. With an alt-azimuth mount, the telescope must be nudged both horizontally and vertically in a step-like fashion to keep up with the sky, which is less convenient than the single motion of an equatorial mount.
If you are thinking of trying your luck with astrophotography, then you almost have to get an equatorial mount. When tilted at an angle to match the observer's latitude, an equatorial mount will track the sky with a single, fluid motion rather than in steps. Perhaps their greatest benefit is being able to attach a motor drive so that the telescope will follow the sky automatically. On the down side of equatorial mounts, however, is that they are almost always larger, heavier, more expensive, and more cumbersome than alt-azimuth mounts. And their diagonal motions (compared to the horizon) can take some getting used to.
There are several different equatorial-mount designs, but the two most popular among amateur astronomers are the German equatorial mount and the Fork mount. The German equatorial mount is shaped like a tilted letter "T." The telescope is mounted to one end of the cross bar, while a weight for counterbalance is secured to the other. If properly designed, German equatorial mounts are very sturdy, but at the same time, can be very heavy. A German equatorial mount designed to hold a 6-inch reflector might weigh between 30 and 100 pounds, while a Dobsonian mount would weigh 15 pounds or less.
Fork equatorial mounts, usually supplied with catadioptric instruments, support their telescopes on bearings set between two short "prongs." The prongs usually extend from a rotatable circular base, which is usually tilted match the observer's latitude. The biggest pluses to the fork mount are its light weight and compact design. But if the design of the fork's prongs is too lightweight or compact, then they will transmit every little vibration to the telescope, like a tuning fork. Most of today's fork-mounted telescopes offer a compromise between the two.
Within the past two years, a new breed of computerized altazimuth mounts are been introduced to the beginner telescope market niche. They have been nicknamed "GoTo" mounts because that is exactly what they are designed to do. After the telescope is set up outside and the power turned on, a lighted display on the mount's computer will begin to initialize the telescope to the sky. Using a handheld controller, the observer is asked to input the date and time, as well as select the time zone and location from the onboard computer's city database. When this data has been entered, the telescope will slew to a known alignment star that is up in the sky at the time. After the telescope stops, the observer uses the hand controller to center the reference star in the field of view. The same step is repeated for a second star and the telescope is then ready to locate hundreds, even thousands of sky objects all by itself that the observer selects from its pre-programmed database.
GoTo mounts incorporate some amazing technology. But they also present a dilemma. For the same amount of money that you might spend on, say, a 2.8-inch refractor on a GoTo mount, you could purchase a 6-inch Newtonian reflector on a Dobsonian mount. The GoTo refractor has great appeal, including automatic finding and tracking. The Newtonian also has great appeal, with about 460% more light-gathering surface area! That translates to an increase of about 1.5 magnitudes, and can mean the difference between seeing an object and missing it entirely. Indeed, only a choice that you can make!
There are many other things to look for when purchasing that first telescope. Eyepieces and finderscopes are especially critical. A finderscope is a small, low-power refracting telescope mounting on the telescope tube to help the observer aim the instrument, like a gun sight. Finderscopes are specified in the same manner as binoculars. A 6x30 finder, for example, has an aperture of 30mm and yields 6 power. Most experienced observers agree that this is the smallest useful size for a finderscope. Smaller finders, such as 5x24 finders that come on many department-store telescopes, will show fewer stars, making it more difficult for the observer to find his or her way.
A popular alternative to traditional finderscopes is a so-called "unity finder." Several are marketed today, though the original, the Telrad, remains the most popular. Unity finders don’t magnify the view. Rather, they let the observer aim the telescope by looking through a clear window that has superimposed on it a red bull's-eye or dot. Simply align the dot or bull's-eye with your target and you're there!
If this is your first telescope, then eyepieces are an especially important consideration. Eyepieces are rated by focal lengths (usually in millimeters), not magnification, since that will also depend on the telescope. Magnification is simply a ratio of focal lengths, the telescope's divided by the eyepiece's. A 25-mm eyepiece, for instance, will produce 48x in a 6-inch telescope with a 48-inch focal length, but 80x in an 8-inch instrument with an 80-inch focal length. In general, you will want two or three eyepieces for viewing. One should produce low power, another for moderate magnification, and a third for high-powered views. Select the appropriate focal lengths based on the aperture of your telescope, remembering the rules of over-magnification cited earlier. A good mix might be a 22-mm to 30-mm eyepiece for low power, 12mm to 17mm for moderate power, and a 7mm to 10mm for high power.
While an eyepiece must be selected based on telescope aperture, the eyepiece barrel must be the proper size to fit into the telescope's focusing mount. All of the telescopes listed here use 1.25-inch diameter eyepieces, a standard that has been around for years, but many department-store telescopes only accept subdiameter, 0.965-inch eyepieces. Sadly, these are frequently very poor in quality, greatly reducing telescope performance.
When shopping for a telescope, be sure to shop at the right stores. Most toy stores and department stores do NOT sell high-quality telescopes. Many camera stores sell a few top-notch telescopes intermingled with an overabundance of poor imitations, making it difficult to tell one from another. Ask the salesperson's advice, but remember they are trying to sell you something, so take their counsel cautiously.
Some telescopes are sold through networks of dealers, while others are only sold directly from the company to the consumer. If you plan on purchasing an instrument from a dealer, make your decision on more than just price alone. Ideally, it is best to purchase from a local outlet, but if one is unavailable, then mail order may be your only alternative. In that case, make certain to ask about "hidden costs," such as exorbitant shipping and handling charges.
The best way to select a good telescope is to seek out the help of a local amateur astronomy club. Chances are good that at least one member already owns the telescope that you are considering and will happily share personal experiences, both good and bad. Plan on attending a club observing session, where members bring along their telescopes and set them up side-by-side to share with each other the excitement of sky watching. To find out if there is a club near you, contact local planetariums, museums, and nature centers. An organization guide to astronomy clubs is also available on Astronomy magazine's web site.
I would also humbly recommend my book Star Ware. Look for the third edition in bookstores or on-line. Star Ware reviews the telescope market, dissecting more than 400 individual models. One is bound to be right for you.