By Jon Stewart-Taylor
This pages addresses two of the most important decisions begining astronomers make; Whether to buy a telescope, and if so, what kind. This page will discus telescopes and mountings. Other pages in the Recreational Astronomer will discuss accessories such as eyepieces and finders.
The single most important thing to remember is; Don't buy a telescope at a department store or from a deep-discounter catalog. These telescopes may have decent optics. However, they usually have inadequate finders and shaky mounts. This means you can't find anything other than the moon, and that the telescope won't stay where you point it for you to look at something if you can find it.
Department store 'scopes are often advertised as giving magnifications of 300x or even 600x. An easy rule of thumb is not to buy any telescope promoted as giving high magnififcation. The laws of physics put an upper limit on the maximum useful magnification a telescope can give as about 2x per millimeter, or 50x per inch, of the objective (primary mirror or lens). This means most small scopes don't have useful magnifications above about 120x. Anything more and the view becomes increasingly blurry.
Department store telescopes are frustration in a box.
Small telescopes can be a good choice, but should be of good quality and purchased with reasonable expectations.
With Rule Number One firmly in mind, here's a quick checklist fo find out whether you're ready for a telescope.
If you don't know the answers to these questions, it's probably time to do more research and get more experience. The rest of this page will try to help with the research part, but the experience is up to you.
The best way to find out what kind of telescope you would be happy with is to go to club star parties and observing sessions and get hands-on exsperieince with different types of telescopes. Most telescope owners will be happy to explain what they like and don't like about their instruments, and many will be willing to let you "test drive" them. There's no substitute for hands-on experience, and it's even better if you can have fun while getting it.
So, what kind of telescope should you get? First and foremost, the best telescope for you is the one you will use most often. A number of factors influence how often a telescope is used:
Consider your experience level when making decisions about fancy equipment. If you've used telescopes before, you're more likely to be able to take advantage of equatorial mounts, motorized drives, and digital setting circles or goto features. If you are just starting out in astronomy, complexities like those will probably just confuse or annoy you until you get enough experience to understand when and how to use them.
if you have never used a telescope before, consider starting out with binoculars. Binoculars are really just a pair of small telescopes joinned together, one for each eye. They are an excellent way to learn your way around the sky, and will take you about half way from unided eyes to small telescopes. A decent pair of binoculars costs less that a telescope. You may even already have a pair. The practice and experience gained finding celestial objects with binoculars can be transferred to a telescope when you get one. Finally, the field of view of most finderscopes is comparable to that of binoculars, which makes binoculars a handy aid for finding objects to view with your telescope.
The various types of telescopes all have common features: an objective lens or mirror to gather light, and eyepiece to magnify the image, a finder to point the scope to an object, and a mount to support the telescope and hold it in position while you view.
The light-gathering ability of a telescope is a function of how big the objective mirror or lens is: the bigger the objective, the more light it gathers.
Magnification is simply making small images bigger. Magnification (m) is determined by the focal length of the objective (Fo) and the focal length of the eyepiece (Fe): m = Fo / Fe. In theory, any telescope can be made to give any magnification by using an eyepiece with the right focal length. In practice, using too much magnification will just make the image blurry.
Focal Ratio, often abbreviated with "f/", is defined as the focal length of the objective divided by the diameter of the objective. for example, if a telescope has an 8-inch diameter objective, with a focal length of 48 inches, the f/ratio is 48/8, or f/6. Focal lengths from f/4 to f/6 are considered short. From f/6 up to f/8 are "normal", and f/8 and up are long.
Short f/ratio telescopes have shorter tubes for a given objective diameter, give lower magnifications with a given eyepiece, and usually produce wider fields of view. This is usually what you want for deep-sky objects.
Longer f/ratios require longer tubes, but tend to be of higher obtical quality because it's eaiser to make longer f/ratio mirrors and lenses. Long f/ratio scopes give higher magnifications with a given eyepiece, and usually a narrower field of view. Long fratio scopes are usually what you want for planets and the moon.
There are three basic types of telsecopes:
Newtonian reflectors may be the best telescope for beginners. They have a primary mirror at the bottom of the tube, which reflects light up to a small diagonal mirror. The diagonal mirror sends the light out the side of the tube to the eyepiece. Due to their simple two-element design, Newtonians are usually the least expensive per inch of objective aperture. Prices var dramatically based on quality and sophistication, especially of the mounting. Newtonians come in a wide range of f/ratios, from short (f/4) to long (anything over f/8).
Refactor telescopes use a lens as their light gathering objective. Because there is no secondary mirror, all of the lens collects light (in most reflectors the primary mirrors are partly shaded by the secondary). There are no diffraction effects to reduce the clarity of the image. On the other hand, since lenses are more difficult to make than mirrors (they have twice as many optical surfaces) and a high-quality refractor objective may contain 3 pieces of glass for 6 surfaces, refractors are much more expensive than reflectors of the same size. A well made refractor can give superieror views for its size, paticularly on brighter objects such as planets. Inexpensive refractors may have decent optics, but often the mountings are flimsy and make the telescope almost unusable.
Catadioptric telescopes have both a lens at the front and a mirror at the back. In the two kinds found most frequently in amateur telescopes (Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MCT)) there's a secondary mirror at the front as well, so the light path is folded back and passes through a small hole in the primary mirror. All this folding makes these telescopes very compact and portable compared with standard refractors or Newtonians.
SCTs are complex to manufacture and align. They're mass-produced with sometimes limited quality control, and produce ocaisional lemons. MCTs tend to have higher quality optics, but are generally made with smaller apertures than SCTs.
Catadioptrics are intermediate in price between reflectors and refractors. Under some weather conditions, the lens at the front is susceptible to dew. Due to their relatively stubby tubes, moderately large apertures, and the fact that they are usually on motor-driven mounts, SCTs are usually good for astro imaging. If photography or portability are primary considerations, a catadioptric may be a good choice.
There are two types of telescope mountings: alt-azimuth, and equatorial. Alt-azimuth mounts rotate around a vertical and a horizontal axis. They are usually fairly simple, and simple alt-azimuth mounts cost the least. The drawback to an alt-azimuth mount is that in order to follow objects in the sky, you must move the mount in both axes at the same time.
Equatorial mounts have a polar axis which you must align with the earth's axis of rotation (i.e., to point to the North Star) and a declination axis which rotates perpendicular to the polar aDobsonian telescopes are a basic Newtonian telescope on a very simple, stable, and easy to use alt-azimuth mount. The mounts are usually made of solid but very inexpensive materials such as plywood and Formica®. This means that most of the money you spend buys optics, and therefore you can get a larger aperture (or higher quality optics) for a given price. The drawback is that the mount cannot be made to track the sky (although computerized mounts are changing that). Most people don't find this to be a problem in practice, and many find the simple motions of the alt-azimuth mount easier to use than an equatorial.xis. Once you've got the polar axis aligned, you can track the movent of the sky , and therefore objects in your scope, moving only the polar axis. This makes it easier to motorize the tracking, and most telescopes with motorized tracking use equatorial mounts.
There are some variations on these two themes:
Once you've bought the telescope, you're all set, right? Nope. People with lots of observing experience sometimes get by with a bare scope, but there are many accessories we lesser mortals can't do without. You'll need a finderscope or unit-power finder, an eypiece or two (make sure your telescope can accomodate 1.25" eyepieces rather than the smaller .965" size), decent sky charts, and maybe a red flashlight. All of these accessories should be of good quality also, and depending on the cost of your scope, may total as much as 1/3 to 1/2 the price of the telescope itself.
So, what do beginner's telescopes cost? That depends on the type and quality. Prices vary quite a bit, from less than $200 for small refractors to over $2000 for large equatorial mount scopes with lots of accessories. Most good beginner's scopes sell for less than $500, although trade-offs of quality, accessories, and price can drop it as low as $250, or raise it to around $1000.
You can find current prices in several ways. if you have a telescope store handy, go ask. Look in current issues of Sky and Telescope or Astronomy magazines. Get catalogs from mail-order stores. In general, you should expect to get what you pay for, and try to pay only for what you need.
One thing to keep in mind is that you can improve most telsecopes over time. Accessories such as eyepieces and the finder can be replaced one at a time as funds permit. An imperfect mount can be reinforced, re-balanced, or replaced. Focusers, mirror mounts, and even the primary objective itself can be improved or replaced. Compromising on the quality of "replaceables" can lower the initial cost of a telescope considerably. However, even if you can improve substandard mounts or finders doesn't mean you should buy a scope like that. There's very little which can ruin a scope's usabilty than a poor finder or mount. Buying a scope with both is asking for trouble.
The bottom line recommendation from many experts for a first telescope is a 6"-8" f/6 or longer dobsonian. If price is the primary consideration, a small refractor or a pair of binoculars may serve. If restricted storage space or eventual astroimaging is your main concern, an 8" catadioptric may be a good choice.
If your budget is less than $200, i strongly recommend binoculars and a tripod. If you must get a telescope, go for a well-made 60mm refractor, with 1.25" eypieces if you can find one.
In the $200-$400 range, i recommend a 4" Dobsonian reflector with a unit-power finder such as a Telrad®. That is enough scope to keep you busy for at least 2 years, and the accessories and experience will transfer to your next scope. Both Orion Telescope and Stargazer Steve make very good scopes. The Orion is currently priced at about $250 including shipping, and includes decent eyepieces and an above-average finder.
For $400 and up, get a Dobsonian reflector in the 6" to 8" size. That should keep you busy for many years, and might even be the only scope you ever need. There are several manufactures who make scopes in this size and price range. If storage, portability, or astroimaging are primary considerations, an 8" SCT may be a good choice. However, if you're considering astroimaging, you may want to get a beginner's scope first to learn the sky and get your imaging scope later.
Local telescope store: In the Triangle area, the closest 'real' telescope store is Camera Corner in Greensborough. There's a lot to be said for being able to look over the scopes up close, and for having a knowledgeable sales staff. In Chapel Hill, Wolfe Camera and Hungates carry some acceptable small scopes (but avoid the ones claiming overly high magnification).
Mail Order: If a low price is more important to you than service and the chance to check out a piece of equipment before you buy it, mail order may be the way to go. Buying things mail order is always something of a risk. In general there is no way to be certain you won't be incovenienced, lied to, or even ripped off by mail-order suppliers. However, if you do some research, you should be able to minimize your risk by choosing your supplier carefully.
In summary, before you buy a telescope, make sure you are ready. Check out different scopes at observing sessions and star parties. Keep your intended usage in mind, and make sure it's a telescope you will use. Buy from a reputable source (whether mail order or retail), and keep the quality vs. price trade-off firmly in mind. Don't skimp, but do understand the difference between needs and wants. Save about 1/4 to 1/3 of your budget for good accessories. Eyepieces and finders contribute as much to the usability of a scope as the primary optics.
If you take a little care and do your research, you should be able to get a scope you can use, and will help you enjoy the night sky. And that's what Recreational Astronomy is really all about.
The info and especially the opinions i've expressed here are offered in good faith, based on what works well for me, and what i wish i'd known when i was starting out. However, everybody's different, so what works well for me might not be right for you. Here are some other people's takes:
At least one of Sky and Telescope or Astronomy magazines are usually considered essential for amateur astronomers. They often contain articles describing telescopes in general, and evaluations of specific brands and models. They contain (many!) ads.
Phillip Harrington's Starware reviews many, many telescopes and accessories. The cost of the book is less than 10% of the cost of the scope you'll eventually buy. It's a very good investment.
Thanks to Bill, Steve, Desmond, and Linda, for letting me play with your 'scopes.
© 2006 Jon Stewart-Taylor
This page last updated March 24 2006.