How to Buy a Telescope
You may be considering buying your first telescope. Before you decide on a telescope, here are three things to do:
1. Don't rush. You'll have to do a little research before you can pick out a telescope that suits your needs. Come to a couple of Star Parties with the club and look through other people’s telescopes. In amateur astronomy, this is equivalent to taking a “test drive.” Consider buying a good pair of binoculars first *$50 - $150). Take a look at the following sites for additional information.
2. Don't blow your money. A telescope is, and should be, a big investment. Saving money on a cheap setup will only disappoint you. Sure, you can spend under a hundred dollars on a starter scope for a nine-year-old (who's liable to break it anyway), but we strongly recommend that you don't waste a dime on one like that for yourself. Consider buying a used telescope. Consider having a person in your astronomy club help you pick out a telescope that is in your budget. Consider saving until your budget is $250 or more (think about the cost of a new mountain bike, golf clubs or decent sewing machine). Astronomy club members often know about unadvertised telescopes for sale.
3. Don't buy your telescope at a toy store or department store. The prices will sound good, but the quality of the optics, mounts and materials is inferior. Stick with a reputable brand. Please read this particular recommendation again.
I strongly encourage you to come to a local Star Party and write down the type of telescopes you like to look through and what specific eyepieces you think give you the best views. You can then begin your cost research and save for what you really want.
Components of a telescope
That said, no one can tell you what telescope to buy. The choice depends on what you want to do with it. One setup will prove best for seeing the planets, another for exploring faraway galaxies. By setup we mean that a telescope is more than just a tube you look through. It's a system that includes the main scope, a finderscope, more than one eyepiece, a tripod, and perhaps a camera adapter.
Get the biggest aperture you can. The aperture is the diameter of the lens or mirror that collects light. The magnification of your telescope is limited by the aperture. A larger aperture collects more light, allowing for greater resolution and higher magnification. Some cheap telescopes feature large magnification, but images are often fuzzy because the aperture is not large enough to support the magnification. Although small telescopes little affected by the atmosphere may give pleasing images even up to 100x per inch of aperture, no more detail is seen than at 50x per inch. On the other hand, large instruments, more affected by atmospheric seeing, may top out at 20x or 30x per inch. In practice, a 3 or 4 inch refractor may work well at 200x, but it is rare indeed that any size instrument benefits from more than two or three times that magnification.
Focal length, eyepieces and magnification
The distance light travels inside the telescope is the focal length. The eyepiece, which will be a separate piece on a good telescope, has its own focal length. The power of a telescope, often called magnification, is determined by dividing the focal length of the telescope by the focal length of the eyepiece. Be sure and purchase a telescope that allows for changeable eyepieces, as this is how you easily change the power of your telescope. Why would you want to? Some objects, such as planets, are better viewed at lower magnification, whereas others require more. Every telescope's power has a limit, typically about 50 times its aperture in inches. A telescope that has a 3.5-inch aperture won't support clear images beyond about 175 power (3.5 x 50).
How much power do you want? Almost any telescope will allow you to see Saturn's rings and Jupiter's larger moons. With 48 power, you'll see faint color bands on Jupiter. Beyond 100 power (still achievable with a 3.5" aperture), you can start tracking the changes in Jupiter's cloud structure. To see the structure of distant galaxies you may want to move up to a telescope with at least a 6-inch aperture.
For more telescope definitions, click here.
A finderscope is crucial to a good setup; it helps you find and center objects that would be elusive in the higher-powered main telescope. The finderscope must be adjustable so that it can be aligned with the main scope. Smaller telescopes will often come with a 5 x 24 finder scope, ample for finding the planets and bright stars. Searching for distant objects is better accomplished with a 6 x 30 finder scope. You should consider a Telrad finder if you enjoy observing relatively bright objects or if you just want a finder that's incredibly easy to use.
Focuser and controls
All of the knobs that control focusing and movement of the telescope must operate smoothly to insure a crisp image and accurate positioning. As the Earth rotates, your target moves quickly out of view, and nothing is more frustrating than clunky controls.
Mount, tripod and photography
A solid base, either a table-mount or a tripod similar to that used by a camera, is critical to steady viewing. The least expensive tripods (called alt-azimuth) move horizontally and vertically, requiring adjustment in both directions to keep an object in the scope. An alt-azimuth mount saves money and works fine for casual viewing. One helpful feature to look for is a "slow-motion" knob, which allows you to make smooth, minor adjustments to the position.
If you plan to get serious, consider an equatorial mount to automatically track the stars as they move across the sky. The equatorial mount provides one axis of motion that matches that of the Earth. Only one adjustment is required. For more money, you can buy a motor drive for the equatorial mount, and the object is tracked automatically.
For photography, an equatorial mount with a motor drive is a must, as is a camera adapter. If you plan to eventually do astrophotography, make sure the telescope you purchase supports an adapter. For more on these options, see the Astronomy Buyer's Guide advice on buying a telescope.
And one more thing...
Do more research! Check out online (and print) magazines, such as Sky & Telescope or Astronomy and ask around at a local observatory or astronomy club. Books can be a great resource, too!! Each telescope is better at some things than the other kind. It's a tough choice. I strongly suggest looking through other people's telescopes and binoculars at a Star Party to decide what's best for you! It's free to gather the information. Check out the Astro Links.