How Telescopes Work

Look Out!

When purchasing a telescope, there are several things you should keep in mind to avoid buying one that won't meet your needs. Here are some of the most important ones:

Do not be lured by claims of high power magnification on the box or in the ad! This consideration is perhaps the most misleading to novice telescope buyers. Often, manufacturers of "cheap, department store" telescopes will display "200x power or more" on the boxes of their products. The magnification or power has little to do with the optical performance of the telescope, and is not a primary consideration. The telescope's ability to enlarge an image (magnification) depends upon the combination of the lenses used, usually a long focal length objective lens or primary mirror in combination with a short focal length eyepiece. As the magnification of an image increases, the field of view and the brightness of the image decrease. A general rule about magnification is that the telescope's maximum magnification is 40x to 60x (average = 50x) per inch of aperture. Since any magnification can be achieved for almost any telescope by using different eyepieces, aperture becomes a more important feature than magnification. Furthermore, most astronomical objects are best viewed on a low magnification or power to gather the most light possible.
Avoid aperture fever! Aperture is probably the most important consideration when buying a telescope, but it is not the only consideration. You want to purchase as much aperture as you can reasonably afford, but you must also keep in mind other factors such as size, weight, storage space, portability and sky conditions. Big telescopes are heavy to carry, and take up a lot of storage space in your house and your car. The biggest telescope is not always the best one! For most observers, the following aperture sizes will be more than sufficient: refractors: 3 inches / 80 millimeters; reflectors: 4 to 8 inches / 10 to 20 centimeters; compound telescopes: 6 to 8 inches / 16 to 20 centimeters
Check the focuser. The focuser moves the eyepiece holder up and down slightly, adjusting the focus of the eyepiece for each individual observer. There are friction focusers or rack-and-pinion focusers. Regardless of the type on your telescope, your focuser should move smoothly without causing your telescope to shake. If it does not move smoothly, then when you have to focus the eyepiece (change eyepieces, or when someone else looks in the telescope), the object is likely to move out of the field of view, especially at high magnification. You may not have the choice of focuser on your telescope, but it is nonetheless helpful to know.
Does your telescope come with eyepieces? Next to the telescope itself and the mount, the eyepieces will be your most important purchase. Most telescopes come with one eyepiece (low power), some telescopes come with none. Therefore, you may have to purchase eyepieces so that you can vary the magnification of your telescope.
Make sure that your eyepieces fit your telescope's eyepiece holder. Eyepieces come in three diameters: 0.965 inches (2.45 cm), 1.25 inches (3.18 cm), and 2 inches (5.08 cm). However, the eyepiece holder in your telescope is fixed. The "cheap, department store" telescopes tend to have 0.965-inch holders. Most telescopes have 1.25-inch holders. Some have 2-inch holders. Eyepieces of differing diameters are not usable in all telescopes. Make sure that the eyepiece holder in your telescope matches with the eyepieces you wish to purchase. You typically do not have an option on the size (the manufacturer sets the size with the telescope model), but you should know what it is so you can pick out the right eyepieces.
Look for a steady telescope mount. Regardless of the type of mount that you use, the most important thing is its stability. It should have a low center of mass so that it does not tip over easily, and should be able to support the weight of the telescope adequately. It should not vibrate when you touch it lightly. Finally, it should place the telescope at a comfortable height for you, whether you decide to stand or sit.
Do not be tempted to buy the most expensive telescope! Telescopes vary widely in price. They can range from a few hundred to several thousand dollars, depending upon the type. Two things to remember: No matter how good the telescope quality is, you probably won't enjoy it if you have to bankrupt your savings or re-mortgage your house to pay for it. You will have to purchase other things to complete your observing equipment (eyepieces, finders, filters).