Equipped to observe: Telescope basics
If you're thinking about buying a telescope, there are important factors to consider before you invest. In general it depends on your budget and what you want to observe, usually in that order.
Buying the most powerful telescope you can afford isn't necessary, and might not even be a good idea for a beginner. Smaller telescopes will allow you to view a variety of nearby and deep-space objects without spending more than a few hundred dollars.
If you're completely new to the field, you could start with some relatively inexpensive 50mm binoculars and a book like "NightWatch" by Terence Dickinson before you move on to a telescope. A few reputable telescope (and binocular) brands are Celestron, Meade, Bushnell and Orion.
A computerized telescope offers the most convenience. You'll spend much less time fussing around trying to find an object and more time observing it. After entering the date, time and your location into the built-in handset and aligning your telescope to any two or three random bright stars, you'll receive a list that includes thousands of objects visible from your location. Select one and the autofinder slews the telescope to it while the tracking feature locks it in view.
You can change the magnification of any telescope simply by switching out the eyepiece. To determine the magnification, divide the focal length of the telescope by the focal length of the eyepiece. These measurements will be displayed on the box and in the booklet that comes in it. The focal length of the eyepiece will be clearly displayed right on the eyepiece.
For example, a telescope with a focal length of 1500mm paired with a 25mm eyepiece will give the telescope a 60x magnification (1500/25 = 60). If you swap the 25mm eyepiece with a 10mm eyepiece, the same telescope's magnification increases to 150x (1500/10 = 150).
The diameter of the telescope helps determine what magnification is best for viewing different objects. (When someone refers to a "6-inch" telescope, they are referring to the diameter.) Remember that clarity is compromised as power increases - kind of like when you zoom in too far on a digital picture. Using the same example, as you approach 300x magnification on a 6-inch telescope, most deep space objects, like galaxies, become too fuzzy to see.
A 6-inch scope has a maximum useful power of about 150 to 225x on an average night and a 4-inch scope tops out around 100 to 120x.
There is not enough room to explore every aspect of the observing equipment out there. The most important point is that larger and more expensive is not necessarily better. Don't skimp on cost, but you don't have to break the bank either. I also recommend subscribing to Astronomy or Sky & Telescope magazine, which offer a regular dose of interesting tidbits, observing ideas and tips, and sky maps, and also advertise really good telescopes and binoculars.