Choosing an Astronomical Telescope

6 Things To Know About How To Buy a Telescope From Nick Greene 

1. Power - A good scope will not talk about its "Power": If the box blares "300X" or other numbers about the "Power" the scope within has, Caution! High power sounds great, but, there's a catch. While high magnification makes an object appear larger, light gathered by the scope is spread over a larger area creating a fainter image. Also, "high-powered" scopes have restrictions of the eyepiece design, which may limit how much of the large image you can actually see. Sometimes, lower power provides a better viewing experience.
2. Refractor/Reflector - Advantages and disadvantages to each type: A refractor uses two lenses. At one end, is the larger lens, called the objective. On the other end is the lens you look through, called the ocular or eyepiece. A reflector works a bit differently. Light is gathered at the bottom of the telescope by a concave mirror, called the Primary, which has a parabolic shape. There are many ways the primary can focus the light, and how it is done determines the type of reflecting scope. Read more about the types of scopes: Telescope.
3. Aperture - Aperture size is the true key to the "power" of a telescope: The aperture of a scope refers to the diameter of either the objective lens of a refractor or objective mirror of a reflector. The aperture size is the true key to the "power" of a telescope. Its ability to gather light is directly proportional to the size of its aperture and the more light a scope can gather, the better the image you will see.
   OK, so you're thinking, "I'll just buy the biggest telescope I can afford." Unless you can afford to invest in your own observatory as well, don't go too big. A small scope you can transport will probably see a lot more use than a larger one you don't feel like hauling around.
   Typically, 2.4 inch (60mm) and 3.1 inch(80mm) refractors and 4.5 inch and 6 inch reflectors are popular for most amateurs.
4. Focal Ratio - Know a Telescope's Focal Ratio: The focal ratio is calculated by dividing aperture size into its focal length. The focal length is measured from the main lens (or mirror) to where the light converges to focus. As an example, a scope with an aperture of 4.5 inches and focal length of 45 inches, will have a focal ratio of f10.
   While a higher focal ratio does not always mean a higher quality image, it often means as good an image for similar cost. However, a higher focal ratio with the same size aperture means a longer scope, which can translate into transportation woes.
5. Mount - Necessary for steady viewing: It's likely you never even considered a mount when you thought of buying a telescope. Most people don't. However, the mount is a very important part of a scope.
   Some kind of stand to hold the scope steady is essential. It is extremely difficult, if not impossible, to view a distant object if the scope is not very steady. Most people do not have that steady of a grip.
   
   There are basically 2 types of mounts, altazimuth & equatorial. Altazimuth is similar to a camera tripod. It allows the telescope to move up and down (altitude) and back and forth (azimuth). The equatorial is designed to follow the movement of objects in the sky. Higher end equatorials come with a motor drive to follow the rotation of the Earth, keeping an object in your field of view longer. Many equatorial mounts come with small computers, which aim the scope automatically.
6. Eyepieces - Power is not the object: Your new scope should have at least 1 eyepiece, and often 2 or 3. An eyepiece is rated by millimeters (mm), with smaller numbers indicating higher magnification. A 25mm eyepiece is common and appropriate for most beginners.
   Earlier, discussing power, I said a telescope's power or magnification is not the best indicator of a good scope. As with the whole, so the parts. A higher power eyepiece does not mean better viewing. Higher and lower power eyepieces each have their place in observing.
   
   While a higher magnification eyepiece may provide more details, it may be harder to keep an object in view, unless you are using a motorized mount. They also require the scope to gather more light to provide a clearer image.
   A lower power eyepiece makes it easier to find objects and keep them in view. Lower magnification eyepieces require less light, so viewing dimmer objects is easier.
   
   Excerpted from: "Buying and Using a Telescope" From Nick Greene on About.Com