Choosing a Telescope
 
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Often, beginning astronomers and non-astronomer friends ask me "What power telescope should I buy." Actually, power is not the factor to consider when choosing a telescope. To understand why not, I will explain the components of a telescope and the functions they perform. When considering a telescope one should also know what they will be looking at with it. The three basic types of telescopes, refractor, reflector, and catadioptic, fulfill different needs..

A telescope consists of two basic components. The job of first part of the telescope is to gather light from planets, stars and galaxies. The second part, the eyepiece is used to magnify the image that is gathered by the first part.

The most important factor for gathering light is aperture. When you look the tube of a telescope, a "fatter" tube has a larger aperture. In other words the bigger the circle at the end of the telescope closest to the stars, the bigger the aperture. Telescope manufacturers usually advertise telescope aperture measured in millimeters or inches. You may see an add in a magazine for a 8" LX200, or a 12.5" inch Newtonian. On the other hand you may see a 80mm refractor, or a 125mm mak-newt. These numbers weather in millimeters or inches represent the diameter ot the primary mirror in a reflector or the primary lens in a refractor. When it comes to aperture a reflector gives the most for your dollar. This is because it is easier and thus less expensive to create a large mirror than a large lens.
Focal Ratio
  Another term you will hear astronomer mention concerning their telescopes "this is an F5". The term F5 or F6 refers to the focal ratio. The focal ratio is the focal length of the telescope divided by the aperture. Thus a short fat telescope will have a lower F ratio than a longer telescope of the same width.
Eyepieces
  The second part of the telescope, the eyepiece system magnifies the image the the first part captures. The magnification is determined by dividing the focal length of the telescope by the focal length of the eyepiece. So if your telescope has a 1000mm focal length and the eyepiece has a 10mm focal length you will have 100x magnification. So as you can see if you have a set of eyepieces, say 32mm, 26mm, 9mm, and 6mm, you have four different magnifications or "powers" for your telescope.
Mount
 

Another thing to consider when buying a telescope is the type of mounting system. The two primarily found mounting types are called equatorial and alt-azimuth. Each of these methods have benefits depending what you are trying to do. While both systems have two axis of rotation, their orientation is what sets them apart.

An equatorial mount has one axis lined up with the axis of the earth and the other with the earth's equator. The benefit of this set up has to do with keeping an image in the eyepiece. Since the earth is constantly turning on its axis, when we magnify a planet, star, or other object, it tends to drift out of the eyepiece. The more magnification we use the faster it will drift out of the field of view. With an equatorial mount, once an object is located, you only need to move one axis to keep it in the eyepiece. With a alt-az mount, we need to adjust both axis to keep the object in the eyepiece which can be a difficult task.

The most popular alt azimuth mount, designed by John Dobson, is called a dobsonian mount or "Dob". While these mounts are not ideal for tracking and object, they are the best arrangement for finding and pointing to an object. To many amateur astronomers finding the object is more rewarding than looking at it. Because a dob mounted telescope is easy to set up and maneuver, it is a popular telescope among amateur astronomers.
Manual or Go-To
  Other modern "go-to" scopes use an alt-az mount caled a fork mount. Because these mounts are computer controlled, the drive system can accurately move the scope on two axis to keep an object accurately aligned.
Try them out!
  The most important thing I can stress when thinking about the purchase of a telescope it to visit a local amateur astronomy club. These clubs exist in every state. Every club I have visited has several telescopes for use by members. Clubs also have "public nights" where they bring the telescopes out for the public to use. If you attend, don't be afraid to ask questions about the equipment. Usually there are telescopes outside and a line to look at something in the eyepiece. While this is fun, it is just the tip of the iceberg when it cones to a night observing. If it is not crowded, ask if you can't try finding something in the scope. Ask if they have a suggestion for an object to find. Ask them to step you through the process of hunting for the object and zeroing in on it. Ask them what types of telescopes they own and which ones the use most often. Becoming a member also has it's rewards. Clubs have "member nights" which are less crowded and give you an opportunity to work hands on with a seasoned astronomer.

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