A Guide to Telescopes
Telescope designs
There are three main types of telescopes, each with their own advantages and benefits.
Refractor
The refractor is what most people think of when they hear the word "telescope". Today's designs are essentially the same as those developed centuries ago. They consist of a long tube with a lens at one end (the objective lens), and an eyepiece at the other. This simple and reliable design is easy to use and requires almost no maintainance. The main advantage of this type of telescope is its extremely low distortion when viewing planets.
Reflector
The Newtonian reflector, or simply "reflector", uses a concave mirror to focus light rather than a lens. The light is focused on a second, flat mirror that reflects the image sideways out to the eyepiece. The advantage with this type of telescope is economy. You get a lot more light-gathering power per dollar spent. To put it simply, mirrors are much easier (and less expensive) to produce than lenses. You can get a much larger reflector, which can gather more light and provide a brighter, sharper image than any refractor for the same money.
Catadioptric
The Catadioptric scope is the most versatile, using a combination of mirrors and lenses to get the advantages of both. The most common type is the Schmidt-Cassegrain telescope. As you can see in the illustration, this scope uses a front correcting lens before being focused by a set of two mirrors. The result is the best all-around telescope; good for lunar, planetary and binary star observation as well as terrestrial viewing and photography. It is also very compact compared to the other types.
Telescope mounts
There are two basic types of mounts avaliable for your telescope, the altazimuth mount and the equatorial mount.
Altazimuth mount
The altazimuth mount is more economical and is adjustable for altitude (up and down) and azimuth (left and right) to achieve a full range of motion. Although not as easy to use as an equatorial mount, the altazimuth mount is substantially less expensive, making it well suited to either entry level telescopes or the large "Dobsonian Reflector" telescopes.
Equatorial mount
As the Earth rotates on its axis, objects in the night sky appear to travel in an arc across the sky. By aligning the mount relative to the Earth's rotation, the controls on an equatorial mount allow you to easily follow an object and keep it in the field of view. Some equatorial mounts are motorized with a clock drive. This allows you to "lock on" to your celestial object and view for extended periods hands-free.
Polar alignment
In order for an equatorial mount to work correctly, the telescope must be properly aligned. The method described here is fine for the casual observer, but as you grow more experienced you may want a greater degree of precision. You can find more precise methods of polar alignment in astronomy textbooks or other sites on the Internet.
First, you need to set the latitude on your mount. If you're in the Lehigh Valley area, your latitude is 41 degrees North. Adjust the latitude on your scope to 41 degrees.
Next, the mount itself should be level. If your mount has a level built in, you're all set, just adjust your tripod until the bubble in the level is right in the middle of the bulls-eye. If you don't have a level on your mount, make sure your scope is set up on a reasonably level surface.
Finally, the right ascention axis (that is, the polar axis) should be pointed toward the North Celestial Pole. Easy enough, just point the mount toward Polaris, the north star. If you're not sure how to find Polaris, just take a look at this map.
That's it, you're done! Once the scope is oriented in this way you'll only have to use the ascention adjustment on your telescope to find objects, and you will be able to track objects continuously with the right ascention cable. If you are using a motor drive, it will perform the tracking for you, keeping the object in view continuously.