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Reflector vs. Refractor Telescopes
There's a difference between refractor and reflector telescopes. Refractor telescopes use a series of lenses to magnify the images of stars and planets, nebulas and galaxies, and focus them into the eyepiece. It uses the same technology as binoculars and rifle scopes. Reflector telescopes, on the other hand, use a set of two mirrors --- either two parabolic (curved) mirrors, or one parabolic and one flat --- that reflect and aim the image into the eyepiece.
Reflecting Telescope Mirrors
Reflector telescopes are the best type of telescope for scientific and research astronomy and are widely available for amateur astronomers as well. Many reflector telescopes use complimentary optics to enhance the quality of the image. There are two main types of reflecting telescopes: Newtonian and Cassegrain. A Newtonian reflector telescope has a large, curved mirror at the base of the telescope tube and a flat mirror near the front of the tube. Cassegrain reflector telescopes have a large, curved mirror at the base of the tube, like the Newtonian, and a smaller curved mirror at the top of the tube facing the large mirror. In both cases, the light comes through the top of the tube and down through a small hole in the center of the top mirror. The light reflects off the back mirror and onto the top mirror, which aims and reflects the image through a tiny hole in the back mirror and into the eyepiece.
Benefits Over Refracting Telescopes
The key benefit that reflecting telescopes has over refracting telescopes is that they don't produce chromatic aberration, a distortion where the lenses tend make the light fuzzy at the edges and cause the light to break down into its individual colors, like a prism. Another benefit is that mirrors cost less than lenses, which have to be specially ground for the individual telescopes. Mirrors also allow for much larger telescopes, because they can gather more light and produce a sharper image than lenses.
Disadvantages of Reflector Telescopes
Reflectors do have a few disadvantages, however. One problem is that the larger the mirrors used, the larger the tube needs to be. Larger research telescopes require larger domes to house them, all of which adds to the cost of the scope. The larger size also makes it more difficult for amateur astronomers to store and transport the telescope to viewing sites. Also, weather and other natural phenomena (i.e. bird droppings) make mirror recoating or replacement necessary from time to time.