There are two basic types of telescopes, refracting and reflecting. Refracting telescopes use lenses to focus the light, and reflecting telescopes use mirrors. Iíll talk first about refracting ones.
Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is. Both lenses are in a shape thatís called 'convex'. Convex lenses work by bending light inwards (like in the diagram). This is what makes the image look smaller. The biggest refracting telescope in the world is located at the Yerkes Observatory of the University of Chicago at Williams Bay, Wisconsin. It has an objective lens thatís 1.02 meters (40 inches) across, and itís total power is 195x.
(The 40 inch telescope at Yerkes Observatory)
Reflecting telescopes, on the other hand, donít use lenses at all. Instead, they use mirrors to focus the light together. In this case, the type of mirror that they use is a concave mirror. Mirrors of this shape also accomplish the goal of bending light together, except that they do it by reflecting the light instead of bending it as it passes through (like lenses do).
The problem with these telescopes is that in order for a person to see the image at the focal point, the telescope has to be so big that a person can actually sit in the middle. (Or they have to put a small mirror in the middle to reflect the image out the side of the telescope, instead.) This is why the reflecting telescope built in the 1780ís by the famous astronomer William Hershel was so big:
Some more complicated telescopes work by combining both lenses and mirrors, but there are so many different ways of doing this that Iím not going to try to get into them. The other way you may have heard of telescopes being categorized is by what kind of light they pick up, rather than how they work. This is what theyíre talking about when they say things like Radio Telescopes and X Ray Telescopes
, which are still made by some combination of refracting and reflecting.
(republished on 07/29/06)