What Are the 2 Types of Telescopes?

Video Transcript

"Hi, I'm Steve Jones and I'm going to tell you something about the two basic types of telescopes. First of all you have reflecting telescopes and you have refracting telescopes. Reflecting telescopes use mirrors as the objective, a refracting telescope uses an objective lens. The reason for the difference between the two, they both do the same job, the objective creates the first image from the distant object, whether it's an objective lens or an objective mirror. Then the eye lens magnifies that image. The reason for using a reflecting telescope is purely a practicality. First of all the curvature of it can be closely controlled, and it is very accurate and you get a single reflection. With a lens, because the lens is quite thick, you get a double refraction and therefore to get a good image is very difficult if the lens is big. So there is a limit to the size of an objective lens and the limit is something like twelve inches at the most. Whereas an objective mirror can be many many, fifteen, twenty feet across. It can be much much bigger and therefore the amount of light it can collect is much greater. It can be built on to a very solid platform. So those are the two basic types of telescope. A refracting telescope normally used on earth, but can be used for astronomy but only of a small kind, up to a foot in diameter, and then the reflecting telescope, up to fifteen feet in diameter, used in all of the big optical telescopes throughout the world. So there we are, two types of telescope, reflecting and refracting."