Figure 3

Figure 3: Examples of the two major telescope classes

Look at the refractor illustration. Light from the tree is bent by the objective lens so that it is concentrated onto the eyepiece. The eyepiece magnifies the image of the tree. Remember that I told you that any telescope may be made to have any magnification you wish? Well changing a telescope’s magnification is merely a matter of changing its eyepiece! An observer will usually have several eyepieces to give him/her the range of magnifications he/she prefers.

Do you see that the refractor’s objective has two parts or elements? The front-most element is convex on both sides while the other element is concave on one side. To understand why two elements are needed instead of one, you need to recall that a refractor’s lens works by bending light rays in the same manner as a prism does. However, this bent light also wants to break up into rainbow colors just like sunlight through a prism! Imagine seeing every object through your telescope covered with all of the colors of the rainbow! Such an image would hardly be realistic. False color in an image is called chromatic aberration. Chromatic aberration is the reason there are two elements in the objective lens. The front element does most of the bending while the other element causes almost all of the false rainbow colors to recombine back into the original colors of the object. Such a two element objective is said to be achromatic, a term derived from the Greek words meaning without color. Thus, this type of telescope is referred to as an achromatic refractor because it will remove almost all of the false color from an image.