Optical telescopes may be divided into two general categories: (1) refracting telescopes that use lenses to gather and focus light, and (2) reflecting telescopes that use mirrors to accomplish the same purpose. We discuss the principles of refracting telescopes in this section and of reflecting telescopes in the following section.

Principle of Refraction

As we have discussed in earlier sections, the direction of light propagation is changed at the boundary of glass and air by refraction. By designing lenses having the right curvature, this principle can be used to gather and focus light. The following figure illustrates the use of a lens to gather and focus light, and the use of two lenses to make a simple refracting telescope.

Principle of refraction and the refracting telescope

Chromatic Aberration

One problem with refracting telescopes is that there is a frequency dependence for refraction, so the amount of refraction at each surface of the lens depends on the wavelength. Thus, different wavelengths focus at slightly different points. This is called chromatic aberration, and causes objects like stars to be surrounded by fuzzy, rainbow colored halos. Chromatic aberration can be corrected by using a second carefully designed lens mounted behind the main objective lens of the telescope to compensate for the chromatic aberration and cause two wavelengths to focus at the same point. This is called an ACHROMATIC lens, but with a second such lens one only gets two wavelengths to focus at the same point, e.g. red and yellow. Then the blue wavelengths would be off.