Mirror Telescopes

Mirror telescopes are also known as reflecting telescopes. They use mirrors to concentrate and reflect light in order to see a distant object. Mirror telescopes were invented to replace refracting telescopes, which used lenses to concentrate and reflect light to view objects. Refracting telescopes had terrible distortion, resulting in severe chromatic aberrations. Mirrored telescopes have their own distortion problems, but they allow for larger lenses and a clearer view.

Isaac Newton is credited with having developed the first practical reflecting telescope in 1668. It used a concave primary mirror and a flat secondary mirror set at a 90 degree angle to the primary mirror to focus the light. With this telescope, he could see the moons of Jupiter. Reflecting telescopes work by gathering the light that objects in space put out and focusing that light onto the parabolic mirror of the telescope. This mirror concentrates the light into a single point, which is then reflected by the secondary mirror to the eyepiece of the telescope.

The angle of the secondary mirror determines where the eyepiece will be. Some telescopes have more than one secondary mirror to position the image for viewing. Since mirrors weigh much less than lenses, and since the support for the parabolic mirror is in the base of the telescope, these telescopes can be built extremely large to view objects that are very far away. The famous telescopes at the Palomar Observatory and the W. M. Keck Observatory are both mirror telescopes.