Reflecting Telescope

The reflecting telescope is an optical type which employs both curved and flat mirrors to create an image, as opposed to the refracting telescope with uses lenses to bend light to create an image.
Originally an idea of the Italian religious Niccolo Zucchi, the first reflecting telescope was invented on 1670 b y Sir Isaac Newton. Because of the inferior quality of the refracting telescope, Newton designed the reflecting telescope, which is traditionally double-mirrored.

Focus designs of reflecting telescopes:

1. Newtonian focus - by Newton, is the focus for reflecting telescope for which the focal point lies to the side of the telescope. The reflecting telescope receives the light and is reflected from the first mirror to a second mirror midway the telescope tube. The second mirror of the reflecting telescope directs the light to the side of the telescope.

Nowadays, this focus design by Newton is only popular in amateur and hobby astronomy.

Professional astronomers and scientists prefer to ulitize the Cassegrain focus and Prime focus designs of the reflecting telescopes.

2. Cassegrain focus - has a parabolic first mirror, and a hyperbolic second mirror that bounces off the light back to the hole in the first mirror.

3.Prime focus - here, the observer sits inside the reflecting telescope, at the focal point of the reflected light.

4. Coude focus - is quite the same as the Cassegrain, except that there is no hole in the primary mirror, and there is a third mirror that reflects the light to the side, and other optics deliver the light to a fixed focus point. The Coude focus design as utilized on observatory telescopes.

Source: Wikipedia - License: GNU FDL