Telescope Basics
A telescope works by bending light rays to form a larger image. The focal length of the telescope determines the image size. The focal length is the measure of the distance between the eyepiece and the point where the rays of incoming light meet.
There are three main types of telescope: refracting , reflecting and radio telescope.
A refracting telescope is a tube with lenses at each end. Light enters the telescope through the objective lens ( primary optic). This light refracts, or bends, the light towards the eyepiece lens. The image as seen by the observer is upside down. Some of the best known refracting telescopes in the world include the 40-inch refractor at Yerkes observatory, and the 36-inch refractor at Lick Observatory.
(Picture of refracting telescope)
In a refracting telescope, the blue light at shorter wavelengths sometimes refracts (bends) more than red light at longer wavelengths. This is called chromatic aberration, and may cause images to appear to have colored edges.
Sir Isaac Newton invented the reflector telescope to solve the problem of chromatic aberration. A reflecting telescope uses a curved mirror as its objective. The mirror gathers light and focuses it on a flat mirror suspended in the center of the telescope tube. The flat mirror reflects the light to the eyepiece lens.
(Picture of reflecting telescopes)
A radio telescope collects & measures faint radio waves (for details on radio waves, check section on "Details on magnitude system") given off by objects in space. The waves are collected in the reflector dish, focused onto the receiver, and deflected into the vertex box. The waves are converted into electric signals and are sent to the computer, which transcribes them into a graph showing the signal strength of the waves.
All the basic types of telescopes used by amateur astronomers are of the reflecting & refracting types or a combination of both. The reflecting type is also called a Newtonian telescope. The combination type telescopes are of two types, namely, cassegrain & dobsonian.
I have a picture sketch of the cassegrain type of telescope :
This arrangement allows the length of the telescope to be shortened considerably.
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