Telescopes, Page 17: [Previous Page] [Next Page] [Class Home Page] Textbook pages 97 - 100

The Basics of an Optical Telescope

Since Galileo's time, telescopes have been the most important tool of the astronomer. There are many kinds of telescopes, depending on the needs of the observer and the kind of light one wishes to view! The most basic optical telescope design, a Refracting Telescope is shown below:


Light from distant objects comes in as parallel lines. The objective lens (or) primary lens bends (refracts) the light and brings it to a focus to form a small inverted image. A second lens, the eyepiece lens, then magnifies this inverted image. The light after passing through the telescope is more concentrated and the image magnified. In the design above, the mirror serves no purpose other than to make the telescope easier to use.

How does a lens work? Parallel rays of light arrive at a lens from an object a great distance away and refract as they pass through the lens. A properly shaped lens causes the light to converge at the focal point. The distance from the lens to the focal point is called the focal length.

Reflecting Telescopes
Instead of a lens, reflecting telescopes use a concave mirror to concentrate the light to a single focal point. It has all the same basic elements of a refracting telescope:



The possible hazard with using a mirror for your objective (or primary) mirror is the light comes right back at you! In order to keep the observer from blocking the light path, a small mirror is placed within the telescope to bounce the light to the side for viewing and placement of the eyepiece. The above design is called a Newtonian Focus.