Telescope (Refractor and reflector)

A telescope is an optical device which makes far objects appear closer: it samples a small area of view, a field, and then magnifies it so that distant objects appear larger.
Parallel light rays entering the telescope are focussed to a single point (called the focus or focal point) and then magnified with a very powerful set of lenses (called an eyepiece) to give enlarged views of distant objects. The eyepiece acts in the reverse direction to the telescope lens, taking the focussed rays and sending them to the eye as parallel rays.

There are two basic types of telescopes, refractors and reflectors, whether the magnification is based upon refraction or reflection (see optical laws).

Refractor telescopes:

The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. Today, excellent small refractors are still made for amateur telescopes, but the disadvantages of this kind of instrument have stopped the construction of very large refractors for use in astronomical research.
Here is a drawing of the path light follows inside a refractor:

Refractor telescopes present many advantages:

Refractor also suffer from a series of disavantages:

Because of these disadvantages, the largest refractor telescope built is the one at Yerkes Observatory, with an objective 1.02 meters across at one end of a 19.2 meter tube.

Reflector telescopes:

The reflector telescope uses a parabolic-shaped mirror to gather and focus light. All modern research telescopes and large amateur ones are of the reflector type... (click here to know more about the different types of reflector telescopes).
Here is a drawing of how light behaves in a reflector telescope:

The advantages of reflector telescopes can be resumed as follows:

Despite the advantages of the reflector telescope, astronomers must accept some minor problems:


Back to the
general dictionary

Back to
current issue