Telescope Making/Thinking About Telescopes

People have been making telescopes for 400 years, and it's no surprise they've built a vocabulary of specialized terms. In this section we'll take a quick look at some of the most important, and provide links to pages that explain them in more depth. Follow those now, or come back and follow them later when you feel the need for more detail.

Refracting Telescopes

The first telescopes were refracting telescopes. In a refracting telescope, light passes through two or more lenses. The right combination of lens shapes brings the light from a small area to focus on the retina of your eye, giving you a magnified image.

A basic problem with a refracting telescope is that different colors of light are bent by different amounts. The red light comes to a focus at a very slightly different point than the blue or green light does. The result is a fringe of rainbow color around bright objects. Camera designers tinker endlessly with lens materials and coatings to avoid this false color problem.

Small inexpensive refracting telescopes can often be found at surplus and used merchandise stores very cheaply. Amateurs can make refracting telescopes. You need a magnifying lens of long focal length, called the objective lens, paired with one of short focal length (the eyepiece). A usable objective lens can be salvaged from many types of equipment that use projection lenses and other optical components.

Refracting Telescope (english)

Reflecting Telescopes

The reflecting telescope is designed around mirrors, not lenses. The light does not pass through a lens and is not refracted. It is simply reflected. Since there is no refraction, all colors of light are affected equally, hence there is no problem of false color.

A reflecting telescope has its own problems. Its mirrors must be shaped precisely, within a fraction of the wavelength of light, in order to avoid distortion. (Historically, the main mirror of the Hubble Space Telescope was improperly curved, due to a failure of communications between the designer and manufacturer. A space shuttle trip had to be made to introduce a compensating lens in the system to clear up the image.)

It is not as hard to shape a perfect mirror as you might think. Our first project, the Newtonian Reflector, requires mirrors, and we will make them. It is really very easy to do because (as you will find), physics is on our side.

Reflecting telescope (english)

Catadioptric telescopes

Catadioptric telescopes are a sort of hybrid. They use both lenses and mirrors to focus the light.

Technical Terms

We need a few technical terms. The first two are focal point and focal length.

Focal length for a lens or a mirror

The focal length of a lens, or of a curved mirror, is the distance at which it brings incoming light to a focal point, or simply focus. The curvature of the lens or mirror surface determines the focal length. The less the curvature, the longer the focal length.

The greater the focal length, the greater the effective magnification of the image. As a general rule, the greater the focal length, the less the impact of imperfections in the curvature. The less the focal length, the more critical it is to have a perfect curve.