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Principles of Telescopes

By — Pearson Allyn Bacon Prentice Hall
Updated on Nov 18, 2010

All telescopes are designed to do one thing: collect light. By using a telescope, you are condensing the light from a larger area through a system of mirrors and or lenses to a small area that can be seen through the pupil of your eye. For instance, using a 6-inch optical telescope will increase the amount of light that reaches your pupil by approximately 200 times. Thus, a very dim object that typically cannot be seen with the naked eye in the nighttime sky will appear much brighter through a telescope.

Except for close objects, such as the moon and planets, and for viewing nebular objects such as galaxies, the magnification of a telescope is of little value. Even with the world’s largest telescope, all stars appear as just a pinpoint of light. But through the telescope, the pinpoint will be much brighter. Thus, when purchasing a telescope you should be much more concerned about how much light a telescope collects than its magnification.

Optical telescopes are divided into two basic categories: refracting telescopes and reflecting telescopes.

Refracting Telescopes  Refracting telescopes utilize a system of lenses that collect and refract light to magnify the image (Figure ). They have a larger diameter lens called the objective lens and a small lens called an eyepiece. Good refracting telescopes utilize multiple lens systems to correct for color aberrations. Chapter discusses lenses in the section on “Stations for Sight”.

Reflecting Telescopes  Reflecting telescopes utilize one or more concave mirrors for the objective lens to condense the light and a small lens called an eyepiece.

Criteria to Consider When Selecting Telescopes for Use with Children

Unfortunately, small inexpensive telescopes are difficult even for adults to use. The result is that the initial excitement of viewing objects through a telescope disappears with the frustration of trying to sight a particular object, focusing the lens, and keeping the image in the field of view.

These exasperating experiences can often dim the excitement of viewing the sky and children’s curiosity about astronomy in general. Children’s first experience with telescopes must be a positive and rewarding one.

Let’s look at some basics to consider when selecting a telescope.

Mount or Tripod  The support for the telescope is very critical. Quality telescopes require at least as much money for the mount as they do for the telescope itself. The mount must be sufficiently heavy and sturdy enough to support the telescope without movement.

Because telescopes typically magnify an object from 50 times to several hundred, the vibration of a mount is also magnified. For example, a vibration of 1/50 of an inch will be viewed through a telescope as a vibration of a full inch, making the image impossible to view.

In addition, unstable mounts make the aligning and focusing of the object to be viewed very difficult. The first time a tripod is bumped by a foot or arm, you will lose the object and focus and will have to start all over.

Field of View  The field of view is the angular diameter of the viewing area of the telescope. Inexpensive telescopes have a small field of view, which makes it much more difficult to find the objects that you are looking for.

In addition, the lower the power of the telescope, the greater the field of view. Therefore, when viewing an object through a telescope, view first in the lowest power and then change eyepieces to work your way up to the higher powers.

Size of Objective Lens  The diameter of the objective lens or mirror determines the amount of light that you collect. Obviously, the larger the objective lens, the more light you collect and, unfortunately, the higher the cost of the telescope. A quality telescope requires a significant investment.

When selecting a telescope, it is critical that you do not sacrifice the quality of the mount and the size of the field of view for the size of the objective. In, general, avoid telescopes that advertise the magnification as their greatest quality.

With young children, you might find that viewing the moon with a good pair of binoculars is a more rewarding experience than using a telescope. With a good-quality pair of binoculars, other beautiful objects, such as star clusters and galaxies, can also be seen. A good basic suggestion for using binoculars for looking at celestial objects is to focus your eyes on the object you want to see and then bring the binoculars up to your eyes, rather than trying to locate the object with the binoculars. A tripod to steady the binoculars can be a valuable asset.

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