Astronomy > Telescope > Ultra-Simple Telescope


Grade Category: Middle School

Subject Category: Astronomy

Sub Category: Telescope

ULTRA-SIMPLE TELESCOPE        (c) 1996 William J. Beaty

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A telescope with a tube is nice, but it's more complicated than necessary. A telescope with adjustable focus is useful, but it's hard to build. A project that's too complex and difficult will drive people away, when the goal is to tempt them into building it.

Here is an extremely easy version of a Telescope Build-it project. No cardboard tube or adjustable focus mechanism is required. All that you need is a pair of lenses. Tempting?


Two lenses are needed to build a telescope. We call these the "objective" lens and the "eyepiece" lens.

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               OBJECTIVE           EYEPIECE
               LENS:               LENS:
               * large             * small
               * weak              * powerful
               * convex only       * convex OR concave

The "Objective" lens should always be a convex lens. Convex lenses are thicker in the middle, and can be used as magnifying glasses or for concentrating sunlight. Try to find one which is large and weak. The weaker it is, the more powerful your telescope will be. The thinner it is in the center, the weaker it is.

   _--       --_
  <_           _>    Side view of convex lens
   _/          \_
  /______________\   This type of convex lens also will work

The "eyepiece" lens can be either a convex or concave lens. If you use a convex eyepiece, your telescope will turn everything upside-down. This kind of telescope is called a "Newtonian." And if you use a concave lens as your eyepiece, your telescope will not turn things upside-down. This type of scope is called a "Galilean."

For your eyepiece, try to find a lens which is small and powerful. A small, powerful magnifying loupe makes a good telescope eyepiece.

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    |      _____      |   Powerful concave lens.
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    |    _________    |   Weak concave lens.
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Face a distant, well-lighted object such as a lamp, or distant trees outdoors.

Hold your Eyepiece Lens right on your eye and look through it. It's OK to close your other eye.

Hold your Objective lens right in front of your eyepiece.

Slowly move your Objective lens forward until the scene comes into focus. Sometimes it's hard to find the right distance, so try many different places. Look through your lenses and find the blurry edge of trees or lightbulb, then move the objective lens in or out so that the blurry edge looks sharper.

Your lenses are now a telescope!

Now that you know the trick, you can make a telescope whenever you find two different lenses lying around. If a friend happens to have two magnifying glasses, grab them, put the more powerful one right on your eye, move the other in and out, and you'll have an instant telescope.


I've read many different explanations of telescopes. Most of them are confusing and complicated. Some are even wrong. So, if you read an explanation and don't understand it, don't blame yourself. Blame the author of the book or encyclopedia for not being a good explainer!

Having said this, do I think I can do better? I don't know. A good explanation of a telescope should be easy to understand. I've never seen a really good one, so all I can do is try to explain things in a different way than books usually do, and see how well it works.