(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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* large
* weak
* convex only

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* small
* powerful
* 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.

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Side view of convex lens

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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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You can buy cheap lenses from the suppliers below.


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.



If you put a lens right on your eye, it makes things blurry, but it does not magnify distant scenes. This is how eyeglasses work. They change the blurry-ness or sharpness of what you see, but they don't act as magnifiers when used normally.

Now if you move a lens away from your eye, and keep looking through it, everything WILL change size. If the lens is concave (thinner in the center,) everything you see in the lens will get smaller and smaller as you move the lens farther away from your eye. If you use a convex lens instead, everything will get bigger and bigger as you move it away.

The convex lens is the interesting one because it makes things bigger when you move it farther from your eye. Keep moving it farther and farther away. You'll find that everything will become VERY big, even infinitely big. And infinitely blurry too. Move the lens a little farther, and things get small again, but now everything seen through the lens is upside-down.

By moving the convex lens in and out, we can change the size of everything, or make it all go upside-down or rightside-up. Unfortunately everything is very blurry when you're looking through the lens. If only there was some way to remove the blurryness, we could hold a convex lens in front of our eye and use it to magnify distant scenes.

There is a way to remove the blur: wear glasses! Glasses are used to correct blurry vision, and they can fix this blur too. Put another lens right upon your eye. It acts like eyeglasses and makes the image sharp. If you do this, you have constructed a telescope. The objective lens creates a magnified view of distant objects, while the eyepiece lens removes the blur. That's how telescopes work.


Here's an interesting question. If human beings could focus their eyes better, could we build telescopes with only one lens? Suppose you were able to focus your eyes on an object that was 1/10 inch away from your eye. Couldn't you hold an "objective" lens a few inches away, look through it, then focus really hard with your eyes and create a telescope? The answer is yes!

Even if you don't have a superhuman ability to overfocus your eyes, you can still make a simple one-lens telescope. Here's how. Hold a weak convex lens in front of your eye. Close your other eye. Move the lens far away so that everything turns upside-down. Move the lens a bit closer so that everything stays upside-down, but gets bigger and blurry. Now focus your eyes really hard by crossing them. (This might take a bit of practice! Crossing your eyes while one eye is closed is not that easy to do.)

The image you see in the lens will become sharper. If it doesn't become completely sharp, then move the lens farther away. Also try moving the lens closer while focusing really, REALLY hard. Everything you see in the lens will be clear, sharp, and magnified! You have made a telescope with nothing but a single lens! Tell your friends about it and they won't believe you. Then show them the trick and they'll be amazed.


It is also possible to make a telescope using aluminum foil and one lens. The lens will act as the telescope's objective lens. To make an eyepiece, we just poke a tiny hole in the foil, and use this pinhole as the telescope eyepiece lens. The tiny hole in the foil removes the blur. But it also makes the image very dim. That's alright, just use this telescope to watch a bright outdoor scene.

To make a good pinhole, stack up several layers of aluminum foil, poke the stack with a pin, then separate the layers and choose whichever one has a very round, very small hole. Experiment with different holes; put one on your eye, look through it a brightly-lit scene, and see how sharp everything looks. Smaller holes generally give sharper, dimmer images, but VERY small holes cause blur because of "optical diffraction" effects. You want your pinhole to be very small, but not so small that things become blurry.

To make a telescope, hold the best pinhole against your eye and look through it. Look at a brightly lit scene, such as the sunny outdoors outside a classroom window. Now place your objective lens against the pinhole, then move it slowly outwards. When you see a magnified scene, your telescope is working! You can hold your lens in different spots so the scene is either upside down or right side up.

An aluminum foil pinhole can be made sturdier by using rubber cement to glue it to a piece of cardboard which has a 1cm hole punched in the center (don't get cement in the tiny pinhole though!)

You can use your pinhole-telescope to create a "zoom lens" effect by moving the objective lens towards the pinhole or away. And depending on the distance between the pinhole and the lens, the scene you see can either be upside-down or right-side-up. It's very complicated to build a zoom-lens telescope with real eyepiece lenses, but if you use a pinhole it becomes simple.

Get yourself some lenses:

American Science and Surplus sells miscellaneous lenses.
Contact them at , get a big weak magnifier, and also a tiny powerful loupe as eyepiece.

Another supplier is Surplus Shed, with "educational" lenses costing about two dollars each.

OPTICS DISCOVERY KIT $22.95, from Optical Society of America OSA (Edmund Sci.)

Edmund Scientific sells lenses too. Check out their website at Get two convex lenses, one with over 200mm focal length, and one with under 75mm focal length.


  • Telescope kit $16 Am. Sci. Surplus

  • Starlab Refracting Telescope for classroom, $10 (or $6)

  • DIY microscope $10

  • Surplus Optics (links)

  • Exploratorium: Make your own Telescope

  • Bizarrelabs: DIY simple telescopes

  • DIY Advanced telescope, Fun Sci Gallery

  • Galileoscope $50 Telescope kit for classroom

    STUPID BILL B. VIDEOS! educational too. But no less stupid!





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