This article will show you the complete process for making a small refractor telescope. I also have a video that goes along with this tutorial. The picture below shows the telescope that I make and that you can make too.
Making a telescope is an extraordinarily rewarding experience because it crosses several different realms of experience. The first reward for you is the fun of making something. And the second reward is the sheer pleasure of using what you made to explore the universe. And you also get the added benefits of saving some money and learning about telescopes and building things.
The Simplest telescope you can make
There are a lot of different types you can make and the absolute simplest one you can make is composed of just two lenses. One large lens called an objective and a second smaller lens called an eyepiece. You hold one lens near your eye and another lens at arms length. You adjust the difference between the two by moving your arm in toward you. A point will be reached where everything comes into focus. It is as simple as that. You have a telescope. Now you can make it much better by creating a tube around it. Get two cardboard tubes with one that can slide in and out of the other. You mount one lens on the end of one tube and the other lens on the end of the other tube. Insert one tube into the other and then slide them in and out to find the focus.
You can get very inexpensive lenses from a variety of surplus and online stores and they generally will only cost a couple of dollars each. I recommend you get for your primary lens a convex lens at least 50 millimeters in diameter and a focal length of between 200 and 300 millimeters. For the secondary lens I recommend you get a convex lens that is between 10 and 25 millimeters wide with a focal length of less than 75 millimeters.
There are several things you can do to make this telescope even better. First off you could make a tube out of something more durable than cardboard tubes. Secondly, this type of telescope will invert images so everything appears upside down which is ok for viewing the sky but is very awkward for viewing things on the earth. You can solve this by using a concave lens for the eyepiece rather than a convex lens.
You can purchase the lenses individually from online surplus stores and there are several kits readily available that give you all the necessary materials to make this kind of telescope. They also come in bulk units for classroom and group use. A kit like this typically gives you all the materials needed to make ten or more telescopes.
Let's Make this Telescope
In this part of the telescope making tutorial I show you the nuts and bolts of how to make and assemble the telescope. I also have a video that shows you this and how to find the focal length of a lens. The picture below shows the parts needed to make this telescope.
Parts and tools
- Two cardboard tubes around 12" each; one should slide into the other
- Two lenses of differing focal length
- Two small pieces of corrugated cardboard
- Scissors, pencil, razor knife, paint and glue
In part two you figured out the focal lengths of your lenses and you cut your tubes to appropriate lengths now all you really have to do is cleanly attach the lenses to the tubes. This can be a bit tricky and I went through a whole bunch of trials before I found a way that was easy and worked well. Here is what you do:
Put a lens down on the square of corrugated cardboard and draw a line around it. Do this whole process for both lenses. (make two of these)
Using an X-acto or razor knife cut the circle out. You can use scissors but it is rather difficult because you will tend to bend and distort the cardboard. Try to cut the circle so the hole is just a tiny bit smaller than the lens. Maybe about 1/32 of an inch
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Pop your lens right into the cardboard. Because of the thickness of the cardboard you should be able to squeeze the lens right in there and it will hold tight. If you have trouble or it doesn't hold tight then just use a little bit of glue and let it dry
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Now, with a pair of scissors trim the cardboard ring so it is the same diameter as the tube. Remember that you have two tubes and each one is a different size because one slides into the other. Trim the ring with the long focus lens so it is the same size as your thicker outer tube. And trim the ring with the short focus lens so it is the same size as your thinner inner tube.
Now glue each ring onto its respective tube.
Slide the thinner tube into the thicker tube and your telescope is complete. Have fun!
Understanding the Optics of the telescope and lenses
In this part of the tutorial on telescope making I will give you some specific advice on how to make a simple telescope with small lenses. There are several different configurations that you can use and I will explain them to you. This way you have the flexibility to scrounge up or purchase various parts.
In the following drawing there are five different telescope configurations you can make. This way you can make use of a variety of lenses and a variety of lens sizes. If you only have one lens you can also make a successful and useful telescope by using a pinhole.
And here is a closer look at the types of lenses:
If you already have lenses check to see what type they are so you can use the appropriate set. If you don't yet have lenses I recommend you get a double convex and a plano convex. This will give you the best performing set and this is the same lens set that Galileo used to make his telescope.
Now lets talk a little bit more about the lenses and specifically about the focal length.
The focal length of the lens is important. This is the distance it takes the lens to bend light down to a point. For a home made telescope you need two lenses; one with a long focal length (this is the main lens furthest away from you and called the Objective). and a second lens with a short focal length (This is the eyepiece lens closest to your eye).
You have a lot of wiggle room when it comes to selecting lenses of various focal lengths because you are making a sliding tube telescope that will adjust to the focal length.
Typical focal length: If you are purchasing lenses here are the focal lengths I recommend: For the objective or main lens you should get something with a focal length of anywhere between 200 and 300 mm. You can go as high as 1000 mm if that is all you have. For the eyepiece lens you should pick something with a focal length less than 75 mm. 25 to 75mm would be good.
One thing to notice about the lenses in the illustration above is that the lens with the sharper and more pronounced curve has the shorter focal length. So, if you are looking at lenses you can get a sense for how long the focal length is by looking at the curve.
Lens Diameter - You have a lot of leeway when it comes to selecting lens diameter too. You can select two lenses that have the same diameter or you can choose One lens of a certain diameter for the objective and another lens that is about half that diameter for the eyepiece lens. When it comes to selecting lenses it is generally better to get the biggest lenses you can get. This means the telescope will gather more light and give you better viewing.
Recommendation: If you are looking for a recommended lens set here is a pretty good set:
- Primary Objective lens: Convex-Concave 25mm diameter with a 300mm focal length
- Eyepiece lens: Plano concave 25mm diameter with a 50mm focal length
Magnification of Your Telescope
The magnification of your telescope will depend on the focal lengths of the lenses you choose. There is an easy formula you can follow: The maginfication = the focal length of objective divided by the focal length of the eyepiece lens. In the project I make here the objective has a focal length of 30cm and the eyepiece is 5 so 30 divided by 5 = 6. The magnification of my little telescope is 6x
