How to make a telescope

How do I make a telescope?

Did you know you can build your own telescope? All you need are two lenses and a few common household items. Depending on your crafting experience, you can complete this fun project in a single afternoon.

The benefit of making your own telescope is it will give you a much better understanding of optics. It is also very rewarding to build your own scientific equipment that really works.

Reflector vs. refractor telescope

There are two types of telescopes: a reflector telescope and a refractor telescope.

Reflector telescope

As it sounds, a reflector telescope works by reflection. Light enters the front of the telescope and reflects off the primary mirror that is located at the back. The light travels to the front of the telescope where a secondary mirror reflects it to an eyepiece. A reflector telescope is easy to identify because the eyepiece is located in the front.

Refractor telescope

For this project, we will be building a refractor telescope. A refractor telescope works by using special lenses that change the angle of light to focus it at a specific distance. This distance is called a focal length. The lens with the larger focal length is located at the front of the telescope and is called the objective lens. The lens with the smaller focal length is located at the back of the telescope and is called the eyepiece lens. For a home model, the objective lens should have a focal length of 8 to 14 inches, while the eyepiece lens should have a focal length of 2 to 4 inches.

Making a refractor telescope: step by step

Materials

The first step to building a refractor telescope is to gather your materials:

• Two convex lenses (1.5 inches in diameter): The telescope’s magnification level is determined by dividing the focal length of the objective lens by the focal length of the eyepiece lens.
• Two empty cardboard paper towel rolls.
• Scissors.
• Tape: Durable tape, such as duct tape, is best.
• Glue: You'll need a fast-setting glue that can hold the glass to cardboard. Alternatively, you can use a hot glue gun and an appropriate glue stick.
• A ruler.
• A pencil: A pen or ultrafine point marker will also work.

Steps

1. Cut one of the cardboard paper towel rolls lengthwise down the middle.
2. Overlap the edges of the cut paper towel roll by one-half inch and tape them in place. This will create a tube with a smaller diameter that can be slid in and out of the larger tube.
3. Carefully make eight evenly spaced one-quarter-inch-deep cuts into one end of each paper towel roll.
4. Fold the eight flaps you just made inward at a 90-degree angle. This will create a ledge on each paper towel roll where you can glue the lenses.
5. Glue the lens with the longer focal length to the paper towel roll with the larger diameter. Be very careful not to get any glue on the part of the lens that you will be looking through or your telescope will not work as expected. You will need to hold the lens in place until the glue sets — follow the instructions that came with the glue.
6. Glue the lens with the shorter focal point to the paper towel roll with a smaller diameter.
7. Insert the smaller paper towel roll into the larger one.

Once you have successfully built your first telescope, feel free to experiment with different aspects of these instructions. For instance, consider trying lenses of different focal lengths. If you use lenses that are larger than 1.5 inches in diameter, however, you will need to use mailing tubes or custom-make your own tubes out of cardboard.

Tips for using your refractor telescope

Now that you have your telescope, there are a few things you’ll need to know to get the most out of it.

• Use the tubes to focus: Slide the smaller tube in and out of the larger tube to focus.
• Never look at the sun: If you use your telescope during the day, you must never aim it at the sun. This can cause permanent damage to your retina.
• Let your eyes adjust: If you use your telescope at night, give your eyes 30 minutes to adjust to the darkness.
• Take breaks: It is easy to get tired when using a telescope. Take frequent breaks to give your eyes a rest.

What you need to make a refractor telescope

Double Convex Lens With 11.81-Inch Focal Length (1.5-Inch Diameter)

This double convex lens has a long focal length (11.81 inches), which makes it ideal for the objective lens of your telescope. Remember, you will need a longer telescope for this to work.

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Double Convex Lens With 7.87-Inch Focal Length (1.5-Inch Diameter)

This double convex lens is a good focal length (7.87 inches) for the objective lens of a homemade telescope. It is a high-quality lens with ground edges for precision.

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Double Convex Lens With 3.93-Inch Focal Length (1.5-Inch Diameter)

This lens has a 3.93-Inch focal length. It works best as the eyepiece lens of a large telescope.

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Double Convex Lens With 1.97-Inch Focal Length (1.5-Inch Diameter)

With a focal length of just 1.97 inches, this lens makes an ideal eyepiece lens for most home telescopes.

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Gorilla Black Duct Tape

This durable tape is tough, weather-resistant, and can securely hold to smooth, rough or uneven surfaces.

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Gorilla 3-Ounce Clear Grip Tube

This fast-setting clear grip glue is a good option for your assembling needs. It is waterproof, paintable and suitable for use with glass.

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Tru Red 8-Inch Stainless Steel Scissors

If you don’t have a quality pair of scissors, you will need to buy a pair for this project. These right-handed scissors have ergonomic grips for comfort.

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Celestron Portable Refractor Telescope

For individuals who prefer to use a store-bought model, this compact Celestron refractor telescope is a solid entry-level option. It comes with two eyepieces, a travel bag and a tripod.

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Allen Foster writes for BestReviews. BestReviews has helped millions of consumers simplify their purchasing decisions, saving them time and money.