### How to Make a Telescope: 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.

Now that you understand all about the optics let's make this telescope

I Made My TELESCOPE - Want to make your own telescope? -- Build-It Yourself! -- Its so easy, now even an eight-year-old can build an 18" long, 3X refractor telescope in less than an hour. -- Includes objective lens, eye lens, glare stops, kraftboard tubes, instructions and an Edmund Star and Planet Locator. Finished product is powerful enough to show moon craters, Jupiters moons and many stars not visible to the naked eye. -- For ages 8 and up. --

Need lenses? These are the exact lenses I purchased for this project:

LENS DBL CONVEX 3.75CM DIAM 30CM FL (This is the objective or long focal length lens)

LENS DBL CONVEX 3.75CM DIAM 5CM FL (This is the lens I used for the eypiece, the short Focal length lens)