How to build your own telescope

Learning about lenses, refraction and optics in general has now enabled me to make my own telescope. I wanted to build one because I have always loved space and astronomy. Another day I must tell you about a hobby of mine, launching water rockets. 

There are two main types of telescopes: refractors (made out of lenses) and reflectors (use mirrors too). Galileo Galilei made one of the first refractor telescopes and Isaac Newton made the first reflector. The one I built is a refractor, which is a bit cheaper to build. 

How it works

In this diagram you can see that the telescope has two lenses:

• The first is called the "objective". The function of the objective is to create an image with the light comes from the celestial body. The light rays come in parallel because the object is very far away. This means the image forms in the focal point. The image is inverted though.
• The second lens is called the "eyepiece" and it typically has a much smaller focal length. The eyepiece needs to be placed so that it has the focal point where the image forms. In other words, the focal points of both lenses need to be in the same place 

The magnification of this telescope is calculated by dividing the focal point of the objective by the one of the eyepiece. So, if you want more magnification you can either get an objective with a big focal length or an eyepiece with a small one, or a combination of both  

For my project I used an achromatic doublet. This a combination of 2 lenses, a divergent one (called the flint) and a convergent one (called the crown). They work together to converge the the focal point of all visible colours together in the same point. 

List of materials

To build a telescope like mine you will need:

• Objective lens. You can search for "50mm achromatic objective lens cemented". They are available in multiple focal lengths. Mine is 600mm. Cemented means glued together
• Eyepiece plossl 12mm. Mine is from a brand called "Celestron"
• Tripod to keep the telescope still. Your parents or a neighbor might have one you can borrow.
• 3D printed parts
• Thick paper
• Nut to mount on the tripod
• Blue tag
• Elastic bands
• Glue
• Zip ties
• Consider making a cap out of plastic bottle to protect the objective from dust 

Build process

We are going to do the tube by rolling paper around the lenses. It is important that the objective and the eye-piece are well aligned. The 3D printed parts helps us align the lenses and give us something rigid to roll on. I have shared the 3D printing files in Thingiverse. It contains the parts to do two different designs: single eye-piece or interchangeable eye-pieces. Read the "evolution" section below to decide what pieces you need to print

• 3D print the parts for the objective, the eye-piece and the tripod mount
• Mount the objective lens in the 3D printed part.
• Make two marks in the paper where the objective and the eye-piece go. They need to be separated by the sum of the two focal lengths, ex: 60cm + 1.2cm = 61.2cm 
• Put some blue tag around both 3D-printed parts and roll the paper. Roll it tightly but make sure the paper doesn't twist or misaligns.
• Once you are finished rolling you can secure it with elastic bands. The tube is now ready for testing
• The telescope magnifies 50 times so you have to make sure it is perfectly still. I have provided the file to print an adapter to mount on a camera tripod. You can 3D print it and then glue the nut so that it doesn't come out
• During your tests you can secure the telescope to the tripod mount with elastic bands. The tripod mount has guides on the sides for this purpose
• Test the telescope to make sure it focuses properly. If not, you can try pulling the eye-piece out a bit. If that is not enough, the focal points are not matching properly and you will have to get the lenses closer or further from each other. 
• Once you get sharp images, mark the positions of the lenses in the paper with a pencil
• Now you can disassemble it and put together with glue to make it permanent

Evolution

At the beginning I had only one objective and one eye piece. I assembled it making the focal points match and secured it with elastic bands and tested it.

There was no Moon that day so I used a table called "Stellarium" to check what I could see and realized that Jupiter was visible in my region. I could not believe my eyes. I was shocked that I was not just able to see Jupiter but also the 4 Galilean moons: Io, Europa (this is the one I signed on he mission for), Ganymede and Callisto. This was special moment because I realized that Galileo must have used a similar telescope to discover them 400 years ago. That's why I decided to call my telescope "Galileo 1". This photo is taken with a cheap webcam I attached to it so is not very sharp, but this is the what you see through it. By the way the red spot was something that was moving quite fast so I guess it was a satellite.

With this setup I also managed to see Mars, Venus and the Moon. Mars is not impressive because it appears blurry. One cool thing about Venus is that I saw it in a crescent. But the Moon was mind-blowing because it was so big and clear and you can clearly see the craters. I took this photo with my dad's phone. It is not good quality because it is hard to keep the camera still. Looking through the telescope much clearer.

With this success I asked my Dad to buy another eye-piece with even more magnification. This was a 4mm for a total 150x magnification, but when I put it in I realized that it wasn't focusing no matter how much I took it in or out. So, if I was going to be able to swap them I would need a way of moving them back and forth more precisely. After some research I found a screwing mechanism in Thingiverse. It is is called a "helical focuser". You can use the screw to move them closer or further apart as needed. I had to design another piece to be attach it to my telescope.

Now both eye pieces were working so I decided to make it permanent by gluing everything together and decorating it ... and of course, a telescope wouldn't be complete without a Minecraft sticker on it. I secured it to the tripod mount with zip ties to reduce vibrations

Lessons Learned

Here are a few lessons I learned:

• When you look at celestial bodies it looks like they are not moving but when you use a telescope their movement is also magnified and once you can see them they get out of sight very quickly. This is specially bad with the 4mm eye piece
• Also, the more magnification you have the harder they are to locate, because a small adjustment of the telescope by hand is a big swing in the sky
• Finally, the design of my telescope is sensitive to vibrations. Any vibrations are magnified as well. So it works better in a place without wind

So, overall the first design with only the 12mm eye piece is very good because you can locate things easier and they don't move so fast. To use more magnification than that you will need a robotic mechanism.