Introduction: Homemade 8 Inches Dobsonian Telescope (Under Construction)

I engaged in this project when I was 15 years old, with the guidance and support of my father, and I want to share it on Instructables so that it can be useful to others.

This project is a culmination of my passion for astronomy and DIY projects. Building a telescope from scratch was a challenging yet incredibly rewarding experience. Throughout the process, I learned valuable skills such as glass engraving, mirror polishing, 3D printing, and woodworking.

I believe that by sharing this project, I can inspire others to explore astronomy and engage in DIY projects. With detailed instructions, including step-by-step guides and downloadable files, I hope to make the process accessible to anyone interested in building their own telescope.

Note: This guide is still unfinished. This project has not been completed and is currently suspended.

This article provides a guide to building a fully functional telescope. Initially, you'll find an explanation with diagrams of how a Newtonian telescope functions. Then, all the steps for building it with a Dobsonian mount will be outlined.

We'll go through the following steps:

  1. Understanding the functioning of a telescope
  2. Choosing the right tool for engraving a glass disk
  3. Figuring out the math
  4. Engraving the future mirror
  5. Checking the result
  6. Melting the pitch
  7. Texturing the tool and polishing the mirror
  8. Applying an aluminum coat or silvering to the primary mirror
  9. Projecting the mount and the whole structure
  10. Horizontal movement and Vertical movement
  11. Attaching the primary mirror and finding the focus
  12. Attaching the secondary mirror
  13. Mounting the eyepiece focusing system
  14. Collimation
  15. Bonuses

Note: Take into account that you may have to polish the 3D printed parts or slightly modify them with some heat (for example, with a heat gun) to better join the parts.

Step 1: Main Kind of Telescopes. What We're Going to Make?

There are mainly three types of telescopes, all of which work on the same principles (collecting as much light as possible and directing it to the eyepiece, where we observe the sky):

  1. Reflector Telescopes (Newtonian Telescopes)
  2. Refractors
  3. Catadioptric

In this Instructable, we'll see how to build a reflector telescope with a Dobsonian mount.

Newtonian Telescopes

Reflector telescopes work by collecting light in a parabolic mirror (almost spherical for diameters smaller than 25 cm) and focusing it on a secondary mirror, which redirects the light into the eyepiece.

The main advantage of a Newtonian Telescope is that the mirror doesn't produce the chromatic aberrations typical of lens telescopes.

Other advantages include:

  • Simple design
  • Easily adaptable primary mirror dimensions

Step 2: Choosing and Buying Primary Mirror and Abrasive Grits Kit

If you want to engrave the primary mirror of your telescope yourself, you'll need some essential tools and materials:

  • 2 x glass disks
  • Abrasive and polishing kit
  • Pitch
  • A lot of patience

You can buy the two disks (with the diameter of your telescope) from a local glassmaker. They don't need to be anything special, just around 1-inch in height. The abrasive and polishing kits will be necessary for engraving the glass disk you've chosen as the primary mirror. Do some research on the Internet (optic abrasive, optic engraving sand, polishing kit for telescopes, etc.), and you'll find many sites similar to the following:

Stellafane - Mirror Grit and Polishing Kit

In most cases, the grits will be made of carborundum (silicon carbide) or aluminum oxide, which are very hard materials, just below diamond on the Mohs scale. You'll also need cerium oxide, a very fine sand, for the polishing phase, along with some black pitch.

Just to give you some basic information, the abrasive grits you buy should have different diameters. (Refer to the table in the "Step title").

To learn the technique of engraving a mirror, watch this video. I recommend watching it entirely before proceeding with the whole project.

Note: Always be careful when cleaning your engraving room when you move from a coarser abrasive to a finer one. Otherwise, you may have to revert to the previous abrasive (a single larger grain can leave a deep scratch in the glass).

Always wash your hands, clean the entire room, and change your clothes if possible (this would be optimal).

Step 3: Figuring Out the Math

Now, you need to determine how much glass you need to remove from the center of the glass. The PDF files uploaded at the end of the article contain chapters from Jean Texereau's book "Construction du télescope d'amateur". Even if you can't read French, skim through it to understand what we are going to do. However, I will include the math in the following lines.

In the equation, 𝑅 represents twice the focal length you want to obtain (where light rays become parallel to each other as they were before hitting the mirror), and 𝑟 represents the radius of the mirror.

If you solve for "e" you will find the amount of glass you have to remove. So if you have a 20cm wide glass-disk and you want to obtain and f/5 telescope (focal length of 1000 mm) you get that e=r^2/2R which in this case (always in millimetres) is e=100^2/(2*2000); e= 2,5mm.

Step 4: Start Grinding the Mirror

The first thing to do is to find a way to securely attach the glass disk to a base that you can easily clean and move. I used some wooden blocks slightly lower than the glass, screwed onto a large wooden base as shown in the picture. The abrasives were all contained in two plastic bags, one inside the other to prevent contamination. Then, I decided to use ketchup bottles for further protection.

Start by grinding a "W" shape onto the mirror, with the lines closer to the edges of the glass and further apart in the center. Each time you move the tool up and down, make sure that only 1/3 of the tool extends beyond the mirror. Gradually rotate the tool every 15-25 seconds, as well as the mirror, at least every 1 or 2 minutes (as you see in the video).

Refer to the images for a better understanding of what you need to do.

Let's say you start with an 80-grit abrasive (the lower the number, the larger the diameter of the abrasive particles). When you have ground away at least 2/3 or something less of the 2.5 mm you want to remove, you can move on to the next grit. At this stage, you would use something like a 100 or 120 grit (always prefer gradual increments).

Step 5: First Results and Testing

After some hours of grinding, you should be able to see the first image on your "mirror" if you cover it with some water and point it at the sun, trying to focus the image on a wall (as shown in the image). Note that the focal length you obtain from direct measurements won't be the same as when the mirror is aluminum-coated, but it's always a good approximation. In my case, I was still far from the desired focal length.

Continue grinding with the abrasive, but always remember to use smaller and smaller grit sizes as you approach the desired focal length. Always check for scratches and imperfections before moving to the next grit size. If you find any imperfections, you need to go back to the previous grit size and continue grinding until they are removed. Then, restart with the smaller grit size and continue grinding.

Step 6: Starting the Polishing Phase (Pitch Melting)

Once you reach the 1200-grit level, you can start the polishing phase, for which you'll need the pitch.

Link: Pitch (resin) - Wikipedia

Now, you must clean everything meticulously, including yourself and your clothes, and dispose of everything that has come into contact with the abrasive kit.

To begin, create a mold or frame on a disk the same diameter as the mirror. This disk can be made of wood, as I did, or you can use the glass tool, which should now be convex (as John Dobson does in the video). Regardless, it's crucial that the pitch you cast into your mold matches exactly with the mirror.

I made a circular mold using aluminum tape on a wooden disk. Then, I melted the pitch in an old pot that my mother wouldn't need anymore (yep, I decided so). When it was completely melted, I poured it into the mold, which immediately became scorching hot; be careful, as the pitch can be sticky.

When it started to cool down and was just slightly warmer but still soft, I placed some backing paper on it and then the mirror. I added an extra 10-12 kg weight so that the pitch would assume a complementary shape to the concave mirror.

Step 7: Texturing the Pitch Tool and Start Polishing

After a few hours, you need to create a sort of grid in the pitch (I used a hot knife). Its function is to prevent small pitch scraps and glass powder from getting onto the mirror. (See the pictures)

Once the pitch is completely solid, you can start polishing the mirror. Now, you need a very strong system to hold the mirror still, because we're going to apply a lot of pressure on it.

Create a mixture of warm water and cerium oxide (or just put them on the mirror separately), and always spread it evenly on the mirror. If you haven't watched the video yet, it's time to do so. You need a visual representation of what you need to do.

I know it's hard, but you must keep polishing it for several hours if you want to have a clear image (graph shown in the images).

Inspect the mirror every 15-30 minutes to make sure there are no scratches or contaminations on the surface. If you notice that some parts are not being polished as smoothly as others, you can create a new pitch tool with a smaller diameter and use it to slightly parabolize the mirror in the center. Then, polish that portion of the glass.

Keep polishing until you are satisfied with the result.

Step 8: Aluminium-coat or Silvering the Mirror.

For this step, my father and I contacted a specialized studio in Venice, which provided us with the aluminum-coating service. We paid €80 for a 20 cm mirror and waited a few weeks for the delivery service.

When the mirror arrived, we noticed some scratches in the glass underneath the aluminum layer, which were caused by contamination from larger abrasives during the engraving and polishing phases.

You can also silver the mirror by depositing the metal through a chemical reaction. It would be cheaper, but not as effective as aluminum-coating. I don't recommend doing so.

Step 9: Projecting the Structure

The telescope I made is divided into 3 main pieces.

  1. The base: It has three wheels, four ball wheels, and a central mainmast. It provides horizontal movement to the telescope and can rotate 360°.
  2. The support for the main body: It has two holes that allow the telescope to move vertically using two threaded rods.
  3. The main body: I built it using metal frames and a wooden cube at the bottom.

Inside the wooden box, there are three screws to center and align the primary mirror..

Step 10: Horizontal and Vertical Movement

To move the telescope, we used three wheels on a wooden base, which can be easily locked down to the ground with their brakes. In the following pictures, you can see one of the ball wheels on which the second base slides.

Later, we decided to build a larger base, but the principle remained the same.

For vertical movement, we used two slots, as mentioned in the last step.

Take a look at the photos for a better understanding.

Step 11: Attaching the Mirror and Finding the Focus

Mount the mirror on your telescope and go outside with a sheet of paper.

Now, you need to find the focus, which is the distance between the mirror and the point where sunlight converges.

Measure the focus and then mark that distance. Next, place the secondary mirror a few inches before that point, so that you can converge the light onto a point outside the telescope body, into the eyepiece.

If this is not clear, look online for the working scheme of a Newtonian Telescope or refer back to the first step.

Important: Do not look directly into the mirror during this operation, as you can burn your eyes.

Step 12: Attaching the Secondary Mirror (Coming Soon)

We bought a secondary mirror for our telescope, which has a minor axis of 50 mm. This dimension depends on the amount of focal length that needs to be redirected into the eyepiece. We paid €50 for it, which is about $60, but if you are lucky, you should find something cheaper. Just make sure it is of high quality.

To mount it on the telescope, we 3D printed the support and used four threaded rods with some bolts. The model we used can be found at

I'm going to upload the STL files just in case the official source is not available.

Step 13: Mounting the Eyepiece and the Focusing System.

We 3D printed a focus mechanism that allows us to move the eyepiece back and forth.

You can always buy one, which would, of course, be more precise. Anyway, I will provide you with the .stl files, which have been designed in OpenScad.

Step 14: Collimation (Coming Soon)

Step 15: Bonus: Paint the Telescope and Printing the Eyepice Holder

I used an undercoat and elegant smooth green paint to improve the visual result.

Then, I 3D printed a holder for the eyepiece, which is available for download.

Step 16: Jean Texereau's Book "Construction Du Tèlescope D'amateur"