Homemade 8 Inches Dobsonian Telescope (Under Construction)




Introduction: Homemade 8 Inches Dobsonian Telescope (Under Construction)

Note: This guide is still in construction. This project is not finished but it will be by the end of June.

This article provides a guide towards the building of a full-function telescope. In the beginning, you will find an explanation with schemes about the functioning of the Newtonian telescope. Then there will be all the steps for building it with a Dobsonian mount.

We'll go through different 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. Aluminium-coat or silvering 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. Mount the eyepiece focusing system
  14. Collimation
  15. Bonuses

Note: Take into account that you may have to polish the 3d prints 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 telescope and all works by the same principles (collecting as much light as possible and collecting it in the eyepiece, which is where we'll place our eye to see the sky).

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

In this Instructable we'll see how to build a Reflector one 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 by focusing it in a secondary mirror which will redirect light into the eyepiece.

The main advantage of a Newtonian Telescope is that the mirror won't generate chromatic aberration that are typical of lenses telescope.

Other advantages are:

  • Simple design;
  • Easily adaptable primary mirror dimensions.

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

If you want to engrave by yourself the primary mirror of your telescope you need to have some assets:

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

You can buy the two disks (of the diameter of your telescope) from a local glassmaker. Nothing special, they just need to be around 1-inch height. The abrasive and polishing kits will be necessary in order to engrave the disk of glass you have chosen as the primary mirror. Do some research on the Internet (optic abrasive, optic engraving sand, polishing kit for telescopes, etc.) you will find a lot of sites similar to the following.


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

Just to give you some basic information, the abrasive grits you buy have to of different diameter. (Give a look at the table in the "Step title").

To learn the technique of engraving a mirror give a look at this video, I recommend watching it entirely before going ahead in the whole project:

Note: Always be careful in cleaning our engraving room when you go from an abrasive to a smaller one.

Otherwise, you will have to go back to the precedent abrasive (a single bigger grain can leave a deep scratch in the glass).

Always wash your hand, the whole room and change your clothes if you want (It would be optimal).

Step 3: Figuring Out the Math

Now you have to figure out how much glass you need to remove in the centre of the glass. The pdf files uploaded at the end of the article are the chapters with Jean Texereau's book "Construction du tèlescope d'amateur" if you can read a français book (also if you are not), skim it to understand what we are going to do, however, I will include the math in the following lines.

In the equation, R represents the double of the focal length you want to obtain (where light rays become distant from each other as they were before hitting the mirror) and "r" 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

If you have watched the previous video about the technique that you need to master when grinding the glass and you are confident with this, you can move ahead and skip this step.

The first thing to do is to find a way to strongly attach the disk of glass to a base that you can easily clean and move. I used some bricks of wood a little lower than the glass with some screw on a large wood base as in the picture. The abrasive were all contained in two plastic bags, one inside each other to prevent contaminations. Then I decided to use some ketchup bottles for further protection.

So you start by doing a "W " shape on the mirror, which lines are closer in the edges of the glass and further in the centre, every time you go up and down you need to be sure that only 1/3 of the tool goes out of the mirror then gradually rotate it every 15-25 seconds as well as the mirror at least every 1 or 2 minutes (as you see in the video).

Watch the images to have a better view of what you need to do.

Let's say you start with an 80, (the lower the number the bigger the dust's diameter will be) when you have engraved at least 2/3 or something less of the 2.5mm you want to achieve you can move on to the next grit. There we are working with something like a 100 or a 120 (always prefer gradual increment).

Step 5: First Results and Testing

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

Keep going with the abrasive but always remember to go with smaller and smaller sand as you approach the focal length you have decided to obtain. Always check for scratches and imperfections before moving to the next sand. If there are some imperfections you need to go back to the previous diameter and engrave until you get it done, then restart with the smaller diameter and keep going.

Step 6: Starting the Polishing Phase (Pitch Melting)

After reaching the 1200 dust you can start the polishing phase, so there we'll need the pitch.


Now you must clean everything meticulously, including yourself and your clothes then get rid of everything which has been in contact with the abrasive kit.

You start by creating a mold or a frame on a disk the same diameter of the mirror, which can be a wood disk, as I do, or the glass tool that should be convex at this time(as John Dobson does in the video). Anyway, it's crucial that the pitch you'll cast into your mold will match exactly with the mirror.

I did a circular mold with some aluminium tape on a wood disk, then melted the pitch in an old pot that my mother wouldn't need anymore(yup, I decided so) and when it was totally melted I cast it in the mold, which became immediately scorching; be careful with it, the pitch can be sticky.

When it started to cool down and it was only a bit more than warm but sill soft I put some backing paper on it and then the mirror. I added an extra 10-12 kg weight so that the pitch would have assumed 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 crawl on the mirror. (See the pictures)

When it's completely solid you can start polishing the mirror. Now you need a very strong system of taking the mirror still, 'cause we're going to apply a lot of pressure on it.

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

I don't care it's hard, you must keep polishing it for several hours if you want to have a clear image (graphic in the images).

You have to inspect the mirror every 15-30 minutes to make sure that there are no scratches or contaminations in the surface. If you notice that some parts are not being polished as smooth as others you can create a new pitch tool with a smaller diameter and use it to slightly parabolize the mirror in the centre and then polishing that portion of the glass.

Keep going with polishing until you are satisfied with the result.

Step 8: Aluminium-coat or Silvering the Mirror.

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

As the mirror arrived we noticed some scratches in the glass underneath the aluminium layer which were caused by some contamination of bigger abrasive during the engraving and polishing phases.

You can also silver it by depositing the metal through chemical reaction, it would be cheaper, but not as good as aluminium-coating. I don't recommend doing so.

Step 9: Projecting the Structure

The telescope I made is subdivided into 3 main pieces. The first one is the base, with three wheels, four ball wheels and a mainmast in the centre; it provides the horizontal movement to the telescope and can rotate by 360°.

Then there is the support for the main body, with two holes that allow the telescope to move vertically thanks to two threaded rods.

In the end, the main body, which I build from some metal frames and a wooden cube in the bottom.

In the wooden box there will be the three screw system to centre 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.

Then we decided to build a bigger base, but the principle remained the same.

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

Give a look at the photos.

Step 11: Attaching the Mirror and Finding the Focus

Mount the mirror on your telescope and go out at the sun with a paper sheet.

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

So you measure the focus and then pin up the length you get to then place the secondary mirror some inches before that point, in a way that you can converge the light in a point out of the telescope body, into the eyepiece.

If that's not clear, look online for the Newtonian Telescope working scheme or go back to the first step.

Do not look directly into the mirror during this operation, or 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 50mm. This dimension depends on the amount of focal length that you have to regains and redirects into the eyepiece. We paid 50€ for it, which is about 60$, but if you are lucky you should find something cheaper, just be sure this is a high-quality one.

To mount it on the telescope we 3D printed the support and used four threaded rods with some bolts. The model we used comes from https://www.thingiverse.com/thing:2705529

I'm going to upload the STL files just in case the official source won't be available.

Step 13: Mounting the Eyepiece and the Focusing System.

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

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

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    Francesco Torsello
    Francesco Torsello

    Reply 2 years ago

    Well, that's actually incomplete. I'm going to finish it in the next month (waiting for the secondary mirror to be delivered)