How to Build a Telescope

Amateur telescope making is a very popular pastime. Whether you want your astronomy telescope but don’t have the budget, or simply enjoy the idea of building your homemade telescope, our guide will help you achieve what you want. So, let’s get started – read on and find out how to make a telescope of your very own.

The most popular amateur homemade telescope is the obsidian telescope, which is a Newtonian reflector with an obsidian mount. It’s possible to build your telescope that is better quality than commercial ones.

You will need two mirrors, a primary mirror, which is usually parabolic, and a smaller, flat secondary mirror. Many amateur astronomers grind their primary telescope mirror that is diffraction limited.

Parabolic mirror

The mirror needs to be ground and polished to a shallow spherical section. Using a polishing lap, it must be figured into a parabolic shape. The depth of the curve determines the focal length of the mirror and the telescope’s f-stop. Making the shape of a paraboloid can be challenging to achieve. So getting the mirror’s focal length long enough, for example, f-12, makes a spherical curve’s performance become roughly equivalent to a paraboloid.

The primary mirror, when used with a given eyepiece, can have its magnifying power improved by having a longer focal length. This does, however, cause the field of view to be smaller.

How to Build a Telescope

The telescope mirror is usually ground from a mirror-blank of low expansion borosilicate glass. Glass ceramics can also be used, resulting in telescope mirrors that have less deformity from temperature change. These do cost more, however.

The mirror-blank needs to be ground against another piece of glass, such as window glass or another mirror blank. The second piece is usually placed in a frame on a barrel to allow you to access it from all sides. Silicon carbide is mixed with water and used between the mirror-blank and the tool, which is the second piece.

Grind a hollow in the mirror

A hollow must now be ground in the mirror, and this is done by stroking the mirror-blank across the tool. The silicon carbide abrasive mixture is ground against the two pieces of glass by the stroking motion, and you should add more as required. Every 10 – 30 seconds, you should make a step around the barrel, which helps create a perfectly concave spherical surface.

As you grind, you will notice the glass pieces becoming concave and convex. Once the mirror starts to get to the desired radius, the depth of focus needs to be checked. Wet the mirror’s surface and see where a light’s image focusses against a piece of card.

Repeat the process using finer abrasives. When the silicon carbide reaches 500 grit, swap to aluminum oxide. Keep checking the focal length of the mirror while grinding.

Polish it!

Now you need to start the polishing process. Firstly, cover the mirror in the rouge. Now you need to make a pitch lap from the tool to be used for polishing. Heat a pitch compound in a double boiler until it liquifies, pour it over the mirror, and press the tool on top of the tool and pitch, so the shape of the lap becomes the same as the mirror.

The rouge on the mirror stopped the lap sticking to it. Once the mirror and lap are separated, cut channels into the lap so the water and abrasives can runoff. Polish the mirror using the lap and rouge or cerium iv oxide.

To test the mirror, you need to make what is known as a Foucault tester. This has a pinhole light source, and a vertical knife-edge mounted on micrometers and mounted at the focus. The light will illuminate the mirror, and you look at it past the knife edge.

Place the mirror vertically in a stand, and set the Foucault tester close to the mirror’s focal point. Adjust the Foucault tester, so the returning beam from the pinhole light source gets interrupted by the knife edge. If you view the mirror from behind the knife-edge, you’ll see a pattern on the surface of the mirror.


If the mirror looks evenly lit across the surface, then it’s a perfect sphere. If it looks donut-shaped, then it’s parabolic. Small imperfections appear as bumps.

Now you need to figure the mirror. Using the polishing lap again, stroke the mirror to make the surface into a spherical shape. Keep stroking the mirror until the mirror has a parabolic curve. Check if the mirror’s surface is evenly lit using the Foucault test.

To make the image less dim, you need to coat the front of the mirror with a highly reflective material such as aluminum. Place the mirror in a vacuum chamber with electrically heated nichrome coils that can sublime aluminum. Exposing the mirror to pure oxygen will form a layer of aluminum oxide. Alternatively, evaporate a layer of quartz in the mirror.

Mount the mirror in a tube made from metal or cardboard. Place the primary mirror at the tube bottom, suspend the secondary mirror in the middle of the tube at the top using a spider, which is a low profile mount. Aim the eyepiece at the secondary mirror on the spider, from outside the tube.

Adjust the position and tilt of the mirrors to focus light through the secondary mirror and into the eyepiece. This is known as collimation. While you’re here read this awesome article on Solar Panels.