Do telescopes use mirrors?
Many do. Reflecting telescopes, including Newtonians and Dobsonians, use a curved mirror to gather and focus light instead of a lens. Large research telescopes use mirrors as well, because a mirror can be supported from behind and avoids some problems that lenses have, such as colour fringing.
The primary mirror in a Dobsonian is a shallow dish, ground and polished to a precise curve, then coated with a reflective layer. Amateurs have made these mirrors at home for generations. This article gives an overview of the process. For detailed measurements and tables, use a dedicated guide such as the one at Stellafane or a mirror-making book, and consider joining a local telescope-making group.
What you need
- A mirror blank and a tool: two glass discs of the same diameter. One becomes the mirror, the other is the tool that grinds it.
- Abrasives: silicon carbide grit in progressively finer grades, followed by finer abrasives for smoothing.
- A sturdy mounting: often a barrel or a stand at a comfortable height, so you can walk around it.
- Polishing material: pitch to form a lap, and a polishing compound such as cerium oxide or rouge.
- A testing setup: a Foucault tester or similar (see below).
- Patience: a first mirror usually takes many hours spread over weeks.
Kits that include the glass and abrasives are available and take away a lot of guesswork.
How the process works
Rough grinding
The mirror and tool are ground against each other with coarse abrasive and water. The glass is moved in long strokes, with regular turns of the glass and walking around the barrel. Over time, the mirror becomes concave and the tool convex. The depth of the curve is measured and compared with the target for the intended focal length.
Fine grinding
You then repeat the process with progressively finer abrasives. Each stage removes the pits left by the previous one and refines the curve. Clean the work area thoroughly between stages, since a single stray coarse grain can leave deep scratches.
Polishing
A pitch lap is made on the tool, and the mirror is polished with a fine polishing compound until the surface is smooth and clear. Polishing can take considerable time, and the pitch needs to be pressed and shaped correctly so it matches the mirror.
Figuring
The mirror must be shaped into a precise paraboloid for a standard Newtonian, as a spherical curve would blur the image on faster mirrors. Figuring means adjusting the surface by polishing in specific ways, testing, and repeating. This is the most delicate stage, and it takes experience.
Testing
Mirror makers test frequently during polishing and figuring.
- Foucault test: a knife edge is moved across the focused light from a point source, showing the shape of the surface as shadows.
- Ronchi test: a grating shows bands whose shape reveals errors.
- Interferometer: more advanced and often used by experienced makers or at clubs.
The Foucault test is simple to build and widely used by amateurs.
Coating
After polishing, the glass needs a reflective coating, typically aluminum, applied in a vacuum chamber. Most home makers send their mirror to a professional coating service. A secondary mirror is also needed, and these are commonly bought ready-made.
Safety
- Glass dust and fine abrasive slurry should not be breathed in. Keep things wet and clean up carefully.
- Glass edges can be sharp until bevelled.
- Pitch is heated, so handle it carefully.
- Read the safety sheets for any chemicals and abrasives you use.
Time, difficulty, and whether to buy instead
Mirror making is accessible, but it is also slow and easy to get wrong. The result can be a high-quality mirror at low cost, and many makers value the craft itself. If you only want a working telescope, buying a mirror or a complete scope is faster and predictable. Our Dobsonian telescope for beginners guide and best Dobsonian telescopes page can help you decide. If you have a mirror ready, see making a Dobsonian telescope for the rest of the build.
FAQ
How do you create a mirror at home?
By grinding one glass disc against another with abrasives, polishing with pitch and a polishing compound, testing the shape, and having it coated. A kit and a good guide make it much more approachable.
How do you build a homemade telescope?
Pair your mirror with a secondary mirror, tube, focuser and mount. The Dobsonian is a popular design for this, as described in our making a Dobsonian telescope overview.
What is a really good home telescope?
A Dobsonian is a strong choice for home use, whether bought or built. Compare options in our best Dobsonian telescopes guide.