How to Grind a Telescope Mirror

Reflector telescopes use mirrors instead of lenses to focus light.

Reflector telescopes, such as Newtonian and Dobson telescopes, use curved mirrors instead of lenses to focus light. Mirrors are much easier to make than lenses and as a result amateur astronomers can build large aperture telescopes after grinding their own mirrors. Simple tools, grinding materials and time allow amateur astronomers to make properly curved astronomical mirrors and to build large reflector telescopes for a fraction of the cost of similarly sized refractor telescopes.

Things You'll Need

• 2 plate glass rounds, desired mirror size
• Rigid 3-legged stand
• 3 wooden blocks, 1/2 the thickness of the plate glass pieces
• Carborundum powder, #80, #120, #220, #320 #400, #600
• Short rule, showing 1/50 and 1/100 inch
• Carborundum stone
• 6 empty plastic detergent bottles
• Paper towels

Instructions

1. Bevel the edge of the round piece of glass that will be the mirror with a fine carborundum stone and water, producing a chamfer of about 1/10 inch. The other piece of glass will be the grinding tool. When beveling the edge of the piece of glass, move the grinding stone toward the edge rather than the center of the glass stock.

2. Attach three wooden blocks to the stool, forming a triangle, such that the blocks securely lock the glass grinding tool in place--this is the other piece of glass, not the mirror. Sprinkle a teaspoonful of #80 carborundum and some water drops on the face of the grinding tool and place the mirror glass disc on top. Press as hard as possible on the work and begin sliding the mirror back and forth across the tool so that the center traverses a distance of about 2/3rds the diameter of the glass.

3. Take slow steady steps around the stand as you slide the mirror over the grinding tool. One revolution around the stand every 30 seconds will be good. Revolve the mirror disc by giving it a small twist with your fingers at the beginning and end of each stroke.

4. Replace the carborundum/water mix every two to three minutes. After 30 minutes of grinding you will notice a slight concave surface. Calculate the proper depth of the concave curve on your mirror by squaring the radius of the mirror and dividing by four times desired focal length of the telescope. For example, if you are grinding an 8-inch mirror for a 48-inch focal length 'scope, the desired concave depth will be 16 (4x4) divided by 192 (4x48) = 16/192 = 0.08 inches. See "Tips" for making a depth gauge.

5. Grind using #80 carborundum until the desired concave depth is reached. Use the squeeze bottles with clean water and paper towels to wash away all carborundum each time you change grid size, such as from #80 to #120. Repeat the grinding process with #120 carborundum until you have worn away all large pits.Repeat the process with #220, #320, #400 and finally with #600 carborundum taking care to keep the center concave depth correct.

6. Grind with #600 carborundum until you form a smooth, pit-free surface.Protect the ground mirror from foreign material and store it in a clean area until you can begin polishing and finally coating.

Tips & Warnings

• The amount of carborundum powder you will need depends on the size of your mirror. Buy at least 4 oz. of each grade to begin.

• Make a depth gauge with a piece of wood marked to the proper length and place it against the center of the mirror so that it extends up to a straight edge running across the diameter of the mirror.

• After a mirror is properly ground, it is ready for polishing and coating. Both of these steps are different than grinding but are essential to producing an parabolic telescope mirror.

• Make sure the supporting stool is solid because you will be place a lot of pressure on it while grinding.

References

• "Advanced Telescope Making Techniques, Vol. 1"; Allan Mackintosh; 1986
• "A Complete Manual of Amateur Astronomy"; P. Clay Sherrod; 1981
• "The Amateur Astronomer's Handbook"; James Muirden; 1983

Resources

• Read this Article in Spanish