How to Grind Telescope Mirrors
Before World War II, there was a considerable telescope-making population around the world. Today, would-be observers usually just buy a telescope, but a few still find enjoyment in making their own. Be forewarned, though: Considerable effort is required to make a telescope mirror.
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Things You'll Need
- Two circular discs of glass that are the size you want the mirror to be
- Carborundum stone
- Rigid three-legged stand
- Measuring tape
- Grinding abrasives
- Pitch
- Polishing compound
- Paper towels
Instructions
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1
Bevel the edge of the mirror disc with a fine carborundum stone and water, taking off a chamfer of about 1/16 of an inch. Aim the grinding stone toward the edge rather than the face of the mirror in case a chip of glass is removed from the surface.
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2
Secure a glass disc to the top of the working stand, inside the three wooden blocks, and sprinkle a teaspoon of 80 carborundum and some drops of water on its face (this is the tool). Put the other glass disc face down on top of the first glass disc (this is the mirror blank).
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3
Press as hard as possible and begin sliding the mirror back and forth across the tool so that its center traverses a distance of about 4 inches. While moving the mirror back and forth, take slow and steady steps around the stand (one revolution per 30 seconds) and revolve the mirror disc by giving it a small twist at the beginning of each stroke.
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4
Change the carborundum grinding mixture as it turns gray and the crunchy sound disappears (about 3 minutes). Lay a straightedge across the mirror after about 30 minutes of grinding; a noticeable concave surface will be evident.
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5
As the concave surface of the mirror comes closer to the desired shape, switch to finer grades of carborundum. Continue grinding, using the finer grades for several hours, and clean carefully between switching grades of carborundum so that scratching does not occur.
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6
When the glass disc has been ground to the desired shape, it is ready for polishing using a mixture of pitch and fine polishing material such as jeweler's rouge. Take extra care not to contaminate the polishing mixture.
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7
Apply a reflective coating, usually aluminum, to the polished mirror. In most cases, the mirror must be sent off to a specialist to have this done, as a vacuum chamber and specialized equipment must be used. When the mirror is returned with the reflective surface applied, it is ready for mounting in a telescope.
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Tips & Warnings
Ordinary plateglass will serve well for the mirror, but low-expansion glass, such as Pyrex, will have less trouble with thermal effects.
Corborundum powder (silicon carbide) is universally used for roughing out the curve of the mirror.
Abrasive powders are graded by mesh size and a recommended series is 1 lb. of 80 carborundum, 4 oz. of 120 carborundum, and 2 oz. of 220, 320, 400, 600 and 900 carborundum.
Final roughing is accomplished using powder grades 1500 or by using M305 emery, which is equivalent to 5-micron grade.
Ensure that no contamination of the abrasive powders occurs, as this will cause your mirror to become scratched.
Polishing compounds that can be used are jeweler's rouge (iron oxide), Barnesite and Cerirouge (1 oz. is enough).
A work stand of comfortable height, able to withstand downward pressure and firm enough not to rock, will be needed. Three small wooden blocks are screwed into the top in a triagle shape to hold the mirror during grinding.
Ensure that the work stand is rigid, stable, and has the ability to withstand heavy pressure without collapsing.
