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26 October

How to Grind a Perfect (Scratchless) Mirror

by C. L. Stong
Originally appeared in "The Amateur Scientist", October 1969

A discouraging phenomenon tends to beset those who first attempt to grind and polish the objective mirror for a reflecting telescope. At least one deep scratch mysteriously appears in the glass no matter how carefully the craftsman works. Telescope mirrors are ground to a concave figure by sandwiching an abrasive slurry between two disks of glass and pushing the top disk back and forth. As the grinding proceeds the lower surface of the top disk, which will function as the mirror, becomes increasingly concave and the top surface of the lower disk, which acts as the grinding tool, becomes correspondingly convex.

The worker uses successively fine particles of abrasive and finally polishes the lightly frosted surface of the mirror to a paraboloidal figure by stroking the glass on a disk of pitch coated with a slurry of rouge. All instruction books admonish the beginner to use abrasives of high quality and to maintain scrupulous cleanliness in order to prevent a coarse particle of grit or dust from lodging between the disks and making the unwanted scratch. Although scratches have almost no effect on the optical performance of the telescope, they are irksome to craftsmen who prize excellence in workmanship. T. R. Macfarlane of Regina, Saskatchewan, describes a little-known but reliable method of ending the difficulty:

"Scratches are made by lumps that form in all grades of fine abrasive. The lumps plow grooves in the glass just as though they were solid particles. They can be dispersed by a sedimentation procedure that improves the abrasive in another respect. All grades of abrasive contain powdered grit: particles much smaller than those of the maximum size. When the powder becomes wet, it acts like mud in that it retards cutting action. By removing the powder the time required for the final stages of grinding can be cut in half.

"Abrasives are graded by number, ranging from 80 (particles about the size of granulated sugar) to 600 (microscopic particles). The coarser grades do not clump and rarely cause scratches. The difficulty appears with grade 320 and smaller. To purify abrasives you will need a few jars of clear glass ranging in size from a quart to a gallon, small jars with caps to hold the purified abrasive, four feet of rubber hose a quarter of an inch in diameter and a quart of water glass (sodium silicate).

"I put clean water, to which I have added about two ounces of water glass, in a gallon jar until the level is an inch below the top. The water glass serves as a deflocculating agent: it disperses lumps that remain solid in water alone. One ounce of abrasive is thoroughly mixed with the solution and left to settle for 30 minutes. With the rubber tubing I then siphon all but two inches of the fluid into a clean container. I label the container 600-1 and put it aside.

"I refill the settling jar with water containing one ounce of water glass, thoroughly mix the remaining grit and again let it settle for 30 minutes. All but an inch of the fluid is then siphoned into a clear glass container and labeled 600-2. Thereafter I repeat the procedure, progressively reducing the intervals of settling to 15, eight and three minutes. The stored containers are labeled 600-3, 600-4 and 600-5 respectively.

"Finally, I shake up the settled dregs and pour them into a smaller jar. This material settles quickly. A sharp line appears at the boundary between the clear fluid and the suspended grit. When the upper third of the fluid clears, I carefully pour all but a third of the remainder into a clean jar. When this material settles, I pour off and discard the clear fluid. I then refill the jar that contains the dregs and repeat the procedure three times. The collected material is labeled 600-6. To the remaining dregs I add one ounce of the 600 grit as it comes from the manufacturer, process it by the same procedure and similarly treat the remaining stock. After several days, when the grit in all six labeled containers has settled, I carefully siphon off the clear fluid and dry the abrasives for use.

"What about the accumulated dregs? To them I add one ounce of 500 grit, proceed as described and then switch to 400, followed by 320. I do not process the coarser grades.

"Purified abrasive easily cuts twice as fast as untreated material. During the final grinding stage, when 600-6 grit is followed by 600-5, -4, -3, -2 and -1, the glass emerges unscratched and with a semipolished surface. Other deflocculating agents and techniques of sedimentation that differ slightly from this procedure are described in Amateur Telescope Making (Book Three), by Albert G. Ingalls (Scientific American, Inc.)."