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ROUGH GRINDING THE MIRROR

The heart of the Newtonian telescope is the comparatively long focus, parabolic mirror called the objective. It gathers light from the object and forms an image of the object in the focal plane at the eyepiece. Most of our know-how, our effort, and our expense shall be devoted to fashioning the curve of this concave, objective mirror.

In order to generate this slightly concave curve on the face of the mirror blank we shall grind the mirror blank face down on the tool with Carborundum (or sand) and water in between. The tool should be a couple of inches smaller than the primary mirror (ex. 6" tool for 10" primary).

To facilitate the grinding we may pin the tool glass between three furring nails near the end of a patio bench and sit astride the bench while we work.

The procedure is simple. With the tool pinned between the furring nails so it can easily be turned around but cannot slide from its position, we wet it and sprinkle it with a small amount of grinding compound, usually 60 grit Carborundum. Then, placing the mirror blank face down on the took, we vigorously push it back and forth with long strokes, pressing down very hard and turning it slightly every few strokes and occasionally turning the tool in the opposite direction.

During most of the rough grinding we keep the mirror almost constantly in the overhang position. We push it over the tool, not center over center, but with the mirror overhanging first on one side and then the other side of the tool by about a quarter of its diameter. The mirror is, of course, turned a little every few strokes and occasionally the tool is turned in the opposite direction. Thus, the turning mirror slowly wanders from side to side while being pushed vigorously forward and backward over the tool.

This heavy grinding, mostly in the overhang position and with long strokes, rapidly grinds out the center of the mirror blank against the edges of the tool and generates a curve. However, grinding in the overhang position, as we have just described it, does not generate a spherical curve. The curve must therefore be corrected toward the end of rough grinding by shortening the strokes and curtailing the lateral overhang to make the curve more nearly spherical.

The grinding should make a great deal of noise and should show sparks if done in the dark. When the sound dies down or when the pushing feels heavy because the grits are worn to powder and mixed with powdered glass, we remove the upper glass, taking great care not to scrape its edge on the lower glass. It is alright to scrape the middle of the mirror glass against the edge of the tool, but the edge of the mirror must be kept inviolate. Then we pour a little water on the tool, replace the mirror, and continue grinding for a few strokes to clean out the "mud". We repeat this washing procedure a couple of times and then add a small amount of fresh grinding compound and proceed again with the grinding. The grinding that takes place between cleanings is called one "wet". If the pushing gets heavy before the sound dies down, you're probably using too much grit. About half a teaspoon per wet should do for an eight inch mirror, about one full teaspoon for a twelve inch mirror.

During all this grinding, the hands must carefully be positioned over the center of the mirror blank, or if separated, with the center just between them so that the pressure from the hands acts as if it were over the center of the mirror blank. If the hands press down together near one edge of the mirror blank, the curve on the face of the mirror blank will become shallower instead of deeper, lengthening instead of shortening the focal length.

The rough grinding usually digs a depression in the center of the upper disc which gradually deepens and spreads toward the edge of the disc. It is a fairly simple matter, by regulating the extent of overhand, to arrange to have the curve on the mirror face reach its proper depth just as the edge of the ground out area reaches the edge of the blank. We must not be deceived into thinking the curve has reached the edge of the blank just because there are coarse grinding pits all the way out to the edge. (If the tool has been used before, and has already a convex surface, the edge of the curve on the mirror face remains clear at all times - but not so if the tool is flat.) Lay a straight-edge across the mirror and see. If the depth of the curve should reach target before the curve reaches the edge of the blank, or if the curve becomes too deep, the grinding may be continued with the tool on top. This will increase (lengthen) the focal length and push the curve to the edge. If a large bubble appears between the discs, we shorten the strokes and curtail the lateral overhang until the surfaces more nearly mesh.

The extent of success in rough grinding may be gauged by splashing water on the mirror curve facing into the sun and catching the reflection of the sun from the water film. The distance between the mirror, faced into the sun, and the smallest obtainable spot of reflected sunlight cast on a wall measure the crude focal length of the curve. When this crude focal length is close to our desired focal length the rough grind has reached target. It shouldn't take more than four to six hours to rough grind a ten or twelve inch mirror.