Turned-down Edge
This is a narrow zone around the mirror,- usually about 1/8" or less in width, but sometimes as much as ½ in which the radius of curvature is rapidly lengthening. On the apparent surface of the mirror, it is somewhat like the run-down heel of a shoe.
If left uncorrected in the telescope, a turned-down edge will pitch its rays out beyond the focus of the rest of the mirror, some of them causing an image blur and others passing into the eyepiece and fogging up the field of view.
Under the knife-edge test, there will always be a diffraction ring around the right-hand edge of the mirror, very brilliant if the edge is badly turned. In that case the left-hand edge of the mirror will appear to be soft and dark. If the mirror's edge is perfect, there will be a fine, continuous hairline of light entirely surrounding it, of nearly equal brightness on both sides. The ring on the right should not persist beyond the point where the diffraction disappears from a straightedge suspended across the face of the mirror.
To correct turned-down edge, use short strokes if the edge is badly turned, otherwise the one-third stroke should bring out the diffraction ring, provided proper attention is paid to maintaining contact. Work for five minutes and cold-press for five minutes. After three or four such spells, try the knife-edge test, and if the diffraction ring is making an appearance, continue with that treatment; if not, use a shorter stroke. An excellent aid to securing contact in cold-pressing with the channeled lap (it is' not necessary for a molded lap) is the use of a piece of onion sacking.
An empty onion bag can be bought at a vegetable store for a few cents. Spread a piece of the material over the lap, lay the mirror on top, and then apply the weights. This breaks up the surface of the lap into numerous small facets which, under polishing, more easily conform to the changing surface of the mirror.
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