Parabolizing Strokes
The purpose now is to convert the apparent cross section of the sphere (Fig. 31c) into the apparent cross section of a paraboloid, and as this will be done by removing glass from the center of the mirror, the apparent cross section to be arrived at is the one at a, Fig. 39.
Obviously, if the latter shape is seen on the mirror at some knife-edge setting (the center of curvature of the edge zone), the shapes b and c will also be found at appropriate settings, and the parabolizing will have been accomplished. Any stroke or combination of strokes that will progressively deepen the mirror into that shape may be used. Before proceeding, the lap should be hot-pressed into absolute contact with the spherical mirror, and about a half-hour allowed before commencing the figuring strokes. A similar procedure should be observed at any stage of the figuring, if the mirror has been removed from the lap for test, or for some other reason.
In general, a narrow zigzagging W stroke gives excellent results. This stroke should carry the mirror from a centered position on the lap to about 1¾" overhang on one side, back across the center to an overhang of 1" on the other side, and thence back to center, where the mirror is given a slight rotation, and the sequence repeated from a new position. Too many strokes taken at one position may cause the surface to become astigmatized.
After circling the stand once, test the mirror to locate, if possible, the position of the shadow crests. If they are found too close to the center of the mirror, the stroke length or extent of overhang should be reduced. If too far out, the stroke length or the overhang
should be increased. The temper of the pitch will determine the extent of stroke length, which may have to be varied somewhat from the nominal figures given above.
The harder the lap, the longer the stroke may be. Fig. 41 shows the positions of the mirror at the extremities of the recommended strokes, and the zigzag line is the course traversed by the center of the mirror. The blending action of this stroke is capable of producing a beautiful paraboloidal surface. By proper manipulation, the degree of correction in anyone zone can be varied at will, within limits, provided care is taken to prevent disfigurement elsewhere through deformation of the lap.
next- parabolizing strokes part 2
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