Correcting the Oblate Spheroid
In the discussion of the oblate spheroid, it was stated that any of the three apparent shapes in Fig. 32 could be converted into a sphere. It is obvious that it would be difficult to select a stroke that would correct a.
The apparent shape at b can be handled by the use of a long stroke, but there is quite an amount of glass in that central hill, and excessive long-stroking will turn down the edge. There is less glass in the central hill in c, and enough in its edge zones to offset the effects of the long strokes, so that is the knife-edge setting that ought to be used for correction.
Hot-press for contact. Use a one-half diameter W stroke, extending the overhang on each side to the boundaries of the hill. Polish for five minutes and press for 10, and test after two such spells. If the hill is being planed down evenly, continue the same stroke, first restoring contact by hot-pressing if necessary. If a crater or hole is showing up at the center of the hill, reduce the stroke length. If progress seems too slow, lengthen the stroke, or slightly increase the overhang on one side. Watch out for a lengthening radius of the edge zones, an indication that the stroke is too long.
Hard laps will permit a wider range of strokes than soft ones. If good contact is being maintained through long periods of cold-pressing, the edge will not be impaired, and the blending overhang stroke will be ironing out any zonal rings or roughness that may have been caused by the short strokes used in edge correction.
With the harder pitch of the molded lap, the periods of corrective polishing can be extended to 10 or 15 minutes. Use of a rapid stroke will often bring a hill down fast, but look out for turning the edge. Finally, to wipe out any zonal irregularities remaining from the oblate figure, return to the one-third W stroke (Fig. 30), cold-pressing frequently for long intervals; or hot-press if necessary.
Eventually the mirror will be whipped into a spherical figure, and on the Foucault stand it will appear flat and velvety smooth.
Next- Correcting the Hyperboloid |
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