Correcting
Under the Foucault test at the completion of polishing, the figure on the mirror will probably be some variation of the oblate spheroid, perhaps with a turned-down edge.
This is because, under the work of fast polishing, the lap has sunk more at the center, and the edge of the mirror has been riding into a higher level of pitch. Sufficient cold-pressing, as discussed above, will have helped to alleviate this condition. If the mirror is hyperboloidal at this stage, either the lap is too soft or too long a stroke was used. Regardless of the shape of the mirror, it should be
brought to a spherical figure preliminary to parabolizing.
The first step in the correcting process is to check the condition of the lap. If it appears to be working well and is responding satisfactorily, only a little trimming is in order. If it has spread out around the edges or closed in on the channels, trim it back neatly and carefully with a razor blade.
If the surface appears pretty well glazed, scrape off the wax coating with the flat edge of the blade and rewax. If you are using the molded type of lap, leave it alone if it is satisfactory, but if the facets have sunk down or have run together (an indication of too-soft pitch) chip it off with an old chisel, wipe the surface of the tool clean with turpentine, and make a new lap. A dulled and unresponsive lap can often be revitalized by warming its surface over a stove until the wax shows signs of melting, then recharging it and pressing it into contact with the mirror.
Frequently some roughness, zonal rings, or even a pattern of the lap may be seen on the mirror under the knife-edge test. A drastic polishing action or the use of short strokes is the cause of this. Thinning down the polishing mixture and a slight lengthening of the stroke, together with the blending effect of zigzagging (Fig. 30) should smooth up the surface.
Next- Turned-down Edge |
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