Polishing Procedure
Be sure in polishing that your fingers do not curl over the edge of the mirror to such an extent that they are in close proximity to the concave surface. So placed, they will warm and expand the edge there, which will polish off, and when the mirror has stabilized, its edge will be found to have been turned down.
Also, when one's fingers are hooked too far over the edge, an imperceptible tilt may be imparted to the mirror in drawing it back from the end of a stroke, causing its edge to plow into the pitch —- result, turned-down edge. Work at the rate of about 20 to 40 strokes per minute, depending upon the drag of the lap.
(Each stroke is the combined back-and-forth motion.) Frequent rotation of the mirror is not necessary, and never rotate it during a stroke, as the excessive action thus given to the edge zones will result in a depressed ring and a turned-down edge. A good procedure is to take six to
eight strokes at one position, then shift about 45° to a new position where the strokes are repeated, then another 45° shift, and so on, giving the mirror a slight rotation at each second or third position only.
While the infrequent rotation is perfectly safe in general polishing, and is suggested to avoid excessive edge polishing, in figuring, the mirror should be rotated with each shift of position, as every precaution must then be taken to avoid any possible introduction of astigmatism. This fault results from failure to maintain a perfect surface of revolution. Working for too long a time in one position and taking too many strokes on the same diameter of the lap or of the mirror are obvious causes of producing different radii of curvature on different diameters of the mirror. This is one of the hazards to be contended with in attempting to figure the mirror with a fast-working polishing agent.
After 20 minutes of polishing, wipe the mirror clean with a soft cloth or absorbent tissue, wipe off any lint with the underside of the forearm, and observe your progress. The usual effect of the channeled lap on a spherical mirror is to polish most quickly at the center. A more even polishing action occurs with the use of the molded lap. Imperfect contact may cause zonal polishing. A hyperbolic mirror will polish most rapidly at the edge, in which case the stroke should be lengthened just enough to get the center of the mirror hitting the lap. This will also reduce the hyperboloid. If the polishing falls off abruptly before reaching the mirror's edge, it indicates a flattened or turned-back edge from improper
grinding. If, after another period of polishing, there is no indication of the lap hitting this edge (a remote possibility), a return to No. 220 carbo, grinding with the mirror on top and with short strokes, may be necessary.
Next- How to use the Foucault Device |
|
