Parabolizing Strokes part 2
Too short a stroke will produce apparent shapes like those in Fig. 42. Note that the crests in c are too far out. Note also that practically all of the correction lies in the outer zones, the center being flat and undercorrected. Unless this is discovered and the
Fig. 42. Apparent shapes of an imperfectly corrected mirror, seen when the knife-edge is placed: (a) at the center of curvature of the edge zone; (b) at the center of curvature of the central zone; and (c) at the 50-per-cent setting. This surface is the result of using too short a stroke. stroke lengthened before the correction amounts to more than about 0.05", overcorrection will probably develop by the time the crests have been moved in to the proper position.
If too long a stroke is used, shapes like those in Fig. 43 will result. Here the center zone has received most of the correction. Although knife-edge measurements may show the full correction to be present, the edge zone is undercorrected, or flat, and the
Fig. 43. This diagram is similar to Fig. 42, with the knife-edge settings for the apparent cross sections made at the same locations for each respective apparent shape. This surface is the result of using too long a stroke. crests are too far in. It is possible to alter this figure to a paraboloid by removing glass from the zone indicated by the arrow at a, down to the dotted curve, where the paraboloid is presumed to lie.
In attempting this, a stroke of little more than one-third length is used, with the undercorrected zone overhanging the edge of the lap. With local pressure applied at that point, and a blending action introduced by varying the overhang between that point and the mirror's edge, successful correction can often be achieved.
Despite the tyro's best efforts, he will seldom be able to measure the full amount of correction on his mirror, so if the shadow locations are good, it is advisable to call the job finished when the measured correction is about 0.07" or 0.08", as there is an excellent probability that the full correction is there anyway.
If the edge has suffered slightly through use of the long strokes, it can be improved or restored by grinding the mirror face down with light pressure on a large piece of plate glass, using No. 305 emery and water for about 15 or 20 seconds. This will grind flat an edge area about 1/40" wide, and will result in a light loss of about 11/2 per cent, but with a beneficial gain in darkness of field. Flush the mirror clean under the tap, dry it, and observe the diffraction ring on the testing stand.
It is seldom that a paraboloid will be achieved on the first attempt. The worker may overshoot the mark and hyperbolize the mirror, or gross zonal errors may occur. If the figure gets out of hand, it should be restored to a spherical shape by methods outlined in the preceding chapter for correcting the hyperboloid. Through failure, which the novice must be prepared to experience, not once, but several times, you are finding out what not to do, and your judgment and skill in figuring are improving with practice. After the third or fourth trial, the figure obtained will be decidedly superior to what you might have accepted on the first try.
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