Testing the Paraboloid part 2
Place the mirror on the testing rack with mask A directly in front of and concentric with it. Set up the testing apparatus, bringing the knife-edge block against the guide cleat so that the indicator is over the scale, and shift the whole device about until after repeated trials the knife-edge is at the center of curvature of the zone exposed by the edge openings of the mask. This occurs when the first faint shadows appear simultaneously in each edge opening, and are at the same time of an equal depth of grayness.
The comparison should be made not when the zone exposures have darkened, but just as the knife-edge encroaches on the rays proceeding from these openings, and while the most delicately gray shadings that permit proper observation are present. This will call for some nice judgment of contrast, particularly as diffraction around the edges of the openings may have a blinding effect (the trick here is not to look at this diffraction), and your eye must dart from one opening to the other to check for simultaneous appearance of the shadows.
The center of curvature thus found is not that of the extreme edge zone, but of a zone of 2.8" radius, or the mean radius of the zonal opening. A calculation will show that the center of curvature of the extreme edge zone, which is what is ultimately sought, lies about 0.01" farther from the mirror. Consequently, when it is deemed that the shadow appearances in the edge openings conform to prescription, the knife-edge should be withdrawn from the mirror by that amount.
A magnifying glass will be of assistance in making a precise setting. Now remove the mask and observe the over-all shadow appearance on the mirror. As the knife-edge is again cut in, the first shadow to appear will be seen 0.9" inside the right-hand edge of the mirror (actually at the 70-per-cent zone). With further movement of the knife-edge, this shadow expands, moving somewhat slowly toward the right-hand edge, and more rapidly toward the center of the mirror.
Finally, it reaches the mirror's right edge and center together, and at the same moment a thin wisp of shadow appears on the left edge, the mirror then having the dish-like appearance shown in Fig. 39a. As you continue the knife-edge movement, the shadows at the left edge and the center approach and meet at the 70-per-cent zone on that side, and the whole mirror is darkened. These are the shadow appearances when the knife-edge is at the center of curvature of the edge zone, and unless they behave as described, the mirror does not have the paraboloidal shape. Note carefully the indicator reading on the scale.
testing the paraboloid part 3 |
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