Testing the Paraboloid part 3
Replace mask A; then slowly slide the knife-edge toward the mirror, stopping frequently to cut it into the cone of light, and observe the passage of the shadow on the centrally exposed portion of the mirror. At first, the shadow will definitely be seen to come in from the right, because the knife-edge is, relatively speaking, still
Fig. 39. Shadows seen on the paraboloid when the knife-edge
is at the center of curvature of: (a) edge zone; (b) center zone;
(c) 70-per-cent zone. Why the shadows behave as described in the text can be found from a study of how, in the lower diagram, the rays proceeding from the various zones of the mirror are intercepted by the knife-edge. Beneath the shadowgrams are shown the apparent cross sections of the mirror seen at the respective knife-edge settings, the broken straight lines representing the reference spheres used in testing, and also the relationship of these spheres with the concave paraboloidal mirror.
.........well beyond the center of curvature of the central zone, but as this point is approached, the direction from which the shadow comes in becomes less apparent. When a position has been found where this exposed area is seen to darken all over, instantly and evenly, just as though it were a small spherical mirror, and no perceptible direction of shadow approach is apparent, the setting may be accepted as the center of curvature of that zone. Again note the scale reading shown by the indicator. The difference between this setting and the one found for the edge zone should equal the value of t2/R, or lie within the recommended tolerances.
We must acquaint ourselves with the over-all shadow appearances at all zonal settings, so after removing the mask, we again cut in the knife-edge. The shadow is seen to appear on the left-hand edge of the mirror, moving slowly at first, then with increasing rapidity until upon reaching the 50-per-cent zone (halfway from the center of the mirror to the edge), it suddenly sweeps across the central part of the mirror. The left-hand zones of the mirror are then dark, and the central part, bounded by the mirror's 50-per-cent zone, appears flat and gray.
In appearance, the mirror resembles a bowl turned bottom side up (Fig. 39b). Special attention should be paid to the upper and lower shadow boundaries as the shadow encroaches on the mirror. These should appear to terminate at the "poles" of the mirror, from which points the shadow seems to pivot. If there is any straightening or outward bending of the shadow, the indication is that the outer zones of the mirror are insufficiently corrected. This test becomes more delicate if the knife-edge is tried a small distance inside of focus.
testing the paraboloid part 4 |
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