Testing the Paraboloid part 4
It is entirely possible, as we shall see when we come to the parabolizing strokes, for the over-all correction to be right, and yet for the mirror not to have the paraboloidal shape.
It is true that the difference cannot be great in the case of the 6-inch f/8; nevertheless the zonal aberration from a mirror of sufficiently irregular figure may be of a rather large order. Of course, at either of the above knife-edge settings, any irregularity in the progression of the curve would be instantly detected by the experienced worker through misbehavior of the shadows, but it might be asking too much of the tyro to expect him to match the veteran's skill in observation and judgment. It is therefore necessary to test an intermediate zone of the mirror, the one most suited to the purpose being the 70-per-cent zone, and for this mask B is used.
The value of the mask for the.70-per-cent zone is questionable, but so many beginners feel that it is beneficial in enabling them better to grasp the fundamentals of the test that its use is suggested.
Now, as closely as possible, place the knife-edge exactly halfway between the above two settings, and cut it in. Faintly gray shadows should appear simultaneously in each zonal opening, there being no indication of the direction of approach.
However, if overcorrection is present (or if the mirror has a lesser ratio than f/8), the shadows in each opening may be seen to come in from the left, but they must come in together. If a shadow appears in the left-hand opening first, the indication is that the central part of the mirror is overcorrected; if seen in the right-hand opening first, then there is a preponderance of correction in the edge zones. Here, too, diffraction from the edges of the zonal opening of the mask will have a disturbing effect on the worker in scanning for the shadows, but if these luminous outlines are not looked at, their disturbing effect will be materially lessened.
testing the paraboloid part 5
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