Testing the Paraboloid
To provide a means of isolating the various zones that are to be tested, and thus to simplify the problem of locating their centers of curvature, it will be necessary to prepare two masks similar to those shown in Fig. 38.
These are made of cardboard, each disk being of the same diameter as the mirror. The one at A has a central opening of about l½" diameter, and edge openings about 2" in length and 0.4" in width. The openings in the mask at B are ½" wide (¼" on each side of the 70-per-cent zone). A good length to make these mask openings is l½".
The mirror will be tested in three zones; that is, the intersection points of rays from the three zones shown in Fig. 39 are to be sought, and if found where they ought to be, and the curve is smoothly progressive, the mirror may be adjudged a paraboloid.
Fig. 38. Masks used in conjunction with the Foucault test for making zonal measurements on a paraboloidal mirror. In positioning the testing device preliminary to actual testing, the guide cleat (see Fig. 19) used to guide the longitudinal motion of the knife-edge should, as a matter of convenience, point directly to the vertical diameter of the mirror.
In seeking the center of curvature of any zone, it is a good idea to begin by trying with the knife-edge placed well inside, and then outside of that point — bracketing it, so to speak — and then gradually to close in on it from each side. Knowing how the shadows appear on either side of focus, the observer is able to decide more accurately where the exact center of curvature of the zone lies.
testing the paraboloid - part 2 |
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