Abstract

In the production of 60-inch anti-aircraft searchlight reflectors by the electroforming process, convex parabolic forms are used as molds. To avoid production of reflectors with excessive spherical aberration, it is necessary to know the inherent spherical aberration of each mold. To obtain this information, an apparatus has been devised to measure the spherical aberrations of the convex surface of a paraboloid of revolution. The optical system of the apparatus consists essentially of a collimator, a plane mirror, and a photographic plate used in the following manner. A narrow beam of light parallel to the optical axis of the mold to be tested is projected from the collimator onto a designated spot of a particular zone on the mold which is in a horizontal position. The beam of light is reflected from the mold as if it were coming from the virtual focus for the designated spot. The reflected light strikes the plane mirror, vertically placed in a predetermined position and is re-reflected so that the beam of light intersects the optical axis of the mold at a distance above the mold equal to the focal length for the designated spot in the mold being examined. The photographic plate is placed horizontally at a distance above the mold equal to the nominal focal length of the mold. The beam of light intercepts the photographic plate and produces a specified image. Eight such images are photographed on one plate at equally spaced intervals around the periphery of the zone being tested. A required number of zones are tested. Simple interpretation of the photographs yield the amount of aberration present. An important feature of this apparatus is that the interpretations of the photographs are identical to those obtained for testing concave parabolic reflectors. This apparatus aids reflector production in that it determines the abberation characteristics of the convex mold directly, thus eliminating the previous practice of making a reflector from a mold to determine the mold’s accuracy indirectly.

© 1947 Optical Society of America

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