Abstract

The usefulness of the interferometer in controlling the quality of telescopic systems has been studied. The study consisted of collecting interferograms for individual and mixtures of the common aberrations under a variety of focal adjustments of the telescopic system under consideration. These interferograms were compared with predicted patterns computed from a knowledge of the aberrations and from an “optical-path-difference equation.” Experimental and computed patterns were found to be in excellent agreement, thus constituting a check on the optical-path-difference equation used. The experimental and computed interferometer patterns were used to form a basis for determining the usefulness of the interferometer. Two distinct uses appeared evident from the point of view of quality control. The first of these is to acquaint an inspector with design limitations of optical systems under consideration. This may be done from a knowledge of the form of patterns that should be produced if the design were properly executed. The second use of the interferometer is in connection with a routine procedure for evaluating the quality of an optical system under test. In this connection it was found impractical to attempt to analyze the nterference pattern for individual component aberrations on the basis of the shape and number of interference fringes even in the case of a simple mixture of aberrations. Therefore a method was developed which does not depend particularly on the number of fringes present nor the shape of the interference pattern. This method is based on a study of interferometer patterns of telescopic systems focused visually on large numerals placed optically at a great distance. For such a focal adjustment, the interferometer pattern is found to have the largest possible diameter covered by a light or dark interference area, the size of which depends upon the magnitude and types of aberrations present. A measure of the quality is related to the largest circle that can be inscribed in this area. When the diameter of the inscribed circle is expressed as a percentage of the diameter of the axial entrance pupil, the quantity obtained is referred to as the “Interferometer Quality.” This quantity is found to be in agreement with quality evaluations based on resolving power measurements. The Interferometer Quality may be determined for telescopic systems at various field angles and is likely the most accurate and rapid procedure available at the present time for evaluating the optical quality of telescopic systems.

© 1947 Optical Society of America

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