In the use of a visual telescopic system, the variations in sign and curvature of the wave front of the light emerging from the eyepiece from various points in the apparent field, control the detail which may be seen in an image. In order to be able to measure the sign and magnitude of this curvature, a device, referred to as Dioptometer D-2, has been designed and constructed. The dioptometer is a telescopic system which may be focused on objects appearing to be located at different distances from its entrance pupil. The focusing mechanism of the dioptometer is calibrated in terms of diopters in order that the data obtained using it may be related to the impairment of the performance caused by imperfections in an optical system under test, from the point of view of the human eye. Dioptometer D-2 has been used in image quality studies in which over 500 different telescopic systems of 75 different designs were involved. In these studies, over 100,000 measurements were made. These included measurements of astigmatism, curvature of the objective field, curvature of the ocular field, spherical aberration, chromatic aberration, the diopter settings of reticles, and parallax. It has been shown that the measurements of these quantities, by means of the dioptometer, are in close agreement with measurements made with much more complex and costly equipment presently being used.
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