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1949 OBJECTIVE LENSES 95
f/0.6 by combining a dispersive air-spaced triplet with a front-surface spherical reflector, Fig. 4c.
Use of a substantially zero-power concentric meniscus lens, which is also concentric to the mirror which it corrects, has been made by Maksutov38 and by Bouwers,39' 40 Fig. 4d, to obtain large fields with large apertures, the aperture being limited only by zonal aberrations and longitudinal chromatic aberration. For those applications requiring extreme aperture where curvature of field to a radius approximately equal to the focal length is not objectionable, systems of this type may be preferable to straight refracting objectives in the present state of the art. The simplicity of concentric systems from the viewpoint of both construction and computation, axial tracing and design suffice for the entire field of view, makes these objectives attractive. Application of this principle to television projection has been made by Bennett41 who has obtained an aperture of //O.8. Bouwers42 has further improved the corrections obtainable with the concentric system by providing an aspherical correcting plate in an arrangement in which the mirror is corrected by a substantially concentric double meniscus. Baker43 has utilized as the correcting means in one modification of the concentric system of //0.6 aperture a pair of menisci with their concave surfaces facing an aspheric correcting plate of very weak curvature. A similar approach, but without a corrective plate, has been presented by Wynne.44 Gabor45 has disclosed a reflecting system of //0.9 aperture based upon the same general principles as that of Maksutov and Bouwers. In Gabor a secondary mirror permits the more convenient positioning of the image just at the rear of the primary mirror. Henyey and Greenstein46 have described a concentric system of about //I aperture for photofluorography which utilizes an achromatized correcting meniscus. In another report47 by these investigators, additional systems of various configurations are shown.
The parabolic mirror is ideal for telescope imagery but suffers so badly from coma that it cannot be employed for photography where both large aperture and extended field are requisite. Warmisham48 ~63 has succeeded in combining spherical and aspherical mirrors with and without aspheric correcting plates to produce a family of large-aperture objectives, some of which are suitable for photography.
It should be noted that in each of these mirror systems there is vignetting either from the photographic film or from a secondary mirror which has the effect of blocking the light from the central zones responsible for much of the sharpness in a lens. The rays from the