Small chromatic aberrations and large field of good definition make Schmidt systems particularly suited for spectrograph cameras. The outstanding disadvantages—spherical focal surface and loss of light due to the position of the plate in front of the mirror—play different roles in cameras of large and small focal lengths. For focal lengths above 30 inches, photographic plates on thin glass can be bent to the necessary curvature without great risk of breakage. For cameras of shorter focal length, curvatures necessitating the use of film can be avoided by the introduction of a field-flattening lens. A plano-convex lens placed close to the focus has proved completely satisfactory for cameras of relative apertures f/3 and f/1.5. The use of flat plates without field flattener restricts the field of good definition to an angular extent of degrees (d permissible diameter of the circle of diffusion, A diameter of the collimator); this is sufficient for many purposes. The loss of light due to occulting by the plate can be avoided by an off-axis construction. This is completely satisfactory for cameras of large focal length where the plate can be bent to the curvature of the focal surface. Field-flattening lenses are usable, but the aberrations introduced by them will be those corresponding to the normal system of larger aperture of which the off-axis system is part. Consequently, the aberrations will be relatively large and the usefulness of field-flattening lenses with the off-axis construction will be restricted to relative apertures of f/4 and smaller. The use of flat plates in off-axis systems results in a sidewise curvature of the spectrum on the plate with a radius of curvature kf2/A; the factor k, depending on details of the construction, will usually be between 1 and 1.5. Such a curvature is objectionable. As the use of film generally reduces the accuracy of wave-length measures and is, therefore, equally undesirable, off-axis systems of short focal lengths are of restricted usefulness. For cameras of very short focal lengths, the small scale of the spectrum permits the use of small plates, thus minimizing the loss of light in a normal system, but a range of focal lengths below 30 inches will always remain for which the full advantages of Schmidt systems are not available without some loss of light.
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