The Solid-Glass Schmidt Camera and a New Type Nebular Spectrograph

The complete fifth-order theory of image errors is applied to both the solid-glass Schmidt camera and to the usual kind of Schmidt camera. The fifth-order defect most important in the formation of images by the Schmidt camera is that called the variation of spherical aberration with angle. The Schmidt camera can be made completely aplanatic, but the resultant optical surfaces differ from those for minimum chromatic aberration. The equation of the meridian section of the correcting surface is developed through tenth-order terms in order to obtain the curve of the correcting surface to sufficient accuracy for the fastest cameras. The fifth-order theory permits a solid Schmidt camera of the limiting focal-ratio of f/0.30 and a field of 7° to be made. An exact calculation indicates that the fifth-order terms, even at f/0.30 and over a 7° field, represent within ten per cent of the exact value the numerical size of the image aberrations, and are completely adequate for slower cameras and moderate fields. The solid Schmidt of f/0.30 is to be used as the camera in a new type nebular spectrograph. A prism arrangement is presented that affords the advantages of cheapness, compactness, and low light absorption. In addition to one or more small 60° dispersing prisms, from two to four 30-60° prisms are added in order to spread the collimated beam of circular cross-section into a beam of elliptical cross-section, entering the camera. This device yields the above advantages without loss of speed or definition at a given dispersion. The coating of the glass surfaces in such a system is recommended in order that the total efficiency be within 90 per cent of the theoretical limit. The prism device enables the solid Schmidt to be used off-axis without loss of light and thus affords easy access to the focal surface.