An Improved Type of Schmidt Camera

By substituting for the aspheric plate of the Schmidt camera a combination of an aspheric plate and a meniscus lens concentric with the mirror surface, a system is obtained whose performance at a nominal aperture of f /1.2 is much superior both to that of the Schmidt camera and to that of the Maksutov system with spherical surfaces. The colour-error introduced by the meniscus is compensated by making the plate in the form of a cementable doublet with a spherical interface. The residual colour-error is comparable with that of the classical Schmidt camera. An analysis is made of the off-axis errors of the system and it is shown that, working over an 18° diameter field, the system sends 97 per cent. of the light of each image into a confusion-circle of angular diameter 12 seconds of arc. Over a 30° field, 97 per cent. of the light of each image is sent into a confusion circle 21 seconds of arc in diameter. The remaining 3 per cent. of the light does not sensibly affect the resolution, but it can be eliminated if desired, by introducing a pair of additional stops close to the aspheric surface, at the cost of a 6 per cent. light-loss by vignetting at the edge of the field. The overall error-spreads are thereby reduced to one tenth of those of a classical Schmidt camera in the first case and one sixteenth in the second.