Chromatic Correction of Wide-Aperture Catadioptric Systems

Abstract

In the Schmidt camera system, where the spherical aberration of a mirror for a selected wave-length of light may be completely corrected by a suitably figured plate at the centre of curvature, there remains a chromatic variation of spherical aberration which sets a limit to the performance on the axis and for small angles of field, and which, for large relative apertures, becomes undesirably large. The effect is commonly reduced by introducing first-order axial chromatic aberration by using a corrector plate in the form of a figured lens of finite power instead of a figured plane parallel plate, the form in which the axial and marginal thicknesses are equal being not far removed from the optimum. Fig. 1 shows the departures (measured in d wave-lengths per 25 mm. of focal length) of the C and g emergent wave-fronts from the spherical d wave-front for a Schmidt system of relative aperture f/0·7, using for the corrector plate a borosilicate crown glass, n_d = 1·516, V = 64·1, the maximum aberration for g being 1·5 wave-lengths when the marginal and intermediate zonal errors are equalized. The chromatic difference of spherical aberration is, of course, of the sign corresponding to an increase of glass-path with increasing aperture.