Proportional to phi 2 so wide-angle imaging most affected. Focal length of plane of light perpendicular to focal plane different to that from the mutually orthogonal plane. Also these two planes form lines at their focal points which are orthogonal. Thus produces elliptical images. Schmidt Telescopes Spherical primary with corrector plate placed across aperture, with smoothly varying radial thickness, to remove the aberration.
Stops limit the effective aperture so that angles of off-axis rays are small, minimising coma.
The principle of the Schmidt camera. Thus there are no off-axis aberrations. The right hand image is a schematic of the Schmidt at Siding Spring, with the inset showing the shape of the correcting lens. The lower diagram shows a Maksutov-Cassegrain camera in which the tube is made considerably shorter because light is reflected off an aluminized patch on the back of the meniscus correcting lens and is brought to a focus behind the mirror. Both these designs create field curvature in the focal plane.