Cassegrain reflector

A Cassegrain arrangement places a secondary before the primary focus and returns light through a hole in the primary to a focus behind it. In the classical design the primary is parabolic and the secondary convex hyperbolic. The compact arrangement has a long effective focal length; its classical implementation retains off-axis coma.

A Ritchey–Chrétien telescope has hyperbolic primary and secondary mirrors chosen to cancel third-order spherical aberration and coma. It retains astigmatism and field curvature; wide-field instruments may require additional correction.

A classical Cassegrain reflector uses a concave parabolic primary and a convex hyperbolic secondary before the prime focus. The secondary returns the converging beam through a hole in the primary. It corrects on-axis spherical aberration but retains off-axis coma.