The Ritchey-Chrétien telescope or RCT is a specialized
Cassegrain telescope with a hyperbolic primary and secondary mirror. It
was invented in the early 1910s by American astronomer George Willis Ritchey
(1864–1945) and French optician Henri Chrétien (1879–1956).
Ritchey constructed the first successful RCT, which had a diameter of
0.5 metres, in 1927. The second RCT was a 1-metre instrument constructed
by Ritchey for the United States Naval Observatory.
The Ritchey-Chrétien design is free of first-order
coma and spherical aberration, although it does suffer
from third-order coma, severe large-angle astigmatism,
and comparatively severe field curvature. When focused
midway between the sagittal and tangential focusing planes,
stars are imaged as circles, making the RCT well suited
for wide field and photographic observations. As with
the other Cassegrain-configuration reflectors, the RCT
has a very short optical tube assembly and compact design
for a given focal length. The RCT offers good off-axis
optical performance, but examples are relatively rare
due to the high cost of hyperbolic primary mirror fabrication;
Ritchey-Chrétien configurations are most commonly
found on high-performance professional telescopes.