The basic Ritchey–Chrétien two-surface design is free of third-order coma and spherical aberration, although it does suffer from fifth-order coma, severe large-angle astigmatism, and comparatively severe field curvature. The remaining aberrations of the basic design may be improved with the addition of smaller optical elements near the focal plane. 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 the Ritchey–Chrétien configuration is most commonly found on high-performance professional telescopes.