Catadioptric telescopes skillfully combine the advantages of refractive telescopes and reflective telescopes. Its objective lens contains both a lens and a reflector, and the light of the object will be refracted and reflected in the telescope at the same time. The basic principle is to add a refraction element to correct aberration on the basis of the sphere mirror. For example, the common Schmidt-Cassegrain telescope places a central convex, peripheral concave, complex wave-shaped correction lens at the front end of the lens barrel. After the light enters, it first passes through the correction lens to produce twists, then reflects the focus through the rear sphere main mirror, and then reflects the light to the opening behind the main Another Markustof-Cassegrain telescope uses a lens with the same curvedity on both sides and concave in the direction of the main mirror as a correction mirror. The light passes through the correction lens and produces a twist, which is reflected by the reflection of the mirror, and then reflected back to the image at the central opening of the main mirror.