The refraction-reflective optical system composed of mirrors and lenses can combine the characteristics of reflective and transmissive systems, use spherical mirrors instead of aspherical mirrors, and use compensation lenses to correct the aberration of spherical mirrors, so as to obtain better image quality. However, such systems are often bulky, difficult to process, and relatively high in cost. Typical catadioptric systems are listed below:

 

1) The primary mirror of the Schmidt system is a spherical mirror, and a correction plate is installed in front of it, as shown in Figure 6-8. The aberration of the spherical mirror can be corrected according to the change of the thickness of the correction plate, but the structure size of this system is large, and the processing of the correction plate is difficult.

 

2) The primary mirror of the Maksutov system is a spherical mirror, and a negative lens (called a Maksutov correction plate) is used to correct the aberration of the spherical mirror. If the diaphragm and the Maksutov correction plate are set near the spherical center of the primary mirror, the off-axis aberration of the objective lens can be further reduced.