The corrector plate reduces or eliminates aberration caused by the mirrors.
Finally, catadioptric telescopes combine both lenses and mirrors. Light first passes through a large front lens, called the corrector plate, and on toward the primary mirror at the back of the tube. The corrector plate reduces or eliminates aberration caused by the mirrors. After bouncing off the primary, the light reflects toward a secondary mirror at the front, then back through a small hole in the middle of the primary and into an eyepiece.