Schmidt plate
The Schmidt plate , named after Bernhard Schmidt , is a correction lens that is attached to the front end of the tube of a Schmidt , Schmidt-Cassegrain or Schmidt-Newton telescope .
It is used to compensate for the spherical aberration of the spherical main mirror. This is necessary because the diaphragm arrangement according to Schmidt, which is used to avoid the coma , which images objects off the optical axis in a distorted manner, only works with such a diaphragm. The aspherical cut of this lens allows the production of almost coma-free optics with a large field of view at the same time.