The conventional Schmidt does this with a refractive corrector plate which also defines the entrance pupil.

In the compact form, the corrector plate is located at or near the focus of the primary mirror.

In the non-compact, the corrector plate remains at or near the center of curvature (twice the focal length) of the primary mirror.

The problem was the difficulty in figuring the corrector plate.

The corrector plate was recently replaced using glass that is transparent to a wider range of wavelengths.

He employed a very clever method (the so-called "vacuum pan" method) to make the difficult "corrector plate," so that the system gave superb images.

This allowed for the inexpensive mass production of corrector plates of uniform shape.

Like the Maksutov-Cassegrain, the size of the instrument is limited, due to the mass of the corrector plate.

In 1963, Gascoigne developed a device, known as an optical corrector plate, which allowed wide field photography on the new 40-inch telescope at Siding Spring.

Such corrector plates were subsequently used on many telescopes and became known as Gascoigne correctors.