Telescope reducers, coma correctors and flatteners are used to improve the image quality of telescopes. They are optical devices that are placed between the telescope’s focuser and the camera or eyepiece.
Focal reducers are used to reduce the focal length of a telescope, which in turn reduces the magnification and increases the field of view. They are useful for astrophotography with refractor and Cassegrain-type telescopes.
Field flatteners and coma correctors are used to correct for field curvature and coma of the objective lens or mirror.