Before the image generated by the telescope reaches the camera sensor, additional correction is useful:
- Flatteners smooth the slightly curved image from mirror telescopes and Schmidt-Cassegrain optics.
- Coma correctors eliminate a Newtonian telescope’s coma defect
- Reducers are strictly speaking not correctors. They simply shorten the focal length and therefore reduce the required exposure time.
The properties of a reducer are often included in the flatteners and coma correctors, that is, these lenses also change the focal length.
These correctors are not required for purely visual observation, in this case the quality of the eyepiece is more important.