No telescope is free from optical aberrations. These are noticeable as colour fringes around bright objects as well as a blurring or distortion (coma) toward the edge of the image field. The latter two geometric image errors can be considerably reduced with a corrector downstream of the primary optic.
In general, optics with a long focal length and a small aperture ratio are less affected by aberrations. For astrophotography, however, a large aperture ratio is desirable. Here the primary optic’s aberrations need to be be corrected.
Correctors include field flatteners (or simply flatteners) for refractor telescopes and coma correctors for Newtonian telescopes.
Some telescopes are supplied with correctors already included. If the technical specifications for the corrector specify integrated, it is already installed. This is the case, for example, with Meade’s ACF optics, Celestron’s EdgeHD optics and with Petzval refractors.