Given the above explanation

Given the above explanation, it is obvious that coma is not really caused by any defect in workmanship as some people mistakenly believe. A telescope’s mirror can possess a perfectly shaped parabolic reflection surface, but if this surface is deeply curved it will still have a lot of coma toward the edge of its field of view! That is, unless an extra optical element is added to the system to cancel the coma. If a lens bends a light beam at a very steep angle, it may also cause coma.

When the reflecting surface of a mirror focuses at a different distance (focal length) when reflected from one diameter of the mirror than it does from another diameter, that is a condition known as astigmatism. It manifests itself by showing a sharp in-focus image in one part of the field of view while the rest of the field is out of focus and hazy. Bringing the out-of-focus part into focus will cause the previously in-focus part of the field to go out of focus.

These explanations were to help you understand how telescopes work. The objectives of Schmidt-Cassegrainians and Maksutov-Cassegrainians might show all of these defects if they were by themselves. When such a mirror is in one of these telescopes; however, the corrector plate or meniscus will cancel out the defects. What you end up with is a telescope that behaves as if the objective were functioning perfectly.