Coma is an optical aberration which affects all Reflector Telescopes. Coma makes stars towards the edge of the eyepiece or image appear elongated, and is especially apparent in fast focal ratio telescopes, such as f/4. This issue is easily fixable with a coma corrector, but this extra accessory is an added cost on top of the telescope itself and can get expensive. Generally speaking, you won't need a coma corrector if you're just observing visually unless you're using a fast telescope or prefer excellent edge views in your eyepiece. For deep sky imaging, though, a coma corrector is usually advised, especially if you're using a large sensor camera.