Round concave mirrors are catoptric imaging. Mirrors( including convex) are reflecting back for imaging rather than get through the light, ray of light follows the law of reflection, that is what we called convergence. They always gather the light that shoot to the surface, ray of light that is entered parallelly will be focus on the spot again, ray of light is reflected from different angles because of different directions of surface normal. Round concave mirrors can not only focus parallel ray of light on the spot but also reflect parallel light from spot.
The diagram of a round concave mirror shows focus, focal length, centre of curvature and principal axis. Concave mirrors or gathering mirrors will deflect reflected ray of light. Concave mirrors will have different images because of the different distance between objects and mirrors, which is different from convex mirrors.
Round concave mirrors don’t have chromatic aberration because they are catoptric imaging, this advantage is can’t be compared with any lens. The resolution ratio is in direct proportion to the clear aperture of objective, it is difficult for the production of large aperture lens while concave mirrors that made according to reflection principal are easier. Therefore, round concave mirrors are always used for producing telescope.