Turn centering screws. But this increases focal length, causing telescopes to have to be long and skinny. A solution to the problem of long telescopes. First built by Newton in 1668. Used a parabolic mirror.
Catadiopteric telescopes use lenses and mirrors.
Larger telescopes allow observewrs to see fainter objects. Since mirror or lens is a light collector, a larger mirror or lens collects more light. Mt. Wilson, Cal. Major disadvantage is poor resolving ability. Wavelengths of radio waves are 1000's of times larger than those of visible light.