Many telescopes appear tilted so that the frame lines up with the celestial poles. In recent decades, several new ideas have been tried to solve old and new problems. Light passing from one air parcel to another is refracted. A new technique is adaptive optics. The mirrors move to keep up with changes in the atmosphere.
Sky brightness limits the faintest objects that can be studied.
Traditional solution is mountain sites. Newer solution is to get cities to use low pressure sodium street lights, which only glow in a few wavelengths. Otherwise, go to bed early. Before spacecraft and very high altitude balloons, no astronomy was done outside of these windows. A 5m mirror is difficult to support and move around.