Telescope sizes determined by the diameter of the largest mirror or lens. The largest refracting telescope is 1m. B.2 Reflecting Telescopes Light is reflected off of curved mirrors to a focus. Mirrors can be more easily supported than lenses, only need good surfaces, and have no chromatic aberration. Almost all large telescopes are reflectors.
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.