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. 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.