Turbulence in Earth's atmosphere limits spatial resolution from ground to 1 arcsec. Causes the 'twinkling' of the stars. Most large optical telescopes are reflectors rather than refractors. Large refractors would require large lenses of very high quality - extremely expensive to make. Also can only support the lens by its edges and it sags under its own weight.
Reflectors are much easier.
Only have to grind the front side precisely. Can support along the back to reduce sagging. Important optical telescope technology. Spectrographs - devices that take the light from cosmic objects and spread it out into its component colors. People used to use photographic plates to make astronomical images but now use mainly charge coupled devices.