Sizes

• Biggest optical reflectors - about 20 feet across

• Biggest refractor - one meter across, at Yerkes Observatory

• Reflectors can be made bigger than refractors because
  1. you need only a good surface, not a good piece of glass,

  2. can't support refractors from below, and glass sags if you do not support it,

  3. chromatic aberration is difficult to correct.
• Biggest radio telescopes - about 200 feet across