 Telescopes for radio wavelengths can be much bigger physically, such as the aperture fixed focus radio telescope of the Arecibo Observatory.Freely steerable radio telescopes with diameters up to have been in operation since the 1970s.These telescopes have a number of features in common, in particular the use of a segmented primary mirror (similar to the existing Keck telescopes), and the use of high-order adaptive optics systems.