Imagers produce a 2-dimensional image of the sky. These are best for determining shapes of objects and finding the relationships between different objects. Before the late-19th century, all astronomy was done by drawing what the astronomer saw. Through most of the 20th century, astronomers took photographs of spectra and images. After WWII, many radio telescopes built.
Mirrors only have to be smoother than the light wavelength, so radio dishes can be quite rough.
Achieves resolution of 0.001''. Detectors must be kept very cool. For work between 100000 and 1mm, we need to get out of the atmosphere. Also some molecular transitions. The International Ultraviolet Explorer is a small UV telescope that has been in orbit since 1979.