Different colors focus at different places. It is corrected with a double lens.
Paraboloidal reflectors have a small field of sharp focus.
Radio telescope These are almost all of the reflector type. These differ from optical and radio in that they send radiation as well as receive it. All other types of telescopes are restricted to merely observing what naturally comes from the object under study. Radar astronomy is only usable within the solar system. We don't learn about the stars from radar.