Why do astronomers need large telescopes?
Large telescopes can collect more light
Can detect fainter objects
Have more light for specialized analysis.
Large telescopes can form more detailed images
Waves spread out as they go through an opening.
The larger the opening, the less they spreads out.
The longer the wavelength the more they spread out
Angle of spread /D  where D is Diameter of telescope
Radio telescopes have to be much bigger than visible ones