In the early days of radio astronomy, many radio sources were detected for which no corresponding visible object was known. By 1960 several hundred such sources were listed in the Third Cambridge Catalog and astronomers were scanning the skies in search of visible counterparts to these radio sources. Their job was made difficult both by the low resolution of the radio observations (which meant that the observers did not know exactly where to look) and by the faintness of these objects at visible wavelengths.
— Astronomy: A Beginner's Guide to the Universe (7th Edition)
by Eric Chaisson, Steve McMillan